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Title:
A METHOD AND SYSTEM FOR DETERMINING THE POWER DEVELOPED BY A RUNNER
Document Type and Number:
WIPO Patent Application WO/2018/087654
Kind Code:
A1
Abstract:
A method an accelerometer and a processing module for determining a power developed by a running athlete, comprising: - splitting each stride of the runner into a contact phase during which the foot is in contact with a ground, and a swing phase during which this foot is not in contact with the ground; - determining a midstance instant during each contact phase during which a vertical ground force (GRFV) is maximal, and/or determining a midstance instant when a postero-anterior ground force (GRFPA) changes its sign and becomes positive, and split said contact phase into an absorption phase being the portion of said contact phase before said midstance instant, and a propulsion phase being the portion of said contact phase after said midstance instant; - determining at least one of the following powers: - an absorption power representing the power dissipated along a vertical and/or postero-anterior direction by the athlete, during each absorption phase or averaged over a plurality of absorption phases; and or: - a propulsion power representing the power dissipated along a vertical and/or postero-anterior direction by the athlete during each propulsion phase or averaged over a plurality of propulsion phases.

Inventors:
FLACTION, Patrick (Route du Belvédère 42, 1965 Chandolin-près-Savièse, 1965, CH)
GINDRE, Cyrille (Route des Corbelets 53, 1854 Leysin, 1854, CH)
CORRE, Jérôme (Rue de de la Chapelle 28, 1965 Granois, 1965, CH)
DEVENES, Steve (Rue Pramedzi 12, 1908 Riddes, 1908, CH)
Application Number:
IB2017/056943
Publication Date:
May 17, 2018
Filing Date:
November 07, 2017
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
MYOTEST SA (Rue de la Blancherie 61, 1950 Sion, 1950, CH)
International Classes:
A61B5/103; A61B5/11; A63B24/00; A63B69/00
Domestic Patent References:
WO2015121690A12015-08-20
WO2016044831A12016-03-24
WO2015144851A12015-10-01
WO2015121690A12015-08-20
WO2016044831A12016-03-24
WO2011157607A12011-12-22
Foreign References:
US20080214360A12008-09-04
US20150351665A12015-12-10
US20030013584A12003-01-16
EP0394146A11990-10-24
US20130178958A12013-07-11
US20150351665A12015-12-10
US20080214360A12008-09-04
EP2889853A12015-07-01
US20150057966A12015-02-26
US20150142329A12015-05-21
Attorney, Agent or Firm:
P&TS SA (AG, LTD.) (Av. J.-J. Rousseau 4, P.O. Box 2848, 2001 Neuchâtel, 2001, CH)
Download PDF:
Claims:
Claims

1. A system for determining a power developed by a runner, said device comprising:

- an accelerometer,

- a processing module programmed to process acceleration data from the accelerometer so as to:

- split each stride of the runner into a contact phase during which the foot is in contact with a ground, and a swing phase during which this foot is not in contact with the ground;

-for each foot, determine a midstance instant during each contact phase during which a vertical ground force (GRFv) is maximal, and/or determine the midstance instant when a postero-anterior ground force (G RFPA) changes its sign and becomes positive, and split said contact phase into an absorption phase being the portion of said contact phase before said midstance instant, and a propulsion phase being the portion of said contact phase after said midstance instant;

-determine at least one of the following powers:

- an absorption power representing the power dissipated along a vertical and/or postero-anterior direction by the athlete during each absorption phase or averaged over a plurality of absorption phases; and or:

- a propulsion power representing the power dissipated along a vertical and/or postero-anterior direction by the athlete during each propulsion phase or averaged over a plurality of propulsion phases.

2. The system of claim 1, wherein said processing module is further programmed for determining a stance power representing the power dissipated along a vertical and/or postero-anterior direction by the athlete during each contact phase or averaged over a plurality of contact phases.

