WO2013030709A2

WO2013030709A2 - Portable device, system and method for measuring a caloric expenditure of a person's physical activity

Info

Publication number: WO2013030709A2
Application number: PCT/IB2012/054171
Authority: WO
Inventors: Joyca Petra Wilma Lacroix; Marten Jeroen Pijl; Gijs Geleijnse; Privender Kaur Saini; Aart Tijmen Van Halteren
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2011-08-31
Filing date: 2012-08-16
Publication date: 2013-03-07
Anticipated expiration: 2014-02-28
Also published as: WO2013030709A3

Abstract

The present invention relates to a portable device for measuring a caloric expenditure of a person's physical activity, said portable device (10) comprising: -girth measurement means (12) for measuring changes in girth of a body part (16) of the person (20) over time to generate a girth signal (18, 18'), said body part (16) being a part of the person's body that varies in girth due to muscle activity during the physical activity,and -processing means (14) for analyzing the girth signal (18, 18') to determine a measure for the muscle tension of said body part (16), for determining a level of physical load of the person's physical activity depending on the determined measure for the muscle tension, and for calculating the caloric expenditure of the person's physical activity based on the determined level of physical load of the person's physical activity.

Description

Portable device, system and method for measuring a caloric expenditure of a person's physical activity

FIELD OF THE INVENTION

The present invention relates to a portable device for measuring a caloric expenditure of a person's physical activity. The invention relates further to a corresponding method and a system. Even further, the present invention relates to a processing device for determining a caloric expenditure of a person's physical activity and to a corresponding processing method, as well as to a computer program for controlling said processing device to carry out the steps of said processing method.

BACKGROUND OF THE INVENTION

Due to the growing number of people that live an inactive life, many physical activity promotion products and services have been developed over the last decades, both for research and commercial objectives. Said physical activity promotion products in most cases try to calculate or estimate a caloric expenditure of a person's physical activity. Measuring the caloric expenditure for activities performed in a gym with dedicated gym devices is comparatively easy, since the physical activity is performed in a controllable surrounding. Besides the pulse of the person, activity parameters can be easily measured directly from the gym device, i.e. measuring the speed of a walking machine the participants use for running workout or measuring the weight/force of a weight lifting device.

However, measuring the caloric expenditure in uncontrollable surroundings, i.e. for outdoor sport activities, is much more complicated, since the physical parameters influencing the caloric expenditure are harder and inexactly to measure. Parameters in this case cannot be taken or measured from controllable gym devices. Such outdoor physical activity measurements are often performed using wearable measurement devices. Wearable technology allows for on-body real-time measuring of the amount of physical activity performed and can be used to provide feedback about the monitored data to increase an individual's attention to the desired behavior and goals. Most of the devices measure the pulse and the acceleration of a specific part of the body with an acceleration measurement device with which the participant is equipped. A measurement device of this kind is for example known from US 2008/0090703 Al . This document refers to a computer implemented method and apparatus for automated personal repetition counting and exercise orchestration. A portable computing device is programmed to access a pre-planned exercise regime for a user, the regime including a series of deferring exercise activity sets, each exercise activity set including a prescribed exercise repetition count for a prescribed exercise type. However, the device shown therein only enables to measure a repetition count, for example the repetition during a weight lifting exercise, but is not able to directly measure or calculate a caloric expenditure of a person's physical activity.

Another method and device known in the art is described in WO 2010/046448 Al . This document refers to a method and device enabling an athlete to determine and then control the rate of displacement of a mass. This muscular training exercise performing method for athletes involves measuring series of acceleration values using a portable accelerometer, and calculating modified set point rates by said portable accelerometer.

However, it does, similar to the method and apparatus mentioned before, not indicate how to calculate energy expenditure from the measured values.

A device for determining calories expended by a person while running is known from EP 0 119 009 Al . This device enables for determining the running speed, the distance traversed, the elapsed time and calculates the caloric expenditure therefrom. A pressure switch or transducer located in a show senses when a foot of the runner is in contact with the surface and produces a foot signal having a duration proportional to the time the foot is in contact with the surface. A radio frequency transmitter is coupled to the pressure switch or transducer and transmits the foot contact signal. A radio frequency receiver receives the foot contact signal transmitted by the frequency transmitter and a microprocessor coupled to the radio frequency receiver calculates, solely from the foot contact signal, an output speed signal representing the speed of the runner. The liquid crystal display coupled to the output of the microprocessor displays the speed of the runner in accordance with the output speed signal. The output speed signal, the distance traversed and the elapsed time are then taken to calculate the calories expended by the person during running. However, this device does not enable for a precise caloric expenditure measurement since it does not account for the intensity, i.e. the level of physical load of the activity, meaning that it registers movements only and is insensitive to variations in strength with which the movements are performed.

Therefore, similar movements performed with more strength, e.g. running with a similar speed but with head wind versus running with back wind, will lead to similar caloric expenditure measurements. This is regarded to be a major disadvantage since the measurement results in an inaccurate estimation or calculation, respectively, of the caloric expenditure.

Philips DirectLife, a product produced by the applicant, is a commercially available physical activity lifestyle program aimed at supporting individuals to increase their physical activity level. DirectLife includes a web service, an activity monitor and human coaching support. Several years of experience with a large set of DirectLife users have shown that participants attach high importance to the accuracy of the measurements of the activity monitor. In particular, when participants have engaged in heavy activity or exercise, they consider it rewarding to see this back in the feedback, both on the device and in the activity graphs on the DirectLife website. If the activity monitor has not registered the intensity of the activity accurately, this can be experienced as de-motivating and has a negative impact on the overall perception of the program. Currently, activity monitoring in the DirectLife activity monitor is, similar as described in EP 0 119 009 Al, based on three-axial accelerometry. The activity monitor therefore also only registers movements and is insensitive to variations in strength with which the movements are performed. Thus, according to this method also, similar movements performed with more strength will lead to similar activity patterns, despite the fact that they are experienced as more intense and they require a higher caloric expenditure.

An arrangement and method which accounts for the physical load and intensity of the person's physical activity is not known so far. Such a system would be especially desirable, since feeding back accurate measurements of caloric expenditure for moderate and high intensity activities by taking into account the level of physical load of the activity would lead to a higher satisfaction of activity program participants that engage these activities.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a device, a system and a method of the kind mentioned initially, which enable an improved and more accurate energy expenditure measurement of a person's physical activity, wherein said measurement also takes into account an intensity with which the physical activity is performed.

