EP1754521A1 - Method and apparatus for determination of the efficiency of a running style - Google Patents

Abstract

The method involves measuring instantaneous accelerations of a runner, in frontal, lateral and vertical directions, by using three dimensional accelerometers (10) placed at the proximity of a center of gravity of the runner. An information is obtained which represents a runner`s mean frontal velocity measured by a portable device comprising the accelerometers. An index is calculated which is represented as a ratio between a function of instantaneous accelerations of vertical, lateral and frontal directions and a function of frontal velocity. Information representing the index is produced. An independent claim is also included for a device for determination of an index of an effectiveness or ineffectiveness of stride.

Description

The present invention relates to the field of physical activities. It relates, more particularly, to a method of measuring the efficiency or, which amounts to the same, the inefficiency, the movement of running.

The effectiveness of the sports movement differs from an overall efficiency in which cardiovascular, but also technical or bio-mechanical factors intervene. Regarding the race, the overall efficiency is proportional to the ratio of the speed of propulsion of the athlete to his heart rate. Indeed, the speed is proportional to the output power, while the heart can be likened to the runner motor.

The overall efficiency of a runner may, of course, be interesting, but it does not distinguish between the cardiovascular and bio-mechanical aspects of efficacy, and thus optimize these two aspects independently. one of the other.

The effectiveness of the movement, or bio-mechanical efficiency, involves many parameters, such as the orientation of the supports, the coordination of gestures, parasitic movements, etc., which are difficult to quantify. A conceptual model, described in a book by P. R. Cavanagh (Biomechanics of Distance Running, Human Kinetics Books, 1990), uses the relationship between oxygen uptake, velocity, and various bio-mechanical factors. An estimation of the efficiency based on this model requires the use of a large number of sensors intended to collect all the information, resulting in a large bulk incompatible with a portable solution.

An alternative solution, described in the patent application EP 04405233 , proposes an approach based on the study and the analysis of the acceleration of the center of gravity. Different parameters extracted from the runner's acceleration are compared to the same parameters of reference runners, for a given heart rate. This method, however, has some disadvantages.

First, the method provides the user with an index that does not have a direct bio-mechanical meaning, but that is representative of the difference between the runner's parameters and the parameters of the reference runners. It is admitted that a lower index corresponds to a better efficiency of the runner. However, these parameters being strongly dependent on the morphology of the runner, the approach is correct only if the user has the same morphology as the reference runner.

In addition, the relative nature of the method makes it difficult to establish a goal of overall efficiency.

• Une mesure de l'efficacité de course devrait fournir un indice absolu représentatif d'une réalité physiologique ou bio-mécanique.
• Un tel indice devrait permettre d'évaluer les pertes dues à une foulée inefficace.
• Il devrait, en outre, renseigner le coureur sur l'éventualité d'une foulée dangereuse.
• Il devrait aussi informer le coureur durant la course sur le niveau d'efficacité auquel il évolue.
• La mesure devrait enfin être d'une complexité minimale afin d'être effectuée par un dispositif compact.

From these different considerations on the measurement of the efficiency of the movement of the race, it is possible to draw up a list of the conditions to be fulfilled for the measurement of such a magnitude:

• A measure of race efficiency should provide an absolute index representative of a physiological or bio-mechanical reality.
• Such an index should make it possible to evaluate losses due to an ineffective stride.
• He should also inform the rider about the possibility of a dangerous stride.
• He should also inform the rider during the race of the level of efficiency at which he evolves.
• The measurement should finally be of minimal complexity to be performed by a compact device.

The method according to the invention meets these demands, based on a simple model of determining the inefficiency of a stride from the measurement of the instantaneous acceleration and the average frontal speed of the runner.

• mesure des accélérations instantanées du coureur, dans au moins les directions frontale et verticale,
• obtention d'une information représentative de la vitesse frontale moyenne du coureur,
• calcul d'un indice s'exprimant comme un rapport entre une fonction d'au moins les accélérations et une fonction d'au moins la vitesse frontale, et
• production d'une information représentant l'indice.

More specifically, the invention relates to a method for determining the effectiveness or inefficiency of the stride of a runner, characterized in that consists of the following main operations:

• measuring the instantaneous accelerations of the runner, in at least the frontal and vertical directions,
• obtaining information representative of the average frontal speed of the runner,
• calculating an index expressing itself as a ratio between a function of at least the accelerations and a function of at least the frontal speed, and
• producing information representing the index.