3. The system of one of the claims 1 or 2, wherein said

processing module is further programmed for determining an absorption power, a propulsion power and/or a stance power representing the power dissipated with both legs along a vertical and/or postero-anterior direction by the athlete during the contact phases of each stride or averaged over a plurality of strides.

4. A system according to one of the claims 1 to 3, arranged for determining a Ground Contact Force from said acceleration, and using a model for determining said powers from said Ground Contact Force.

5. A system according to one of the claims 1 to 4, arranged for determining an instantaneous Power from said acceleration, and

determining said powers from Instantaneous Power. 6. A system according to one of the claims 1 to 5, said

accelerometer being adapted for wear on the athlete's torso.

7. A system according to any of the claims 1 to 6, said processing module being mounted in a wristwatch or in a head-worn device.

8. A system according to claim 7, said wristwatch comprising a display for displaying at least one of said determined powers or a ratio between said powers or one of said powers divided by the weight of the runner.

9. A system according to one of the preceding claims, said accelerometer and said processing unit being in a same wearable device. 10. A system according to any one of the preceding claims, said processing module being adapted for computing at least one among

-a stance power,

-an absorption power and/or

-a propulsion power

along a medio-lateral direction during at least one contact phase.

1 1. A system according to any one of the preceding claims, said processing module being adapted for computing an instantaneous power dissipated by the athlete.

12. A method an accelerometer and a processing module for determining a power developed by a running athlete, comprising:

-splitting each stride of the runner into a contact phase during which the foot is in contact with a ground, and a swing phase during which this foot is not in contact with the ground;

-determining a midstance instant during each contact phase during which a vertical ground force (GRFv) is maximal, and/or determining a midstance instant when a postero-anterior ground force (G RFPA) changes its sign and becomes positive, and split said contact phase into an

absorption phase being the portion of said contact phase before said midstance instant, and a propulsion phase being the portion of said contact phase after said midstance instant;

- determining at least one of the following powers:

- an absorption power representing the power dissipated along a vertical and/or postero-anterior direction by the athlete, during each absorption phase or averaged over a plurality of absorption phases; and or:

- a propulsion power representing the power dissipated along a vertical and/or postero-anterior direction by the athlete during each propulsion phase or averaged over a plurality of propulsion phases.

13. The method of claim 12, further comprising:

determining a stance power representing the power dissipated along a vertical and/or postero-anterior direction by the athlete during each contact phase or averaged over a plurality of contact phases.

14. The method of one of the claims 12 or 13, further comprising:

determining an absorption power, a propulsion power and/or a stance power representing the power dissipated with both legs along a vertical and/or postero-anterior direction by the athlete during the contact phases of each stride or averaged over a plurality of strides.

15. The method of one of the claims 12 to 14, said accelerometer being worn on the athlete's torso.

16. The method of any one of the claims 12 to 15, said processing module being mounted in a wristwatch or in a head-worn device.

17. The method of any one of the claims 12 to 16, said wristwatch comprising a display for displaying at least one of said determined powers or a ratio between said powers or one of said powers divided by the weight of the runner.

18. The method of one of the claims 12 to 17, further comprising:

computing a power along a medio-lateral direction during at least one contact phase.

19. The method of one of the claims 12 to 18, further comprising:

computing an instantaneous power dissipated by the runner.

Description:
A method and system for determining the power developed by a runner

Field of the invention

[0001] The present invention concerns the field of performance assessment in sport, especially for measuring the performances of runners with an accelerometer.

Description of related art

[0001] Sport watches and other wearables devices are often used for measuring and displaying various parameters of an athlete, for example during a running or cycling session. It is for example well known for measuring the speed, pace and heart rate of the athlete.