In a first aspect, this object is, according to the present invention, achieved by a portable device for measuring a caloric expenditure of a person's physical activity, said portable device comprising: girth measurement means for measuring changes in girth of a body part of the person over time to generate a girth signal, said body part being a part of the person's body that varies in girth due to muscle activity during the physical activity, and

processing means for analyzing the girth signal to determine a measure for the muscle tension of said body part, for determining a level of physical load of the person's physical activity depending on the determined measure for the muscle tension, and for calculating the caloric expenditure of the person's physical activity based on the determined level of physical load of the person's physical activity.

In a second aspect of the present invention, a corresponding method is presented, which includes the steps of:

measuring changes in girth of a body part of the person over time to generate a girth signal, said body part being a part which varies in girth due to muscle activity during the physical activity,

analyzing the girth signal to determine a measure for the muscle tension of said body part,

determining a level of physical load of the person's physical activity depending on the determined measure for the muscle tension, and

calculating the caloric expenditure of the person's physical activity based on the determined level of physical load of the person's physical activity.

In a third aspect of the present invention, a system for measuring a caloric expenditure of a person's physical activity is presented, which comprises:

a portable girth measurement device for measuring changes in girth of a body part of the person over time to generate a girth signal, said body part being a part of the person's body that varies in girth due to muscle activity during the physical activity, and a processing device which comprises an interface for receiving said girth signal, and a processing means for analyzing the girth signal to determine a measure for the muscle tension of said body part, for determining a level of physical load of the person's physical activity depending on the determined measure for the muscle tension, and for calculating the caloric expenditure of the person's physical activity based on the determined level of physical load of the person's physical activity.

In a further aspect of the present invention, a processing device and a corresponding processing method are presented for calculating a caloric expenditure of a person's physical activity. In a still further aspect of the present invention, a computer program product is presented comprising program code means for causing a computer to control said processing device to carry out the steps of said processing method when said computer program is carried out on the computer.

Preferred embodiments of the invention are defined in the dependent claims. It shall be understood that the claimed measuring method, the claimed system, the claimed processing device and the claimed processing method have similar and/or identical preferred embodiments as the claimed portable device and as defined in the dependent claims.

It has been recognized by the inventors that the measurement of a caloric expenditure of a person's physical activity may be improved by measuring the girth of a body part of the person over time and by analyzing this girth signal in order to determine the caloric expenditure of the person therefrom. The girth is thereby preferably measured at a body part of the person that varies in girth due to a muscle activity, which occurs at said body part when the person performs the physical activity. In other words, the girth is preferably measured on a body part, for example on a leg or an arm of the person, which is most intensively used during the physical activity. Said girth measurement means can, for example, be a mechanical or an electronic sensor which measures the girth of said particular body part over time. It is to be noted that, of course, the girth itself (e.g. measured in cm) as well as the changes of the girth (e.g. the strain measured in % of a reference girth) can be measured over time. Both ways enable to generate a girth-over-time signal which gives information about the muscle activity of the measured body part that occurs during the physical activity of the person.

By analyzing said generated girth signal it is thus possible to determine a measure for the muscle tension of the measured body part. This measure could be the tension of a muscle which is located in the measured body part. This would for example be possible when measuring the girth of the upper arm, since the girth of the biceps, for example, anatomically depends on the muscle tension occurring within the biceps. In this way, it is thus possible to determine the muscle tension from the measured girth at this part of the body.

However, the described measure for the muscle tension does not necessarily need to be an exact value. Instead of determining an exact tension value, it is also

conceivable to use the measured girth signal for determining a comparative value which is a measure for a magnitude of the muscle tension compared to a reference magnitude of the muscle tension in unstressed conditions of the muscle, i.e. in a condition where the measured muscle is more or less relaxed. In this case, the measure for the muscle tension determined from the girth of the body part defines a level of magnitude, such as low, middle or high muscle tension. Of course, different levels of magnitudes for the muscle tension could be defined. Similarly, it is also possible to define a grading scale defining different grades of muscle tension levels. Nevertheless, it is clear that the highest accuracy for the finally determined caloric expenditure is realized when the measure for the muscle tension is as exact as possible.

Depending on the determined measure for the muscle tension, a level of physical load of the person's physical activity may be defined in the next step. This level of physical load denotes the level of intensity the physical activity is performed with. Again, this intensity level can either be an exact value or an approximate grading level on a predefined analysis scale, such as "very low intensity - 1", "low intensity - 2", "medium intensity - 3", "medium to high intensity - 4", "high intensity - 5", and "very high intensity - 6". In this way, the physical activity of the person is kind of assigned to a score level (e.g. 1 to 6). However, it is clear that again an exact value finally results in a most accurate caloric expenditure measurement. An exact value in this context means a value which describes the level of intensity of the physical activity as accurate as possible. Since no scale or units are known in the art describing the intensity of a physical activity, a scale may be proposed that defines the intensity as a function of the muscle tension depending on the girth of the body part where the muscle tension is measured. "Intensity" in the context of the present invention means a value that describes the level of physical load of a physical activity that is derived from a measured muscle tension at a specific body part.

Finally, the caloric expenditure of the person's physical activity is calculated based on the determined level of physical load/intensity that has been assigned to the person's physical activity. In other words, the presented measurement for calculating the caloric expenditure takes into account the intensity, with which the physical activity is carried out. A calculation of the caloric expenditure based on such an intensity level is especially possible for physical activities in which there is a known relation between the muscle tension that is measured depending on the girth of a specific body part used during the activity and the energy that is afforded, respectively consumed by such kind of physical strain. For example, for activities like weight lifting, cycling or jogging it is possible to determine a direct association between the intensity level with which the activity is performed and the caloric expenditure. By measuring the girth on a body part that is most intensively used during these activities, the intensity level can be calculated or at least estimated so that the caloric expenditure can be determined therefrom. In the above-mentioned exemplary activities, the portable device is, for example, attached to the upper arm measuring the girth of the biceps when the person performs a weight lifting activity, or it is attached to the thigh/upper leg, when the caloric expenditure is to be measured during a cycling or running activity.

In the above-mentioned way, the presented portable device and the

corresponding method enable to measure the caloric expenditure of the person's physical activity. The presented device and the corresponding method are thereby sensitive to the strength with which the movements are performed in said activity. In contrast to known devices and methods from the prior which only take into account an acceleration

measurement of a specific body part, the accuracy of activity energy expenditure

measurements is improved, in particular for activities that involve moderate to high muscle strength. This improvement mainly relies on the fact that the presented device and method are able to determine the caloric expenditure based on a measurable intensity of the activity that depends on the muscle tension of a body part used during this activity, which again is measured via a girth measurement device that measures the girth of said specific body part during the time of the activity. In contrast to caloric expenditure measurements that are only based on three-axial accelerometry similar movements performed with more strength, e.g. cycling with a similar frequency, but with head wind versus without wind, will lead to different measurement results, whereas accelerometry measurement devices would provide the same result in such a situation, i.e. the same caloric expenditure.