The invention also relates to a device for determining an index of the efficiency or the inefficiency of the stride of a runner, characterized in that it comprises at least one accelerometer intended to be placed in the vicinity of the center of gravity of the runner, a microprocessor implementing the calculation of the index representative of the efficiency or inefficiency of the stride as defined by the method, and display means of this index.

• les figures 1 et 2 représentent schématiquement un premier et un deuxième mode de réalisation d'un dispositif de détermination de l'inefficacité d'une foulée selon l'invention, et
• la figure 3 illustre les résultats expérimentaux obtenus sur une population de coureurs.

Other features of the invention will emerge more clearly on reading the description which follows, made with reference to the appended drawing, in which:

• FIGS. 1 and 2 schematically represent a first and a second embodiment of a device for determining the inefficiency of a stride according to the invention, and
• Figure 3 illustrates the experimental results obtained on a population of runners.

The device for measuring the bio-mechanical inefficiency shown schematically in FIG. 1 comprises a three-dimensional accelerometer 10 placed on the torso of the runner, as close as possible to its center of gravity. The accelerometer 10 is intended to measure the instantaneous accelerations of the center of gravity in the three directions defined with respect to the rider, namely the frontal, lateral and vertical directions. As a variant, the device may comprise three one-dimensional accelerometers.

It should be noted that most of the information is contained in the measure frontal and vertical accelerations, and therefore the measurement of lateral acceleration is optional.

The signals delivered by the accelerometer 10 are applied to a microprocessor 12 responsible for calculating an index representative of the inefficiency of the stride of the runner, according to the model explained below. This index is finally transmitted to a device 14 which delivers it in the form of a written and / or audible message.

The three essential components of the device can be united in a single box attached to the torso of the runner, but nothing prevents that, to facilitate the reading of the message, the index provided by the microprocessor 12 is addressed, by any means of communication at a short distance well known to those skilled in the art, to a receiver worn as a watch on the athlete's wrist.

The calculation of the inefficiency index carried out by the microprocessor 12 is made from the instantaneous acceleration measurements provided by the accelerometer 10 and which are designated respectively at ₁ , a ₂ and a ₃ for the frontal, vertical accelerations. and lateral. Throughout the rest of the talk, the lowercase letters will designate instantaneous quantities, while the capital letters will refer to average sizes.

The calculation is based on a model proposing a definition of the inefficiency of the SrI stride as the ratio of the total kinetic kinetic energy to the kinetic energy of the runner in the frontal direction. Such a definition is based on the following observation.

The movement of the runner is generally a frontal movement at a stable speed, which is superimposed parasitic movements in the vertical, lateral and frontal directions. These parasitic movements, not participating in frontal propulsion, but consuming a significant energy, are characterized by rapid variations in instantaneous speed in the three directions. Intuitively, it appears that the greater these variations, for a constant frontal speed, the more the movement is inefficient. It is why, it is judicious to postulate that the ratio of the kinetic total kinetic energy to the kinetic energy of the runner in the frontal direction is representative of the inefficiency of the movement of the race.

dans laquelle m est le poids du coureur et V_f sa vitesse frontale moyenne.The frontal kinetic energy of the runner is defined by: $$E_{\mathrm{vs},\mathrm{front}}\left(t\right)=\frac{m{V}_f{\left(\mathrm{<figure-callout\; id="2"\; label="t"\; filenames="imgf0002.png"\; state="\{\{state\}\}">t</figure-callout>}\right)}^2}{2}$$

where m is the rider's weight and V _{f is} his average frontal speed.

dans laquelle v_i(t) et a_i(t) sont respectivement la vitesse et l'accélération instantanées dans la direction i qui prend les valeurs 1, 2 et 3 respectivement pour les trois directions frontale, verticale et latérale.The total kinetic kinetic energy, in other words the instantaneous power associated with a velocity variation of the subject, is obtained by derivation of the total kinetic energy: $$E_{\mathrm{vs},\mathit{variational}}\left(t\right)={\displaystyle {\displaystyle \underset{\mathit{i}}{\Sigma }\mathrm{m}{v}_i\left(t\right){\mathrm{at}}_i\left(t\right)}}$$

where v _i (t) and a _i (t) are respectively the instantaneous speed and acceleration in the direction i which takes the values 1, 2 and 3 respectively for the three directions frontal, vertical and lateral.