[0002] Among those parameters, power meters have been tested by professional cycling racers since the late 1980s. They usually use strain gauges mounted in the bottom bracket, rear freehub or crankset in order to measure the power output of the rider. Training using a power meter has become increasingly popular. Power provides instant feedback to the rider about his performance and measures his actual output, whereas heart rate monitors measure the physiological effect of effort and therefore lags behind actual power output. For example, an athlete performing "interval" training with a power meter can instantly see that he has increased his output to produce for example 300 watts, instead of waiting for his heart rate to climb to a certain point. Similarly, he can instantly see that he has reduced his output to say 1500 watts during the recuperation phase, instead of waiting for his heart rate to lower back down.

[0003] Power meters also provide an objective measurement of real output that allows training progress to be tracked very simply.

[0004] It is also known to measure the instantaneous power developed by a runner. There are a multitude of possible applications for power measurement in running. First and foremost, monitoring the power allows a runner to pace his race or training much more accurately than using the currently available heart rate monitoring or pace measurement. Power can also be used to assess the maximum performance of a runner.

[0005] In order to achieve some of those goals, various power metering solutions have been proposed to runners. Among those, W015144851 suggests to base the computation of power of a runner on the heart rate. This method is not very precise. Moreover, it creates a delay between a change of power, a corresponding change of heart rate, and a measure of this heart rate. [0006] US2003013584 and EP0394146 describe methods for computing the instantaneous power developed by a runner, using a running mat with sensors. Such a running mat is not appropriate for measuring the power developed by a long distance runner, such as a marathon runner, during the whole race. [0007] US2013178958A describes a system for determining user performance characteristics. An inertial sensor may be coupled with the user's torso and generates one or more signals corresponding to the motion of the user's torso. The processing system is in communication with the inertial sensor and is operable to use the one or more signals to determine one or more user performance characteristics. The user performance characteristics includes the "running power" and the power used to raise the torso. These parameters may be computed on a step basis.

[0008] US2015/351665 discloses a method for determining the power developped by a runner. It uses a sole with force sensors, in order to determine an instantaneous power, an average power, a peak power or an average peak power.

[0009] US2008/214360 discloses the use of an accelerometer for runners, in order to determine the power. [0010] WO2015/121690 discloses another device for measuring the power developed by a runner along the displacement direction and during the stance phase.

[0011] WO2016/044831 discloses another method for measuring the power developed by a runner.

[0012] EP2889853 describes a method for optimizing running

performance for an individual. The movements of a foot are monitored using one or more acce I ero meters near the foot of the individual. The data is transmitted to a portable electronic processing device, e.g. in the form of a cellular phone, where the data is processed to derive a movement pattern of the foot. The portable electronic processing device determines various parameters, including the duration of impact on the foot and the instantaneous power spent to propel the individual's body. This measure indicates whether or not the individual is spending energy in an efficient manner, and used to provide a running efficiency indicator.

[0013] This document describes a movement pattern of the foot during each stride. However, it only computes the power developed by the individual during the whole stride, or at least during the impact phase. More details about the repartition of power during this impact phase are not available.

[0014] WO2015/121690 discloses another device for measuring the power developed by a runner along the displacement direction and during the stance phase.

[0015] US2015057966 describes a jump sensor that provides an average power for the jump, i.e. the energy divided by the time from the initiation of the jump to the apex of the jump. It also provides an instantaneous power as the energy divided by the time for the acceleration just prior to leaving the ground. [0016] The average power, instantaneous power or power during the contact phase (or stance phase) which is measured in the above described documents are all important physiological information in order to determine the level of training of the athlete and the amount of energy he can produce on average, at each moment or during each stride. However, this information does not really help a runner in improving his running efficiency.

[0017] It is already known in the prior art that a part of the energy produced by a runner at each stride is actually used to brake him. As indicated in US2015142329, at the time of strike, the forces due to propulsion generated from the previous stride are absorbed throughout the user's lower extremities; during this phase, a negative longitudinal force is exerted by the ground which opposes the runner's motion at the time of impact. It is only during the last part of each stride that a positive propulsion force is produced in the vertical and longitudinal direction.