Since the presented device enables to distinguish these different intensities of the activity, it provides more realistic measurement values of the caloric expenditure. Since the participants attach high importance to the accuracy of the measurements, a device that has the ability to register the intensity of the activity will have a positive impact on the overall perception of the user. Feeding back accurate measurements of the caloric expenditure for moderate and high intensity activities will thus lead to a higher satisfaction of the users.

According to an embodiment, the processing means is adapted to analyze the girth signal to determine a measure for the muscle tension of said body part by integrating the girth signal over a time-interval. The time-dependent integral of the girth signal over time signal gives a good approximation of the physical load to which the muscles are exposed at this part of the body. If the measured body part is a part that is most intensively used during the physical activity, this measure is a good indication of the overall intensity of the person's physical activity. Even though integrating the girth signal over time seems to be the most accurate method to determine the level of intensity of the physical activity, the measure for the muscle tension of said body part may also be determined based on the full width at half maximum of a peak in the girth signal. Furthermore, it is conceivable to simply determine the intensity of the physical activity by identifying the peak values within the girth-over-time signal.

In order to further improve the measurement accuracy, acceleration measurements may additionally be taken into account. According to an embodiment of the present invention, the portable device therefore further comprises an acceleration

measurement means for measuring an acceleration of a part of the person's body in at least one spatial dimension during the physical activity, wherein the processing means is adapted to calculate the caloric expenditure of the person's physical activity based on the determined level of physical load of the person's physical activity and on the measured acceleration of said part of person's body.

According to this embodiment, the acceleration means may be realized by an accelerometer or a gyroscope measuring the acceleration at a significant position on the person's body in at least one spatial dimension. Preferably, the acceleration measurements are performed similarly as this is done according to the prior art. In running appliances, for example, this accelerometer may be attached to the running shoe measuring the accelerations occurring at the feet of the user. As this is proposed by EP 0 119 009 Al, the acceleration measurement means is, according to this embodiment, adapted to perform a three-axial accelerometry during the time of the activity. The processing means is adapted to determine the running speed, the distance traversed, the elapsed time and to calculate the caloric expenditure therefrom. The acceleration measurement means is therefore realized by a pressure switch or a transducer located, for example, in a show of the user. The pressure switch/transducer senses when a foot of the user is in contact with the surface and produces a foot signal having a duration proportional to the time the foot is in contact with the surface. The processing means may, according to this embodiment of the present invention, be adapted to calculate an output speed signal from the foot contact signal that represents the speed of the user. The processing means is further adapted to calculate the calories expended by the user, by analyzing the output speed signal, the distance traversed and the elapsed time. Even further, the processing means is adapted to calculate a "corrected/adapted" caloric expenditure based on the level of physical load that has been determined by the help of the girth measurement means.

In this case, the user wears the portable device including the girth measurement means, i.e. the above described girth sensor, together with the accelerometer throughout the engagement in the activity. Based on the accelerometer data and the girth measurement data that are both measured over time, a caloric expenditure value may be determined that does not only take the occurring accelerations, but also takes the level of intensity of the physical activity into account. In other words, in this embodiment an acceleration-based caloric expenditure measurement is further improved by a measurement of the intensity of an activity which is derived from a girth measurement of a significant body part measured with a girth measurement means.

It is to be noted that the girth measurement means and the acceleration measurement means, depending on the type of the physical activity, not necessarily need to be attached to the same part of the user's body. For example, in an appliance used for measuring the caloric expenditure of a runner, the girth measurement means is preferably attached to the thigh, since the thigh muscles give the most significant information of the intensity of the running activity, whereas the acceleration measurement means is in this case preferably attached to the shoe. However, there are also appliances, such as weight lifting, where the girth measurement means and the acceleration measurement means is preferably attached to the same body part, in this case near or on the biceps.

By applying a combination of an accelerometer-based and an intensity-based measurement using the acceleration measurement means and the girth measurement means, as this is done in this embodiment of the present invention, it is possible to adapt the acceleration-based calculation of the caloric expenditure with the intensity levels of the physical activity that are determined from the girth signal that includes information about the muscle tension at a measured body part of the user.

The accelerometer-based calculation of the caloric expenditure may for example be adjusted to a more accurate value, if the girth or strain measured by the girth measurement means exceeds a pre-given personalized threshold value. In other words, if the generated girth signal exceeds a value denoting a high level of intensity of the physical activity, the processing means adjust the value calculated for the caloric expenditure by the accelerometer-based measurement to a corresponding higher value. If on the other hand the girth signal measured by the girth measurement means underruns a pre-given personalized threshold value, the results of the accelerometer-based measurements will be scaled down, accordingly. Taking the above-mentioned analysis scale as an example, the accelerometer based value for the caloric expenditure could be scaled as follows:

scaling the (accelerometer based) calculated caloric expenditure by 0.9 for

"very low intensity - 1"; scaling the (accelerometer based) calculated caloric expenditure by 0.95 for "low intensity - 2";

scaling the (accelerometer based) calculated caloric expenditure by 1 for "medium intensity - 3";

scaling the (accelerometer based) calculated caloric expenditure by 1.1 for "medium to high intensity - 4";

scaling the (accelerometer based) calculated caloric expenditure by 1.3 for "high intensity - 5"; and

scaling the (accelerometer based) calculated caloric expenditure by 1.5 for "very high intensity - 6".

It is to be noted that the above mentioned values only represent exemplary numbers. Correct values need to be found by way of experiment. Of course, the scaling values may differ depending on the position of the girth measurement, depending on the type of the physical activity as well as depending on other personal and physical parameters, such as age gender, etc.

To even further improve the accuracy of the caloric expenditure measurement, the portable device, according to an embodiment, further comprises an input interface means for receiving the person's personal data, in particular an age, a gender, a body weight and/or a pulse-over-time measurement, and the processing means is adapted to calculate the caloric expenditure of the person's physical activity based on the determined level of physical load of the person's physical activity and on the received personal data.