pour i=1 à 3 correspondant aux directions respectives frontale, verticale et latérale. En tenant compte d'un phénomène de stockage et de restitution de l'énergie dans les tendons et ligaments, l'inefficacité Srl(t) s'écrit encore: $$SrI\left(t\right)=\frac{2{\displaystyle \underset{t-T}{\overset{t}{\int }}\left|{\displaystyle \sum _i\mathrm{\alpha }\left(i,,\mathrm{\tau }\right)v_i\left(t\right)a_i\left(\mathrm{\tau }\right)}\right|d\mathrm{\tau }}}{V_f{\left(t\right)}^2}$$

où α(i,τ) est un coefficient de pondération permettant de prendre en considération le phénomène de recyclage énergétique. Dans le cas le plus simple, α(i,τ) est une constante dépendant de l'axe pondéré. Dans une implémentation plus avancée, α(i,τ) est une fonction de la variable d'intégration τ, à estimer en fonction de chaque coureur.From equations (1) and (2), it follows that the inefficiency Srl (t) is expressed as follows: $$SrI\left(t\right)=\frac{2{\displaystyle \underset{i}{\Sigma }{v}_i\left(t\right){\mathrm{at}}_i\left(t\right)}}{V_f{\left(\mathrm{<figure-callout\; id="2"\; label="t"\; filenames="imgf0002.png"\; state="\{\{state\}\}">t</figure-callout>}\right)}^2}$$

for i = 1 to 3 corresponding to the respective directions frontal, vertical and lateral. Taking into account a phenomenon of energy storage and restitution in the tendons and ligaments, inefficiency Srl (t) is still written: $$SrI\left(t\right)=\frac{2{\displaystyle \underset{t-T}{\overset{t}{\int }}\left|{\displaystyle \underset{i}{\Sigma }\mathrm{\alpha }\left(i,,\mathrm{\tau }\right)v_i\left(t\right){\mathrm{at}}_i\left(\mathrm{\tau }\right)}\right|d\mathrm{\tau }}}{V_f{\left(\mathrm{<figure-callout\; id="2"\; label="t"\; filenames="imgf0002.png"\; state="\{\{state\}\}">t</figure-callout>}\right)}^2}$$

where α (i, τ) is a weighting coefficient making it possible to take into account the energy recycling phenomenon. In the simplest case, α (i, τ) is a constant that depends on the weighted axis. In a more advanced implementation, α (i, τ) is a function of the variable integration τ, to estimate according to each runner.

From this basic formula of the ineffectiveness of the stride, several options are possible for the calculation of the index Srl (t).

As shown in FIG. 1, according to a first embodiment, the microprocessor 12 performs first, at 16, the calculation of the average frontal speed V _f from the instantaneous vertical acceleration a ₂ and according to one of the methods known, such as that used by the firm Polar. The following operation is, at 18, the computation of the instantaneous velocities v _i (t), which consists of an integration of the instantaneous accelerations a _i (t). The speeds are obtained at a constant, equal to zero for the vertical and lateral directions and equal to V _f , the measured frontal speed, for the frontal direction. The index Srl (t) is finally calculated at 20 according to formula (3) or (4). It is this information that the microprocessor 12 provides to the display device 14.

pour i=1 à 3, dans laquelle les a_i (t) sont les accélérations instantanées filtrées par un filtre passe-bas, dans les directions respectivement frontale, verticale et horizontale. Ainsi, dans ce deuxième mode de réalisation, l'opération 18 consiste à filtrer les accélérations instantanées a_i(t) à l'aide d'un filtre passe-bas, tandis que le calcul de l'indice Srl(t) selon la fonction (5) se fait en 20.The second embodiment, also shown in FIG. 1, differs from the first one by the operations carried out at 18 and 20. In fact, it is possible to advantageously express the inefficiency of the stride Srl (t) as a function of the instantaneous accelerations and frontal speed only, two quantities measured directly. The speed being limited by the acceleration, we can approximate the inefficiency of the stride by the following simplified equality: $$SrI\left(t\right)=\frac{2{\displaystyle \underset{t-T}{\overset{t}{\int }}{\displaystyle \underset{i}{\Sigma }\left|\mathrm{\alpha }\left(i,,\mathrm{\tau }\right){\mathrm{at}}_i{\left(\mathrm{\tau }\right)}^2\right|}d\mathrm{\tau }}}{V_f{\left(\mathrm{<figure-callout\; id="2"\; label="t"\; filenames="imgf0002.png"\; state="\{\{state\}\}">t</figure-callout>}\right)}^2}$$

for i = 1 to 3, in which the a _i (t) are the instantaneous accelerations filtered by a low-pass filter, in the respectively frontal, vertical and horizontal directions. Thus, in this second embodiment, the operation 18 consists in filtering the instantaneous accelerations a _i (t) with the aid of a low-pass filter, whereas the calculation of the index S r (t) according to the function (5) is done in 20.