[0018] US2015142329 suggests to calculate vertical-to-horizontal force ratios during the impact phase, so that the runner can learn the situations when the ratios deviate from their optimum values. However, an indicator based only on force does not take into account the duration during which those forces are produced; a strong braking force during a small period could be less detrimental than a lower braking force over a longer period.

Brief summary of the invention

[0019] In order to improve their overall running efficiency, many runners would like to know if the power they produce is used in an efficient way, i.e., to generate a positive propulsion force in the horizontal direction.

[0020] In order to achieve this goal, it would be useful to have a system and a method for measuring the power developed by a runner during important phases of each stride. [0021 ] According to the invention, those aims are achieved with a system for determining a power developed by a runner, said device comprising:

- an accelerometer,

- a processing module programmed to process acceleration data from the accelerometer so as to:

-for each foot, split each stride of the user into a contact phase during which the foot is in contact with a ground, and a swing phase during which this foot is not in contact with the ground;

-for each foot, determine an instant during each contact phase during which a vertical ground force (GRFv) is maximal, and/or determine the instant when a postero-anterior ground force (G RFPA) changes its sign and becomes positive, and split said contact phase into an absorption phase being the portion of said contact phase before said instant, and a

propulsion phase being the portion of said contact phase after said instant;

-determine at least one of the following powers:

- an absorption power representing the power dissipated along a vertical and/or postero-anterior direction by the athlete during each absorption phase or averaged over a plurality of absorption phases; and or:

- a propulsion power representing the power dissipated along a vertical and/or postero-anterior direction by the athlete during each propulsion phase or averaged over a plurality of propulsion phases.

[0022] According to one aspect, this system is thus able to distinguish automatically between the absorption phase and the propulsion phase in each stance.

[0023] According to another aspect, the system is able to compute independently the power developed by the runner during each phase.

[0024] Therefore, a runner using this system can learn the power he is using during the absorption phase and the power he is producing during the propulsion phase, and improve his running efficiency accordingly. Brief Description of the Drawings

[0025] The invention will be better understood with the aid of the description of an embodiment given by way of example and illustrated by the figures, in which: Fig. 1 shows the swing and contact phases of a running's gait cycle.

Fig. 2 shows the contact and the swing phase of one stride, as well as the absorption phase and propulsion phase within the contact phase. Figures 3A-3C show three foot strike types.

Figure 4 shows a runner reference frame, illustrating the vertical axis, the postero-anterior axis, and the mediolateral axis.

Figure 5 shows the forces exerted by the ground on the runner's foot. Figure 6 shows a runner equipped with an example of the system according to the invention.

Figure 7 shows the running ground reaction force.

Figure 8 shows the vertical running ground reaction force and the vertical stance power. Figure 9 shows the vertical running ground reaction force, the vertical absorption power and the vertical propulsion power.

Figure 10 shows the vertical running ground reaction force and the vertical instantaneous power. Detailed Description of possible embodiments of the Invention

[0026] In mechanics, the average power P avg during an event is the amount of work (or energy) produced or consumed over the duration T of the event: P aV g=W/ T = (F d) I T where

Pavg is the average power during the event, in Watts

W is the work in Joules

F is the force in Newtons

D is the displacement in meters

T is the duration of the event in seconds.

[0027] The force F produced by a runner with a mass m in order to change its speed can be determined from the well-known Newton formula:

F = m a where m is the acceleration which can be measured with an accelerometer.

[0028] The instantaneous power P/nst is the derivative of work over time t:

Pinst = dW/dt

[0029] Therefore, if the instantaneous power Pi ' nst changes during an interval T, the average power Pavg is [0030] As illustrated on Figure 1, running is a type of gait characterized by a contact phase CR (also called stance phase) where one right foot of a runner 1 is in contact with the ground 2, followed by a flight phase F (or swing phase) during which both feet are above the ground, and then by another contact phase CL where the left food is in contact with the ground, etc... This is in contrast to walking where one foot is always in contact with the ground.