According to this embodiment, the portable device does not only include a girth measurement means for measuring changes in girth and determining a level of intensity of the physical activity therefrom, but also includes an input interface means which enables the user to enter his/her personal data. A conceivable way of implementing this is, for example, providing a small keypad that the user may use to enter his/her age, gender and body weight. It is to be noted that also other personal data may be entered and processed by the presented device in order to calculate a caloric expenditure therefrom. Instead of a keypad, also other types of interfaces are conceivable, such as, for example, a USB-interface or a wireless interface (e.g. a Bluetooth-interface) that is used to copy the personal data of the user to the portable device.

The received personal data may be used by the processing means for a so- called standard caloric expenditure measurement. The processing means may particularly be adapted to calculate the Basal Metabolic Rate (BMR) which is a known estimation value for the amount of daily energy expended by the body.

Several prediction equations exist to determine this BMR value. The processing means is according to this embodiment preferably adapted to calculate the BMR according to one of the following equations:

A formula known as Harris-Benedict equation:

f U.15 \6m 5.0033/z 6.7550a _{rr Λ}^_η ^Λ kcal _r

■ + + 66.4730 lor men

lkg lcm \year day

where P is a total heat production at complete rest, m is the weight, h is the height, and a is the age. The difference in BMR for men an women being mainly due to differences in body weight.

A further known formula to calculate the BMR is known as Mifflin-equation:

where P again is the total heat production at complete rest, m is the weight, h is the height, a is the age, and where s is +5 for men and -161 for women.

Other formulas exist which take into account a lean body mass, two of which are known as Katch-McArdle formula and Cunningham formula.

In Katch-McArdle formula:

P = 370 + (21.6 x LBM) ,

where LBM is the lean body mass in kg.

Or the Cunningham formula:

P = 500 + (22 x LBM) ,

where LBM is again the lean body mass in kg.

Of course, the above-mentioned equations are known in the art and used in many known appliances. The values determined with these equations are only rough estimations for the daily caloric expenditure of a person and, standing alone, are not capable for determining an accurate caloric expenditure of a person's physical activity. Nevertheless, these formulas can be implemented in the processing means of the presented portable device and used in combination with the above-mentioned girth measurements that take into account the intensity level of the physical activity of the user. A device that combines these two calculation methods for determining the caloric expenditure is so far not known in the art. Additionally taking into account the muscle tension that, as explained above, gives an information about the intensity of the physical activity enables a further improved estimation of the caloric expenditure.

It has shown that the measurement accuracy can be further increased by additionally equipping the portable device with acceleration measurement means in the way explained above. The user in this case wears the girth sensor together with the accelerometer throughout the engagement in the activity. Based on the intensity of the activity that has been determined with the girth sensor and the acceleration data delivered by the accelerometer, the processing means is adapted to calculate a personalized caloric expenditure value that can be taken to recalculate the standard caloric expenditure measurement that is based on the BMR.

The processing means may, for example, be adapted to calculate the calories expended during the activity based on the measured acceleration data and on the BMR corresponding to the entered personal data of the user, and then apply an adaption factor to the received amount of calories that represents the intensity of the activity. This adaption factor may be determined by the girth measurement means and the processing means in the way explained above.

By measuring the changes in girth of a specific body part of the user over time, a girth signal is generated by the girth measurement means. This girth signal is processed in the processing means to determine a measure for the muscle tension of said specific body part in order to determine a level of physical intensity therefrom. The results of this determination may, as this has already been mentioned above, be either an exact value or a rough grading, e.g. "low intensity", "medium intensity", "high intensity". These intensity levels may be mapped by the processing means to the above-mentioned adaption factors, e.g. a determined low intensity level corresponds to an adaption factor of 0.95, a medium intensity level to an adaption factor of 1.0, and a high intensity level to an adaption factor of 1.3. It is to be noted that such a simple mapping is only illustrated by way of example, wherein in practice other values and a more accurate grading scale may, of course, be taken.

According to a further embodiment of the present invention, the processing means is further adapted to analyze the girth signal to determine a measure for the muscle tension of said body part per time-interval, to determine a level of physical load of the person's physical activity per time-interval depending on the determined measure for the muscle tension in the respective time-interval, and to calculate the caloric expenditure of the person's physical activity per time-interval from the determined level of physical load of the person's physical activity in the respective time-interval. In other words, the processing means is adapted to apply a sample rate and to determine the caloric expenditure based on the measured girth signal per time-interval. The processing time-interval may either be set manually, for example by the user, to a constant time-interval measured in seconds or minutes. On the other hand, the processing means may also be adapted to automatically set the sample rate. The time-intervals in which the girth signal is analyzed may thus also be adapted to the properties of the girth signal, meaning that the processing means automatically adapt the time-intervals to shorter time-intervals when frequent variations in the girth signal are recognized, whereas the measurement is automatically adapted to longer time-intervals when the girth signal varies only slightly over time. In particular in weight lifting appliances where due to the properties of the activity many repetition movements occur and depending on that the measured girth signal will strongly vary over time, preferably shorter time-intervals are set. In a running appliance, where more or less constant movements occur during running and depending on that the girth signal will vary only slightly over time, larger time-intervals may be chosen.

It is to be noted that by decreasing/shortening the time-intervals, the measurement accuracy and, thus, the resulting accuracy of the caloric expenditure is increased. However, also the processing time increases with shorter time-intervals. If the processing means is adapted to automatically adapt the time-interval depending on the variation in the girth signal, as this has been explained above, a rather good balance between accuracy and processing time may be established. It is to be noted that of course also time- intervals may be recognized where no girth signals are received, i.e. where the device is not in use. According to an embodiment, the processing means may thus further be adapted to switch off the device when such a situation is recognized, and to automatically switch the device on again when a girth signal is received from the girth measurement means.

According to a further embodiment of the present invention, the girth measurement means is attachable to said body part and comprises a stretch sensor, in particular a strain gauge, for measuring the material strain of at least a part of said girth measurement means over time. The girth measurement means is thereto preferably realized by a stretchable band which at least partly surrounds said body part. The stretchable band can be worn unobtrusively around a leg, an arm, the hip or around any other part of the user's body. The muscle tensions at these body parts may, for example, be measured by a strain gauge that is attached or included into said stretchable band. However, there is a number of further conceivable methods for measuring the girth, respectively the strain of said body parts. Mechanical methods measuring the girth with a simple parameter sensor measuring the parameter of said body part are conceivable. On the other hand, the strain can also be measured by applying a stretchable band made of rubber with a known density and calculating the strain of said band with a simple mechanical calculation method. Furthermore, it is also conceivable to measure the strain within the stretchable band based on camera vision. Independent of the measurement method either the strain or the girth itself are measured.