pour i=1 à 3. Les a_i,j(τ) sont les accélérations harmoniques instantanées dans les directions frontale, verticale et horizontale, et les coefficients f_o,j(t) en sont les fréquences centrales, soit la fréquence de foulée ainsi que ses harmoniques. Ainsi, dans ce troisième mode de réalisation, l'indice Srl(t) est calculé directement sur la base des accélérations instantanées mesurées et à partir de la relation (6) dans une unique étape 22.
La dernière égalité (6), qui est exploitée dans le troisième mode de réalisation, fait apparaître l'influence d'un certain nombre de paramètres sur l'inefficacité de la course, influence par ailleurs déjà observée et établie par diverses études.

• Tout d'abord, une valeur de fréquence de foulée élevée tend à diminuer l'indice Srl(t) de l'inefficacité.
• Par ailleurs, la vitesse frontale, qui est le produit de la fréquence de foulée par la longueur de la foulée, doit également être élevée afin de minimiser l'indice Srl(t). II en résulte que la longueur de la foulée, comme sa fréquence, doivent être élevées.
• Enfin, les accélérations instantanées dans les trois directions, sources de perte d'énergie, doivent être minimisées.

Finally, a third embodiment, represented in FIG. 2, differs from the first two in that the operations 18 and 20 are reduced to a single operation 22. It is based on a reasoning also leading to the elimination of the instantaneous speeds v _i (t) in relation (4). The instantaneous speed being a periodic function of frequency equal to the stride frequency, it can be written in the form of a harmonic decomposition. The inefficiency of the stride is expressed by the following relation: $$SrI\left(t\right)=\frac{{\displaystyle \underset{t-T}{\overset{t}{\int }}{\displaystyle \underset{j}{\Sigma }{\displaystyle \underset{i}{\Sigma }\left|\mathrm{\alpha }\left(i,,\mathrm{\tau }\right){\mathrm{at}}_{i,j}{\left(\mathrm{<figure-callout\; id="2"\; label="\tau "\; filenames="imgf0002.png"\; state="\{\{state\}\}">\tau </figure-callout>}\right)}^2/f_{o,j}\right|}}d\mathrm{\tau }}}{V_f{\left(\mathrm{<figure-callout\; id="2"\; label="t"\; filenames="imgf0002.png"\; state="\{\{state\}\}">t</figure-callout>}\right)}^2}$$

for i = 1 to 3. The a _{i, j} (τ) are the instantaneous harmonic accelerations in the frontal, vertical and horizontal directions, and the coefficients f _{o, j} (t) are the central frequencies, ie the frequency of stride as well as its harmonics. Thus, in this third embodiment, the index Srl (t) is calculated directly on the basis of the instantaneous accelerations measured and from the relation (6) in a single step 22.
The last equality (6), which is exploited in the third embodiment, shows the influence of a certain number of parameters on the inefficiency of the race, an influence that has already been observed and established by various studies.

• First, a high stride frequency value tends to decrease the sI (t) index of inefficiency.
• Furthermore, the frontal speed, which is the product of the stride frequency by the length of the stride, must also be high in order to minimize the index Srl (t). As a result, the length of the stride, like its frequency, must be high.
• Finally, the instantaneous accelerations in the three directions, sources of energy loss, must be minimized.

The very simple model thus proposed of the inefficiency of a stride is corroborated by the general rules established for a long time in the field of the race.

An experimental study also reinforces the validity of this model.

• les coureurs débutants, symbolisés par des cercles pleins et des traits pointillés,
• les coureurs confirmés, symbolisés par des symboles vides et des traits pleins, et
• les coureurs d'élite, symbolisés par des symboles vides et des traits pointillés.