[0031 ] As one could guess, the instantaneous power developed by the runner is not constant during each running gait cycle. While the runner is in the flight phase, he is merely in free fall in the earth's gravitational field and the power developed by the runner is low or null. The runner's ability to apply a force against the ground and to produce a work occurs only when he is in contact with the ground during the CR and CL phases. Therefore, the contact phases are of particular interest when looking at the runner's output power. [0032] Figure 2 illustrates how the contact phase C R of the right foot can be further subdivided into an absorption phase A R and a propulsion phase P R . The absorption phase A R begins with the foot strike and ends at midstance. The propulsion phase P R begins at midstance and ends at the toe-off. The same decomposition can be made with the contact phase C L for the left foot, which can be subdivided into an absorption phase A L and a propulsion phase P L .

[0033] During the absorption phase, forces due to propulsion generated from the previous contact phase are absorbed throughout the lower extremity. The absorption phase begins with the foot strike, the point when the foot 10 makes initial contact with the ground 2. Common foot strike types include heel strike (Figure 3A), midfoot strike (Figure 3B), and forefoot strike (Figure 3C) types. These are characterized by initial contact of the heel of the foot, ball and heel of the foot simultaneously and ball of the foot, respectively.

[0034] During absorption, the ground exerts a backward force against the front foot of the runner who is braked; his horizontal speed decreases. At the same time, his vertical falling speed decelerates since the runner is not in free fall anymore. [0035] Absorption of forces continues as the body moves from foot strike to midstance. Midstance occurs while the foot is fully planted, as the centre of mass of the body travels above the supporting foot.

[0036] As the lower extremity enters midstance, the propulsion phase begins. During this phase, the lower limb produces forces to propel the runner. These forces may be generated by elastic loading from the absorption phases or by stand-alone concentric muscle flexion. During the propulsion phase the body moves forward of the supporting foot. Eventually leading to the point when the heel lifts off the ground. The propulsion phase ends with the Toe-Off, the point when the foot loses contact with the ground. During propulsion, the ground exerts a forward oriented force against the runner, whose horizontal speed increases.

[0037] Figure 4 illustrates a reference frame attached to the runner and centered at his Gravity Center (GC). One of the axes V of this reference frame is aligned with the vertical direction of the earth's gravitational force. The remaining two axes of our reference frame are thus in the horizontal plane. One axis is referred to as "postero-anterior" (PA axis) and is directed from the back towards the front of the runner's body. The third axis ML is mediolateral: pointing out of the runner's body towards the side. [0038] Figure 5 illustrates the ground reaction force (GRF) which is the force exerted by the ground 2 on a runner 1 in contact with it. For example, a person standing motionless on the ground exerts a contact force C on it (equal to the person's weight) and at the same time an equal and opposite ground reaction force GRF is exerted by the ground on the person. [0039] In running, given that firstly power is directly dependent on work and secondly that work is directly dependent on force and thirdly that the runner 1 only applies forces to the ground 2, it is apparent that the ground reaction force is a key component to the measurement of power. Naturally, the GRF only exists during the contact phase, i.e. whilst the runner is in contact with the ground. During flight, the GRF is non-existent. [0040] Therefore, in one embodiment of the invention, the contact phase is determined as being the phase during which the vertical ground force is equal to or higher than the weight of the runner, i.e., the phase during which the acceleration of the runner's body is lower than the acceleration due to gravity. [0041 ] The GRF has both a component GRFv normal to the ground 2, along the runner's reference frame vertical axis, and two components G RF PA and G RF M L parallel to the ground 2, along the postero-anterior axis PA and along the mediolateral axis MA.

[0042] Figure 7 illustrates the Ground reaction forces GRFv and G RFPA during a contact phase C of one foot. As one can see, during the initial absorption phase A, the ground exerts a negative force (braking force) along the postero-anterior axis and an increasing upward oriented vertical force. The runner's horizontal speed thus decelerates during this phase.

[0043] After midstance M, during the propulsion phase, the force exerted by the ground along the postero-anterior axis becomes positive, so that the runner accelerates again. At the same time, the vertical ground force decreases again.