According to a further embodiment, it is preferred that the stretchable girth measurement means is adapted to a specific type of physical activity and to a specific part of the body. In other words, it is, in case of the usage of a stretchable band, preferred that said band more or less exactly fits to the specific body part it is attached to. Similarly, the girth measurement means need to be adapted to the specific type of physical activity. It is thus preferred to provide different dedicated girth sensors with different sizes and measuring sensitivities depending on the type of physical activity they are used for. A girth/strain sensor in cycling appliances may, for example, be less sensitive than a girth/strain sensor used in a running appliance.

In a further embodiment, the girth sensor can be associated with multiple activities. The portable device may therefore comprise an additional activity recognition means that is adapted to recognize the type of activity by analyzing the measured girth signal. This activity recognition may, for example, be done by analyzing the pattern of the girth signal and comparing the received patterns with typical girth signal patterns relating to different types of sportive activities. These exemplary reference patterns of typical girth signals may be stored in a storage unit with which the device is equipped. It becomes clear that such an activity recognition is easy to implement, when for example analyzing the frequency, the upper or lower limits of the girth signal, since these factors mainly depend from the type of activity that is performed. In other words, frequency, peak and minimum values occurring during a cycling activity will differ from frequency, peak and minimum values occurring during a running or even during a weight lifting activity.

According to a further embodiment of the present invention, said portable device further comprises a storage unit which is adapted to store reference measures for muscle tensions belonging to known levels of physical load, wherein the processing means is adapted to determine the level of physical load by comparing the measure for the muscle tension determined in the analysis of the measured girth signal with the reference measures stored in said storage unit. In other words, the storage unit is adapted to store calibration measurements that map measured girth signals and muscle tensions to known levels of physical load. If the portable device comprises such a storage unit including these calibration data, the processing means is enabled to determine a level of physical load by comparing the measured girth signal or parts of it with similar or same girth signals stored in the calibration data set. In other words, the data set comprises reference measurements mapping already measured girth signals to known intensity levels. In a practical implementation, the storage unit may be adapted to store different data sets distinguishing between different physical activities.

The above-mentioned reference data sets may be received from a calibration of the girth measurement means. Such a calibration can either be done manually or automatically, as this will be explained below by example.

To provide a girth measurement means that is personally adapted to the anatomical properties of the user, a calibration phase is required. In this calibration phase changes in girth of a specific body part that is used for the measurement are associated with levels of intensity of the activity which directly relate to the caloric expenditure. This can be done by allowing the user to manually label the intensity on a screen that displays the measurement results of a girth and/or an acceleration signal which have been measured in the above-mentioned way by the girth measurement means and/or the acceleration measurement means during a test activity.

The user has for example performed a test activity while being equipped with the presented portable device that measures the girth and/or the acceleration of a specific body part. After having performed this test activity the received girth and/or acceleration signals may be displayed on a screen in a girth/acceleration over time graph. Given a set of labels (e.g. low-medium-high intensity) and their descriptors, the user can select a time- interval within said graph and attach this time-interval with the appropriate label. By mapping all time-intervals within said graph with appropriate labels of intensity the measured girth/acceleration signal is in each time-interval related to the intensity of the activity in the respective time interval.

It is to be noted that during this mapping process not necessarily all measurement results need to be shown to the user in detail. It is, for example, also conceivable that the measurement signals of the test activity are represented in a simplified bar graph to reduce the complexity and ease the mapping process for the user.

Alternatively, the calibration may also be performed automatically. This may be achieved by the usage of a dedicated gym device, e.g. a cycle or a rowing machine in the gym. Under these controlled conditions, these devices can estimate the caloric expenditure of the user per time-interval. In this way, these machines can be used to calibrate the girth measurement means, the acceleration measurement means or a combination thereof. Either manually or fully automatic, the caloric expenditure measured by the gym device is associated with the measured girth and/or acceleration signal received per time-interval.

Thus, the calibration phase results in a mapping between the measured girth signal and a level of intensity of an activity. It is preferable that separate girth measurement means, i.e. separate girth sensors, are used for different activity types that the user engages in, e.g. one for cycling, one for running, one for weight lifting and one for rowing with a separate calibration for each of these sensors. In order to decrease the number of girth measurement means a user needs, it is preferable that a girth measurement means can be associated with multiple activities.

This is, for example, possible by storing different kinds of calibration results, each kind representing a type of activity, in the storage unit and providing an interface means, such as a small keypad, to the user in order to classify the type of activity before the user uses the portable device.

To implement the above-mentioned procedure, it is according to an embodiment preferred that the girth measurement means are calibrated with measurement results that map known girth signals to known intensity levels of a physical activity.

The above-mentioned embodiments refer to a portable device that is equipped with a girth measurement means and a processing means, and may additionally be equipped with an acceleration means and a storage unit, as this has been explained above. However, the processing means and the storage unit do not necessarily have to be included into the portable device itself. It is also conceivable to provide a system that consists of a separate portable girth measurement device and a separate processing device.

According to an aspect, the present invention therefore also provides a system for measuring a caloric expenditure of a person's physical activity which comprises:

a portable girth measurement device for measuring changes in girth of a body part of the person over time to generate a girth signal, said body part being a part of the person's body that varies in girth due to muscle activity during the physical activity, and

a processing device which comprises an interface for receiving said girth signal, and a processing means for analyzing the girth signal to determine a measure for the muscle tension of said body part, for determining a level of physical load of the person's physical activity depending on the determined measure for the muscle tension, and for calculating the caloric expenditure of the person's physical activity based on the determined level of physical load of the person's physical activity.

The measurement of the caloric expenditure is realized in the same way as explained above. However, the user is in this case only equipped with the portable girth measurement device, which could be exemplarily realized by a stretchable band that is attached to a specific body part of the user and comprises a stretch sensor for measuring the strain/girth at said body part, whereas the processing device is realized by a separate processing device, such as e.g. a laptop or a desktop computer. The portable girth

measurement device according to this aspect of the present invention solely measures the girth of the measured body part in order to generate a girth signal over time. The above- mentioned evaluation, respectively the calculation of the caloric expenditure is in this case provided by a separate processing device which comprises an interface for receiving said girth signal and processing means for analyzing the girth signal in the above-mentioned way to calculate the caloric expenditure of the person's physical activity.