Several riders of all levels have been equipped with a three-dimensional accelerometer and a device to calculate the inefficiency of the race according to the proposed model. The results of this study are grouped together in FIG. 3, which represents the graph of the SrI inefficiency index as a function of the frontal speed, for three groups of runners:

• novice runners, symbolized by full circles and dotted lines,
• confirmed runners, symbolized by empty symbols and solid lines, and
• elite runners, symbolized by empty symbols and dashed lines.

• Les coureurs d'élite ont une inefficacité bio-mécanique cinq à six fois inférieure à celle des coureurs débutants.
• Les pertes énergétiques dues à l'inefficacité bio-mécanique sont de l'ordre de 1% à 5% de l'énergie cinétique.
• Une inefficacité minimale est généralement observée pour une vitesse se situant autour de 80% à 90% de la vitesse maximale d'aérobie.

The following three trends are observed:

• Elite riders have a bio-mechanical inefficiency five to six times lower than novice riders.
• The energy losses due to bio-mechanical inefficiency are of the order of 1% to 5% of the kinetic energy.
• Minimal inefficiency is generally observed for a speed of around 80% to 90% of the maximum aerobic speed.

These experimental results are corroborated by previous bio-mechanical studies of a much greater complexity than the method proposed by the invention.

Thus, it appears that on the one hand mathematical developments, on the other hand experimentation, based on the model according to the invention, are in line with what is commonly known and accepted in the field of racing. This model is also very simple, which allows to exploit it with a portable device comprising a three-dimensional accelerometer, a microprocessor, and display means such as a watch.

It will also be noted that the three embodiments presented determine the index of inefficiency of the stride from a value of the average frontal velocity V _f calculated. Another possibility is to have a device for measuring the commercialized average frontal speed, such as the Polar S625X, or the Forerunner 201/301 from Garmin. In this case, the operation of calculating the average frontal speed effected at 16 can be suppressed. The device for measuring the frontal speed must, however, communicate with the microprocessor 12 in order to transmit its data.

Finally, the present description was made with reference to an index of inefficiency of the stride. Of course, a calculation method similar to that just described and to provide an efficiency index of the opposite step of the proposed inefficiency index, would remain within the scope of the invention.

Claims (10)

A method of determining the effectiveness or inefficiency of the stride of a runner, characterized in that it comprises the following main operations: - measurement of the instantaneous accelerations of the runner, in at least the frontal and vertical directions, obtaining information (16) representative of the average frontal speed of the runner, calculating an index (20) expressing itself as a ratio between a function of at least said accelerations and a function of at least said frontal speed, and - production (14) of information representing said index. Method according to Claim 1, characterized in that the numerator of said ratio is a function of the instantaneous vertical, lateral and frontal acceleration respectively of a point close to the runner's center of gravity, and the denominator of said ratio is a function of at least said frontal speed. Method according to one of claims 1 and 2, characterized in that said accelerations are measured by at least one accelerometer (10) located near the center of gravity of said runner. Method according to one of Claims 1 to 3, characterized in that the said frontal speed is calculated (16) from the instantaneous acceleration measurements. Method according to one of claims 1 to 3, characterized in that said frontal speed is measured by a portable type device. Method according to one of claims 1 to 5, characterized in that said calculated index is proportional to the following function: $${\displaystyle \Sigma \left[{\mathrm{v}}_{\mathrm{i}}\left(\mathrm{t}\right){\mathrm{at}}_{\mathrm{i}}\left(\mathrm{t}\right)\right]/\left[{\mathrm{V}}_{\mathrm{f}}{\left(\mathrm{t}\right)}^2\right]}\mathrm{}\mathrm{i}=1\mathrm{}\mathrm{at}\mathrm{}3$$

where v _i (t) and a _i (t) are instantaneous velocities and accelerations respectively in the three frontal, vertical and lateral, and V _f (t) is the average frontal speed of said runner. Method according to claim 6, characterized in that said instantaneous speeds are calculated (18) by integration of said instantaneous accelerations. Method according to one of claims 6 and 7, characterized in that said function is simplified so as to express said index only from the instantaneous accelerations and the average frontal speed. Device for determining an index of the effectiveness or ineffectiveness of the stride, characterized in that it comprises at least one accelerometer (10) intended to be placed in the vicinity of the rider's center of gravity, a microprocessor ( 12) implementing the calculation of the index as defined in one of claims 1 to 8, and display means (14) of said index. Device according to claim 9, characterized in that it further comprises means for measuring the average frontal speed of the runner.

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