[0044] The midstance instant M between the absorption phase A and the propulsion phase P can thus be determined either by determining the instant when the vertical ground force GRFV is maximal, and/or by determining the instant when the postero-anterior ground force changes its sign and becomes positive-

[0045] As already suggested, power measurement has the potential to revolutionize the training methods for runners. First and foremost, monitoring the power allows a runner to pace his race or training much more accurately than using the currently available heart rate monitoring or pace measurement. Power can also be used to assess the maximum performance of a runner. Power over time can be used to assess the stamina of a runner through his ability to sustain a level of power over a set period of time or distance. Power can be used as a metric to monitor the improvement of a runner over a training period. Power can also be used in non-running physical training exercise (squat, plyometric jump, etc.) to make it easier for runners to relate the physical preparation work to their running capabilities. [0046] According to one aspect of the invention, instead of looking at the average or instantaneous power produced by a runner, we can look at when in time and how the power is output during a runner's stride. This will allow a multitude of new insights into running, such as (but not limited to) optimise the runner's technique for an optimum power output for his running sport (sprint or long distance).

[0047] According to one aspect, power measures during running are split into at least two different components. Firstly, the power along a vertical direction, which includes both the vertical oscillation seen during running and any climb or downhill displacement during the run. Secondly, the power developed in a horizontal plane, which can be further divided into power along the anteroposterior axis (in the direction of travel) and power along the mediolateral movement (in the horizontal plane, perpendicular to the direction of travel).

[0048] Moreover, according to one aspect, power can be measured separately for each foot or leg, or averaged for both.

[0049] We define the Running Power as the amount of work produced to displace the runner's body over the running time. It is the most easily understandable and relevant power parameters for any runner. For instance, if a runner sustains 350 watts for 20 minutes he can directly deduce that he has produced 420 KJ of Work. This value should also closely relate to the one produced by cycling power meters, so 20 minutes running at 350 watts should feel the same as 20 minutes cycling at 350 watts.

[0050] The Running power can be calculated in real time for every step or it can be averaged over a whole or part of a running session. [0051] The Running Power can be divided into left foot stance power and right foot stance power

[0052] The Running power can also be split into both its vertical and horizontal components (postero-anterior and medio-lateral). This is particularly useful to look at how a runner's power expenditure is spread. For instance, if two runners of the same weight both produce a running power of 300 watts they may not have the same efficiency, one may have 280 watts vertically and 20 watts horizontally while the other may be able to produce 240 watts vertically and 60 watts horizontally, since the main aim of running is to move in a horizontal plane, the second runner is much more efficient at doing that.

[0053] Both the vertical and horizontal powers can be divided into left foot running power and right foot running power

[0054] Both the vertical and horizontal powers can be calculated in real time for each leg or for both legs either for every step or averaged over a whole or part of a running session.

[0055] Stance Power

[0056] Unlike cycling, where the rider is constantly applying force on the crankset, a runner only applies forces when in contact with the ground. He is therefore only doing work when in contact with the ground. The work is therefore only being done during contact phases and not over the whole running time.

[0057] For example, let's imagine two runners of the same weight producing the same work and running at the same step frequency, their running power will be identical. They may however have different running styles resulting in different amount of contact and flight time. Let's say that the first runner has a contact time of 140ms and a flight time of 260ms whilst the second runner has a contact time of 220ms and a flight time of 180ms. Their running power will be identical as they are producing the same work over the same total time. However, their stance power will be different as the first runner produces this work over a much shorter time, thus, his stance power will be higher.

[0058] Therefore, the system of the invention can determine the stance power, i.e. the energy produced by the runner during the contact phase.

[0059] This stance power can be determined separately for the left foot and for the right foot.

[0060] The stance power can also be divided into both its vertical and postero-anterior components, to form the vertical stance power and the postero-anterior stance power

[0061] The stance power can be determined in real time for every step or it can be averaged over the whole or part of a running session, or over predetermined intervals.