In a practical appliance a stretchable band comprising a stretch sensor is attached to the body of the user during the physical activity. A girth signal measuring the girth of said body part over time is generated and stored in a storage means with which the portable girth measurement device may be equipped. After having performed the physical activity, the user may then upload the stored girth signal from the storage means of the girth measurement device to a separate processing device, which usually is realized by a processing unit or a computer. This processing device may then calculate the caloric expenditure in the way explained above. Similarly, measured acceleration data may also be included into the calculation as well as personal data to perform a standard caloric expenditure measurement based on the BMR value. The processing device may then recalculate the BMR value based on the intensity level received from the girth measurement and based on the acceleration data received from the acceleration measurement.

The interface between the portable girth measurement device and the processing device as well as between the accelerometer and the processing device can be realized in various ways, e.g. a hardware connection, such as a USB-connection, or any kind of wireless connection, such as a Bluetooth connection or an infrared connection. In case of a wireless connection, it is also possible that the girth signal is sent from interface means of the portable girth measurement device to the processing device in real-time, wherein the caloric expenditure is based on this real-time delivered girth signal calculated also in real-time by the processing device. This real-time calculation may then be sent back again to the portable girth measurement device. In this case, the calculated caloric expenditure may be visualized on a portable display, e.g. on a LED array with which the user is equipped.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. Therein

Fig. 1 shows a schematic appliance of a portable device according to the present invention,

Fig. 2 shows a schematic block diagram to illustrate the components of the portable device according to a first embodiment,

Fig. 3 shows a schematic block diagram illustrating the component of the portable device according to a second embodiment,

Fig. 4 schematically shows an appliance of a system according to the present invention,

Fig. 5 shows a schematic block diagram illustrating the components of the system according to a first embodiment,

Fig. 6 shows a schematic block diagram illustrating the components of the system according to a second embodiment,

Fig. 7 shows exemplary girth-over-time signals that have been measured with the device according to the present invention, and

Fig. 8 shows an exemplary graph representing caloric measurements performed with a device according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Fig. 1 schematically shows an appliance of the portable device according to the present invention which is denoted by the reference numeral 10. A person 20, that is in this figure exemplarily shown as a runner, wears said portable device 10 for measuring the calories expended during his physical activity. As this shown in the schematic block diagram of Fig. 2 said portable device 10 comprises a girth measurement means 12 and a processing means 14. The girth measurement means 12 are adapted to measure changes in girth of a body part 16 of the person 20 over time.

In the example shown in Fig. 1, the girth measurement means 12 measures the changes in girth of an upper leg 16 of the runner 20. Depending on the type of sports activity the portable device 10 may of course also be attached to other parts of the person's body, e.g. around the hip, the arm or the neck. In any case, it needs to be a body part 16 that varies in girth due to muscle activity during the physical activity.

Said girth measurement means 12 is preferably implemented as a stretchable band 22 than can be unobtrusively worn by the user 20. By the help of a strain gauge (for simplicity reasons not shown) it is possible to measure the material strain within the stretchable band 22. It is to be noted that, of course, the girth itself (e.g. measured in cm) as well as the changes of the girth (e.g. the strain measured in % of a reference girth) can be measured over time. Both ways enable to generate a girth-over-time signal 18, 18' (see Fig. 7) which gives an information about the muscle activity, respectively about the muscle tensions of the measured body part 16 that occur during the physical activity of the person 20.

By analyzing said generated girth signal 18, 18' it is possible to determine a measure for the muscle tension of the measured body part 16. This is done by the processing means 14 which is in practical appliances usually realized as a microprocessor 14 that is integrated into the wearable device 10. The measure for the muscle tension that is calculated therewith could be the tension of a muscle which is located in the measured body part 16. Calculating the tension of the muscle itself from the measured girth signal 18, 18' is possible in body regions, where the girth of the measured body part 16 anatomically depends on the muscle tension. An anatomical dependency in this sense means that the occurring muscle tension directly result in a change of the girth of the body part 16, where the muscle tension occurs. This is for example the case at the biceps or the thigh, where a strain of the biceps or thigh muscle results in an enlargement of said biceps or thigh muscle.

Fig. 7 shows exemplary girth-over-time signals 18, 18' that have been measured with the device 10 according to the present invention. Therein, the axis of abscissae shows the time (measured in seconds) and the axis of ordinates shows the girth (measured e.g. measured in centimeters). The first girth signal 18 has been measured on the upper leg of a cyclist who cycled with a low intensity, while the second girth signal 18' has been measured when said cyclist cycled with a higher intensity. By comparing these two signals 18, 18', the above-mentioned effect is apparent. It can be seen that the areas below the girth peaks increase with an increase of the intensity of the physical activity, i.e. the areas below the peaks of the first girth signal 18 are larger than the areas below the peaks of the second girth signal 18'. This mainly results from the higher muscle activity, respectively from the larger muscle tension, in case of a higher physical load. Thus, the integral of the girth signal over a time signal 18, 18' gives a good approximation of the physical load to which the muscles are exposed at this part 16 of the body. If the measured body part 16 is a part that is most intensively used during the physical activity, this measure is a good indication of the overall intensity of the person's physical activity.

conceivable to use the measured girth signal 18, 18' for determining a comparative value which is a measure for a magnitude of the muscle tension compared to a reference magnitude of the muscle tension in unstressed conditions of the muscle, i.e. in a condition where the measured muscle is more or less relaxed. In this case, the measure for the muscle tension determined from the girth of the body part 16 defines a level of magnitude, such as low, middle or high muscle tension. Of course, different levels of magnitudes for the muscle tension could be defined. Similarly, it is also possible to define a grading scale defining different grades of muscle tension levels, e.g. a grading scale from 1 to 6 or 1 to 10, wherein higher numbers denote a more intense muscle activity.

The processing means 14 are further adapted to determine a level of physical load of the person's physical activity depending on the determined measure for the muscle tension. Said level of physical load is also denoted as the intensity of the physical activity giving information about the effort or the intensity with which the physical activity is performed. Again, this intensity can either be indicated by an exact value or an approximate grading level on a predefined analysis scale, such as "very low intensity - 1", "low intensity - 2", "medium intensity - 3", "medium to high intensity - 4", "high intensity - 5", and "very high intensity - 6".

