Method and device for measuring the heartbeat during rythmic sport practice

The present invention relates to the field of signal processing. It relates more particularly to a method and an integrated device for measuring a heartbeat during the practice of a rhythmic sports activity.

Heart rate measuring devices generally include an external probe, fixed, for example, to the chest or ear, and connected to a signal processing and display unit. These devices are relatively bulky and inconvenient.

Recently, integrated devices have appeared. By integrated device is meant a device formed by a single module performing, at the same point, the measurement of a signal, its processing and the display of the result. Such devices are, for example, worn on the wrist like a watch. Their operation is based on an optical technique for measuring physiological data called photoplethysmography (or PPG). The principle is as follows: An optical source emits infrared radiation propagating inside human tissues in which it undergoes diffusion and absorption. The interaction between radiation and tissue depends on the type and volume of tissue passed through. An optical detector measures the signal at the tissue outlet. This signal contains information on the tissues crossed. In the absence of movement of the wearer, it has a continuous component, derived from static tissues, and a periodic component, derived from pulsatile tissues, typically blood. This last component allows the measurement of the heart rate.

A drawback related to the method of measuring the heart rate by PPG is its sensitivity to the movements of the subject on which the measurement is performed. These movements generate artifacts whose contribution to the signal often exceeds the useful contribution of the heartbeat, by an order of magnitude. The optical signal must therefore be processed so as to eliminate the contribution due to movements. We are talking about enhancement of the optical signal. To this end, the devices generally incorporate a three-dimensional accelerometer capable of delivering a signal representative of the movement carried out at the point where the optical signal is measured.

Various methods of enhancing the optical signal using the acceleration signal have been disclosed to date. We will cite, for example, the document<patcit id="pcit0001" dnum="US7018338B"><text>US 7,018,338</text></patcit> which describes a method based on non-linear modeling of the acceleration signal. Another method, described in the document<patcit id="pcit0002" dnum="WO2006044677A"><text>WO 2006/044677</text></patcit>, transposes the optical and acceleration signals from the time domain to the frequency domain by Fourier transformation (FFT), in order to eliminate motion-related artifacts.

Such optical signal enhancement methods are extremely cumbersome and require considerable computing power. For reasons of cost and size, simpler solutions are preferred. The present invention is based on a rigorous analysis of the sports movement in order to propose a simplified, robust and efficient method and device for detecting the heart rate.

More specifically, the invention relates to a method of measuring a heartbeat during a rhythmic sports activity involving periodic movement, consisting mainly of:<ul id="ul0001" list-style="dash" compact="compact"><li>transmit light radiation into organic tissue,</li><li>detecting an optical signal resulting from the interaction of said light radiation with organic tissues, the optical signal comprising a component relating to the heartbeat and a component relating to the movement carried out during sporting activity,</li><li>detecting an acceleration signal containing an indication relating to the movement carried out during the sporting activity,</li><li>perform an enhancement of the optical signal so as to eliminate the component relating to the movement performed, and</li><li>calculate the heartbeat from the optical signal thus enhanced,</li></ul>

According to the invention, the enhancement of the optical signal mainly comprises the following steps:<ul id="ul0002" list-style="dash" compact="compact"><li>determine the fundamental frequency of the movement,</li><li>determine the harmonics corresponding to the fundamental frequency, and</li><li>eliminating the frequencies thus determined from the optical signal.</li></ul>

Thanks to these characteristics, the method of measuring the heart rate according to the invention can gain speed, simplicity and robustness.

The invention also relates to an integrated device for measuring a heartbeat during a rhythmic sports activity, for the implementation of such a method, comprising:<ul id="ul0003" list-style="dash" compact="compact"><li>a light source,</li><li>a photo-detector providing an optical signal comprising a component relating to the heartbeat and a component relating to the movement carried out during the sporting activity,</li><li>an accelerometer providing an acceleration signal containing an indication relating to the movement carried out during the sporting activity, and</li><li>a processor capable of carrying out an enhancement of the optical signal so as to eliminate the component relating to the movement carried out, and calculating the cardiac pulse from the enhanced optical signal,</li></ul>said integrated device being characterized in that the enhancement of the optical signal mainly comprises the following steps:<ul id="ul0004" list-style="dash" compact="compact"><li>determine a fundamental frequency of the movement from the acceleration signal,</li><li>determine harmonic frequencies of the movement, and</li><li>eliminate from the optical signal the determined movement frequencies.</li></ul>

Other characteristics and advantages of the present invention will emerge more clearly from the following detailed description of an exemplary embodiment of a device for measuring a heart rate according to the invention, this example being given purely by way of illustration and non-limiting only, in conjunction with the appended drawing in which:<ul id="ul0005" list-style="dash" compact="compact"><li>FIGS. 1 and 2 are views respectively from below and in section of the device according to the invention,</li><li>FIG. 3 is a diagram illustrating an algorithm for calculating the heart rate used by the device according to the invention, and</li><li>FIG. 4 relates to a step of the algorithm represented in FIG. 3.</li></ul>

The integrated heart rate measurement device shown diagrammatically in FIGS. 1 and 2 and designated by the general reference 10, conventionally comprises a housing 12 comprising a light source 14 intended to transmit infrared radiation to human tissue, and four photos detectors 16a, 16b, 16c and 16d, such as photodiodes, arranged around the light source 14 so as to receive infrared radiation after its interaction with the tissues. The device 10 also includes a motion detection system 18, such as a three-dimensional accelerometer, and a processor 20 arranged to receive and process the signals from the photo-detectors 16a, 16b, 16c, 16d, and the accelerometer 18. A display module 22, such as an LCD screen, integrated into the housing 12, is intended for displaying the measured physiological parameters, in particular the heart rate. The case 12 is, moreover, provided with a bracelet 24 for fixing the device 10 to the wrist in the manner of a watch.