[0062] Figure 8 illustrates the Vertical Running Ground Force GRFv and the Vertical Stance Power VSP over four cycles of contact phases and flight phases. In this example, the vertical stance power is averaged over at least one cycle and thus remains nearly constant.

[0063] Absorption and Propulsion Power

[0064] Since the contact phase can be subdivided into absorption and propulsion phases, in one embodiment the system determines the power for those absorption and propulsion separately. It is particularly interesting to be able to see the power repartition between those two phases.

[0065] Absorption and propulsion power can also be divided into both their vertical and postero-anterior components, to form the Vertical Absorption Power (VAP), the Vertical Propulsion Power (VPP), the Postero- anterior Absorption Power (PAP) and the Postero-anterior Propulsion Power (PPP). [0066] These powers can also be determined for the left foot and right foot separately, or averaged for both feet.

[0067] These powers can also be determined in real time for every step or it can averaged over the whole or part of a running session, or over predetermined intervals.

[0068] These powers may be determined as absolute values, for example in watts. They may also be determined in relation to the mass of the runner, in watts per kilo.

[0069] Figure 9 illustrates the Vertical Running Ground Force GRFv, the Vertical Absorption Power VAP and the Vertical Propulsion Power VPP over four cycles of contact phases and flight phases. In this example, the vertical stance power is averaged over at least one cycle and thus remains nearly constant.

[0070] The system can also determine and display the Instantaneous Power IP, as illustrated on Figure 10. The instantaneous power can be determined at a high sampling rate, for example 100Hz or preferably 200Hz or more. The Instantaneous Power curve gives interesting feedback about the time needed to switch from the absorption phase to the propulsion phase, about the stability and stiffness of the runner's body, and for the early detection of injuries, risks of injuries and asymmetries.

[0071] The instantaneous power signal can be determined for the left foot and the right foot independently,

[0072] The instantaneous power signal can also be divided into both its vertical and postero-anterior components. [0073] The instantaneous power signal also shows the absorption and propulsion phases as well as the peak value in each of these regions. [0074] Power ramping rates (in W/s) are also visible on the

instantaneous power signal.

[0075] Running style

[0076] The system and method of the invention can also be used for categorizing the style of a runner. Each runner has a natural running style, varying from "gazelle" to "glider". Knowing what style a runner is naturally most comfortable with, helps determine what his training should focus on and what type of footwear should be favoured.

[0077] Some runners are more "terrestrial, in the sense that the angle of propulsion when the runner leaves the ground is relatively small, so that the runner stays close to the ground and makes more small steps. Other "aerial" runners propulse themselves in a more vertical direction, resulting in a higher and longer jump, but also in a less efficient use of energy. The method of the present invention can be used for determining a runner's style based on several parameters derived from the acceleration.

[0078] Figure 6 illustrates an example of possible system 4 according to an embodiment of the invention. In this embodiment, the system comprises two components: an acceleration measuring device 3, and a user interface module 5. Both components may also be integrated in one device. [0079] The acceleration measuring device 3 is preferably intended to be worn on the user's body, close or in fixed relation to the runner center of mass, for example on his torso or on his head. It comprises for example a 3- axis MEMS-based accelerometer, for measuring the accelerations of the runner along three axes. The device 3 may also, optionally, include a 3-axis gyroscope.

[0080] The device 3 preferably also comprises a microprocessor unit, or a microcontroller, or a FPGA module, which can read the raw data from the accelerometer and perform some processing algorithms on these data, for example in order to filter noise. [0081] The accelerations measured with the device 3 are preferably converted into accelerations along the vertical, postero-anterior and mediolateral axis previously described in relation with Figure 4; for example, the vertical direction may be roughly determined during the flight phase as the only acceleration to which the runner is exposed, while the postero-anterior direction is the main direction of progression in the horizontal plane.