Finally, the caloric expenditure of the person's physical activity is calculated within the microprocessor 14 based on the determined level of physical load/intensity that has been assigned to the person's physical activity. In this way, the presented portable device 10 enables to measure the caloric expenditure of the person's physical activity in dependency of a muscle tension that occurs at a distinctive body position 16 of the person 20 that has been measured by means of a girth/strain sensor 12. The presented device 10 is thus sensitive to the strength with which the movements are performed in said activity. In contrast to known devices from the prior which only take into account an acceleration measurement of a specific body part, the accuracy of the activity energy expenditure measurement is thus improved. This applies in particular for activities that involve moderate to high muscle strength. The caloric expenditure values that have been calculated by the processing means 14 may be visualized to the user by means of a small display that can be included into a wearable display device 24, e.g. in a kind of watch, so that the user 20 has a direct real-time feedback of the calories he or she expended during the activity. This wearable display 24 may be connected to the processing means 14 either via a hard wire connection or via a wireless connection.

According to an embodiment that is schematically shown in Fig. 3, the portable device may, in addition to the girth measurement means 12 and the processing means 14, further comprise an acceleration measurement means 26, an input interface means 28 and a storage unit 30. It is to be noted that not all of these additional means 26, 28 and 30 need to be provided and that they can also be combined separately with the girth

measurement means 12 and the processing means 14.

The acceleration measurement means 26 may be realized by an accelerometer or a gyroscope measuring the acceleration at a significant position on the person's body in at least one spatial dimension. Preferably, the acceleration measurement means 26 are adapted to perform a three-axial accelerometry during the time of the activity. This may be implemented, e.g. for running appliances, by attaching an accelerometer to a running shoe measuring the accelerations occurring at the feet of the user 20. Providing the processing means 14 with the accelerometer data, the processing means 14 determines the running speed, the distance traversed, the elapsed time and calculates the caloric expenditure based on these data. Even further, the processing means 14 is adapted to calculate an adapted caloric expenditure based on the level of physical load that has been determined by the help of the girth measurement means 12.

In this case, the user wears the portable device 10 including the girth measurement means 12 together with the accelerometer 26 throughout the engagement in the activity. It is to be noted that the girth measurement means 12 and the acceleration measurement means 26, depending on the type of the physical activity, not necessarily need to be attached to the same part 16 of the user's 20 body (see Fig. 1).

By applying a combination of an accelerometer-based and an intensity-based measurement using the accelerometer 26 and the girth sensor 12, it is possible to adapt the acceleration-based calculation of the caloric expenditure with the intensity levels of the physical activity that are determined from the girth signal 18, 18' that includes information about the muscle tension at the measured body part 16 of the user 20. In this way, a personalized caloric expenditure value is determined that is based on the intensity of the activity measured with girth sensor 12 and on the acceleration data measured with the accelerometer 26.

By the help of the above-mentioned input interface means 28, which could be realized as a small keypad, the user 20 is able to enter his/her personal data, i.e. his/her age, gender and body weight. The processing means 14 is adapted to evaluate this information by performing a standard caloric expenditure measurement in one of the ways explained above on page 11. This standard caloric expenditure measurement is then re-calculated within the processing means 14 by the above-mentioned personalized caloric expenditure value that is based on the intensity and on the acceleration data. In contrast to the embodiment

schematically shown in Fig. 2, the embodiment schematically shown in Fig. 3 thus allows for a much more accurate measurement.

Additionally equipping the device 10 with a storage unit 30 allows to include calibration measurements for the girth measurement means 12. In this storage unit 30 calibration measurements may be stored that map measured girth signals 18, 18' and muscle tensions to known levels of physical load. If the portable device 10 comprises such a storage unit 30 (as this is schematically shown in Fig. 3), the processing means 14 is enabled to determine a level of physical load by comparing the measured girth signal 18, 18' or parts of it with similar or same girth signals stored in a calibration data set in the storage unit 30. These reference data sets may be received from a calibration of the girth measurement means 12 that has been explained above on pages 15 and 16. The result of this calibration is a mapping of different kinds of girth signals measured in a test activity with intensity levels corresponding to these girth signals. Or in other words, the storage unit 30 comprises stored data that represent a personalized association between girth signal 18, 18' and intensity levels. "Personalized" in this context means, that this association is adapted to the user 20.

Thus, the processing means 14 are adapted to determine the level of intensity of the physical activity based on the users profile stored in the storage unit 30. In order to increase the measurement accuracy, of course, different user profiles may be stored in the storage unit for different activity types and different users.

Figs. 4 to 6 illustrate a further embodiment of the present invention. In this embodiment, a system 100 is shown which comprises a portable girth measurement device 40 and a processing device 50. The portable girth measurement device 40 comprises a girth measurement means 12 and a communication interface 32. The portable girth measurement device 40 is, similar to the portable device 10 (shown in Fig. 1), preferably realized as a stretchable band 22 that the user 20 wears throughout the engagement in the physical activity. This stretchable band 22 includes a strain or girth sensor 12, which is also denoted as girth measurement means 12, for measuring changes in girth of a body part 16 of the person over time to generate a girth signal 18, 18'. In contrast to the embodiments shown in Figs. 1 to 3, the processing means 14 is not integrated into the portable girth measurement device 40, but realized by a separate processing device 50 which is in practice usually a laptop or a desktop computer. This processing device 50 also includes an interface 34 that is adapted to communicate with the communication interface 32 of the portable girth measurement device 40.

The user 20 is in this case only equipped with the portable girth measurement device 40. The portable girth measurement device 40 according to this aspect of the present invention solely measures the girth of the measured body part 16 by the help of the girth sensor 12 in order to generate the girth signal 18, 18'. The girth signal 18, 18' is generated in the same way as explained above with reference to the first embodiments shown in Fig. 1 to 3. The above-mentioned evaluation, respectively the calculation of the caloric expenditure is in this case provided by the separate processing device 50, i.e. the processing means 14 are not integrated into the girth measurement device 40. However, the calculation of the caloric expenditure itself is done by the processing means 14 that are integrated into the processing device 50, in the same way as mentioned-above, i.e. the same principles are used to calculate the caloric expenditure.

The measured girth signal 18, 18' is transferred from the portable girth measurement device 40 to the processing device 50 via a connection 36 between the communication interface 32 and the interface 34 of the processing device 50. Said connection between the portable girth measurement device 40 and the processing device 50 can be realized in various ways, e.g. a hardware connection, such as a USB-connection, or any kind of wireless connection, such as a Bluetooth connection or an infrared connection.

In case of a hardware connection, the user 20 wears the girth measurement device during his/her physical activity and connects it afterwards to the processing device 50 to upload the measured girth over time signal 18, 18'. As soon as the girth over time signal 18, 18' is uploaded, the caloric expenditure may be calculated in the way mentioned above. On a monitor 38, the measurement results may be shown to the user 20. The calculation of the caloric expenditure may also be provided by a software that is applicable on a website.