[0082] The device 3 further preferably includes a wireless interface, such as a Bluetooth, ZigBee, WiFi or ANT interface, for transmitting the processed acceleration data to a remote device such as the user interface device 5, and for receiving commands from this remote device.

[0083] The user interface device 5 may be for example a wristwatch, a smartphone, a head-up display, a headset, etc. It also includes a processor for further processing the data received from the device 3, and for determining the above described various powers. A display and/or an audio interface can be used for presenting the power information to the runner.

[0084] The system may also store the raw data from the accelerometer and/or the processed power parameters for later use. [0085] Several approaches may be used within the framework of the invention in order to determine the above mentioned running power parameters. The first set of approaches relies on modelled ground reaction force matching, in some ways, the measured acceleration signal; the power parameters are derived from the parameters modelling the GRF. The second type of approach relies on calculating the Instantaneous Power directly from the measured acceleration signal and then deriving all the other power parameters from the instantaneous power signal.

[0086] The advantages of putting forward several approaches is that each may be best suited to different types of user applications. Some approaches require less processing power and lower acceleration sampling frequency. Although they may not be suited to compute all of the parameters detailed here, they will still be suited to many user applications. More complex approaches require a larger footprint, more processing power and higher accelerometer sampling rates and as such will require more power. They, on the other end, will be able to deliver more

parameters and higher accuracy, which may be suited to certain specific user applications where other approaches would be less suited.

[0087] According to the first approach, components of the Ground Reaction Force are determined from several parameters x,y,z,t retrieved from the acceleration values, such as but not limited to, the Ground

Contact Time, the Flight time, the Stride Length, the Horizontal distance covered by the runner's center of gravity during the contact time, the takeoff angle between the horizontal plane, the toe and the center of gravity at take-off, the landing angle between the ground and the leg at the beginning of foot contact, the vertical undulation of the runner's center of mass, etc.

[0088] A plurality of models or equations can be used for retrieving those parameters x,y,z, as described amongst others in WO201 1/157607.

[0089] The Ground Reaction Force is then determined in all directions from those parameters x,y,z using a model function GRF(t)=f(x,y,z,t).

[0090] We can then determine the various running powers as a function of the GRF and/or of those parameters x,y,z,t:

Running ; GRF) [0091] According to a second approach, the instantaneous power is determined directly from the measured acceleration signal. All the other power parameters are then determined from the instantaneous power signal.

[0092] The Instantaneous Power is calculated for every sample of the raw acceleration measurement. [0093] Given that: P=Wt

With:

P the Power in Watts (W)

W the Work in Joules (J) And that W=F -d

With

F the Force in Newtons (N)

d the displacement in meters (m)

We can deduce that: P=F dt [0094] Given that dt is the average velocity V over the period of time t , we can write that P=F -V

With

V the average Velocity in meters per second (m.s-1) over a period of time t [0095] According to Newton's 2nd law of motion, written in terms of an object's acceleration, we have:

F=m -a

with

m the mass of the runner in kilograms (Kg), assumed to be a constant.

a the acceleration in meters per second squared (m.s 2 )

[0096] Still according to Newton's 2nd law of motion, written in terms of an object's velocity, we have:

F=m -dvdt which can also be written as: v= Ja-dt&t [0097] Thus we can deduce that Instantaneous Power as a function of time is:

[0098] The main practical problem with this approach arises from the fact that the numerical integration of the acceleration measurements over time causes inaccuracies and signal noise to accumulate into a large error. This large error is often referred to as "drift". The drift error can become many times larger than the real signal even over a relatively short time. The difficulty is thus to be able to remove the "DC component" error in the acceleration signal to avoid drift in the integrated velocity.

[0099] In order to avoid this problem, the vertical speed of the athlete may be reset to zero during each cycle, for example at midstance. The horizontal speed along the posterior-anterior axis may be determined from a GPS signal, and/or assumed to be roughly constant during each gait.

[00100] The system of the invention can also determine and display the variation of Power of time