In case of a wireless connection, it is even possible that the girth signal 18, 18' is sent from communication interface means 32 of the portable girth measurement device 40 to the processing device 50 in real-time, wherein the caloric expenditure is based on this real- time delivered girth signal 18, 18' calculated also in real-time by the processing means 14 of the processing device 50. This real-time calculation may then be sent back again to the portable girth measurement device 40. In this case, the calculated caloric expenditure may be visualized on a portable display 24 (see Fig. 4), e.g. on a LED array with which the user 20 is equipped.

In the same way as this has been done according to the first embodiments shown in Figs 1 to 3, acceleration measurements may also be included into the calculation of the caloric expenditure. To realize this, the user may be equipped with an accelerometer 26 that is able to perform a three-axial accelerometry. This accelerometer 26 communicates with the interface 34 of the processing device 50 in the same way as the girth measurement device 40 communicates with the processing device 50 (either via a hardwire or via a wireless connection). This is schematically shown in Fig. 6. Using an input interface 28', which is usually realized by a keyboard of the processing device 50, the user 20 may also enter his personal data. In this way, the processing means 14 of the processing device 50 are enabled to calculate the BMR value and recalculate the BMR value based on the intensity level received from the girth measurement and based on the acceleration data received from the acceleration measurement.

In order to ease the examination of the measurement results for the user, the user is finally shown a recalculated calorie expenditure graph that shows the caloric expenditure in a bar graph (exemplarily shown in Fig. 8). The bars in said graph show the caloric expenditure per time-interval. Additionally the calculated intensity of the physical activity may also be shown in a different sub-window, telling the user the different intensity levels that have been evaluated per time-interval.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single element or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. A portable device for measuring a caloric expenditure of a person's physical activity, said portable device comprising:

girth measurement means (12) for measuring changes in girth of a body part (16) of the person (20) over time to generate a girth signal (18, 18'), said body part (16) being a part of the person's body that varies in girth due to muscle activity during the physical activity, and

processing means (14) for analyzing the girth signal (18, 18') to determine a measure for the muscle tension of said body part (16), for determining a level of physical load of the person's physical activity depending on the determined measure for the muscle tension, and for calculating the caloric expenditure of the person's physical activity based on the determined level of physical load of the person's physical activity.

2. A portable device as claimed in claim 1, wherein said portable device (10) further comprises an acceleration measurement means (26) for measuring an acceleration of a part (16) of the person's body in at least one spatial dimension during the physical activity, and wherein the processing means (14) is adapted to calculate the caloric expenditure of the person's physical activity based on the determined level of physical load of the person's physical activity and on the measured acceleration of said part of the person's body.

3. A portable device as claimed in claim 1, wherein said portable device (10) further comprises an input interface means (28) for receiving the person's personal data, in particular an age, a gender, a body weight and/or a pulse over time measurement, and wherein the processing means (14) is adapted to calculate the caloric expenditure of the person's physical activity based on the determined level of physical load of the person's physical activity and on the received personal data.

4. A portable device as claimed in claim 1, wherein the processing means (14) is adapted to analyze the girth signal (18, 18') to determine a measure for the muscle tension of said body part (16) per time-interval, to determine a level of physical load of the person's physical activity per time-interval depending on the determined measure for the muscle tension in the respective time-interval, and to calculate the caloric expenditure of the person's physical activity per time-interval from the determined level of physical load of the person's physical activity in the respective time-interval.

5. A portable device as claimed in claim 1, wherein the girth measurement means

(12) is attachable to said body part (16) and comprises a stretch sensor, in particular a strain gauge, for measuring the material strain of at least a part of said girth measurement means over time.

6. A portable device as claimed in claim 1, wherein the girth measurement means (12) is a stretchable band which at least partly surrounds said body part.

7. A portable device as claimed in claim 1, wherein the girth measurement means (12) is adapted to a specific type of physical activity and to a specific part of the body.

8. A portable device as claimed in claim 1, wherein said portable device (10) further comprises a storage unit (30) which is adapted to store reference measures for muscle tensions belonging to known levels of physical load, and wherein the processing means (14) is adapted to determine the level of physical load by comparing the measure for the muscle tension determined in the analysis of the measured girth signal (18, 18') with the reference measures stored in said storage unit (30).

9. A portable device as claimed in claim 1, wherein the processing means (14) is adapted to analyze the girth signal (18, 18') to determine a measure for the muscle tension of said body part (16) by integrating the girth signal (18, 18') over a time-interval.

10. A portable device as claimed in claim 1, wherein the processing means (14) is adapted to analyze the girth signal (18, 18') to determine a measure for the muscle tension of said body part based on the full width at half maximum of a peak in the girth signal (18, 18').

11. A method for measuring a caloric expenditure of a person's physical activity, including the steps of: measuring changes in girth of a body part (16) of the person (20) over time to generate a girth signal (18, 18'), said body part (16) being a part which varies in girth due to muscle activity during the physical activity,

analyzing the girth signal (18, 18') to determine a measure for the muscle tension of said body part (16),

12. A processing device for determining a caloric expenditure of a person's physical activity, said processing device (50) comprising:

an interface (34) for receiving a girth signal (18, 18') that has been generated by measuring changes in girth of a body part (16) of the person (20) over time, said body part (16) being a part of the person's body that varies in girth due to muscle activity during the physical activity, and

13. A system for measuring a caloric expenditure of a person's physical activity, said system comprising:

a portable girth measurement device (40) for measuring changes in girth of a body part (16) of the person (20) over time to generate a girth signal (18, 18'), said body part (16) being a part of the person's body that varies in girth due to muscle activity during the physical activity, and

a processing device (50) which comprises an interface (34) for receiving said girth signal (18, 18'), and a processing means (14) for analyzing the girth signal (18, 18') to determine a measure for the muscle tension of said body part (16), for determining a level of physical load of the person's physical activity depending on the determined measure for the muscle tension, and for calculating the caloric expenditure of the person's physical activity based on the determined level of physical load of the person's physical activity.

14. A processing method for calculating a caloric expenditure of a person's physical activity, including the steps of:

receiving a girth signal (18, 18') that has been generated by measuring changes in girth of a body part (16) of the person (20) over time, said body part (16) being a part of the person's body that varies in girth due to muscle activity during the physical activity,

15. A computer program product comprising program code means for causing a computer to control a processing device (50) as claimed in claim 12 to carry out the steps of the processing method as claimed in claim 14 when said computer program is carried out on the computer.