The device 10 is intended for measuring a heart rate of an athlete practicing a sport of rhythmic type such as walking, running, cycling, swimming, etc., or any sport associated with a periodic movement. Its operation is as follows: The light source 14 emits infrared radiation which interacts with human tissue. The photo-detectors 16a, 16b, 16c and 16d pick up an optical signal containing information on the tissues crossed. At the same time, the accelerometer 18 picks up an acceleration signal containing information on the sports movement performed. The optical and acceleration signals are processed by the processor 20, in order to extract useful information on a physiological parameter, in particular a heart rate. To this end, the processor 20 implements an original and efficient signal processing algorithm, based on careful observation and analysis of the periodic sports movement.

Such a movement is generally complex. It is formed from a combination of sub-movements: the sub-movements of the legs, arms, torso, etc. It has been observed, in the context of the present invention, that each of these sub-movements is of harmonic type, i.e. characterized by a fundamental frequency f₀, and harmonic frequencies f₁, f₂, ... f_i, multiples of the fundamental frequency f₀. In addition, the sub-movements are not independent of each other: Their fundamental frequencies are themselves multiple of each other, so that the overall movement is a harmonic movement with the lowest fundamental frequency of the fundamental frequencies of the different sub -movements. Such an observation considerably simplifies the processing of the acceleration signal. Indeed, it turns out to be sufficient, on the basis of this observation, to determine the fundamental frequency f₀ of the sports movement, to obtain all the frequencies of the movement by simple multiplication. The calculation of a full frequency spectrum of the acceleration signal is thus unnecessarily heavy and complex, and its high consumption of computing power is unjustified.

Note, however, that only an ideal global movement is rigorously harmonic. In the real case, the overall movement is appreciably harmonic because the fundamental frequencies of the different sub-movements are not strictly multiple from one another, and small variations exist. We will see in the rest of this talk, how such variations are taken into account in the estimation of the heartbeat.

The algorithm according to the invention, illustrated schematically in Figure 3, is based on the previous analysis. It comprises three main blocks respectively of preprocessing 100, enhancement of the optical signal 200, and estimation of the cardiac pulse 300, each comprising a certain number of sub-blocks, and detailed below.

at. Pre-treatment 100

The optical and acceleration signals originating respectively from the photo-detectors 16a, 16b, 16c, 16d, and from the accelerometer 18, undergo first of all a preprocessing, symbolized respectively by two sub-blocks 101 and 102 belonging to the module 100. The pretreatment 101 generally consists, for the optical signal, in a first step of eliminating the continuous component of the signal, followed by a second step of filtering in order to reduce the frequency domain to values ​​compatible with a heart rate. Regarding the acceleration signal, the process for essentially eliminating the DC component is carried out during the preprocessing step 102. It will be noted that the preprocessing of the optical and acceleration signals can be carried out in analog or digital mode.

b. Optical signal enhancement 200

The optical signal enhancement module 200 comprises a first sub-block 201 for determining the fundamental frequency of the movement from the pretreated acceleration signal, according to the basic idea of ​​the present invention that the extraction of this single parameter is necessary.

Advantageously, the auto-correlation function, well known to those skilled in the art, will be used for the estimation of the fundamental frequency of the movement. According to the Wiener-Khintchine theorem, the spectrum of the autocorrelation function of a signal is equivalent to the power spectrum of the original signal. In other words, the auto-correlation function of a signal contains the same frequency information as the original signal without the phase information. Consequently, for a harmonic signal, whose frequencies f_i are multiples of the fundamental frequency f₀, the autocorrelation function r_γγ of the signal can be written in the following form: <maths id="math0001" num="(1)"><math display="block"><msub><mi>r</mi><mi mathvariant="normal">γγ</mi></msub><mfenced><mi>l</mi></mfenced><mo>=</mo><mstyle displaystyle="true"><munderover><mo>∑</mo><mrow><mi>not</mi><mo>=</mo><mn>1</mn></mrow><mi>M</mi></munderover></mstyle><msub><mi>AT</mi><mi>not</mi></msub><mo>⋅</mo><mi>cos</mi><mfenced separators=""><mn>2</mn><mo>⁢</mo><mi mathvariant="normal">π</mi><mo>⁢</mo><mi>not</mi><mo>⁢</mo><mi>f</mi><mo>⁢</mo><msub>&lt;mi /&gt;<mn>0</mn></msub><mo>⁢</mo><mi>l</mi></mfenced></math><img file="EP1908401A1_D0001.tif" /></maths>

It follows from equation (1) that the autocorrelation function of a harmonic signal, and in particular the autocorrelation function r_γγ of the acceleration signal, has a maximum at I = 1 / f₀, where f₀ is the fundamental frequency of the signal.

The sub-block 201 for determining the fundamental frequency therefore proceeds, in a first step, to the calculation of the auto-correlation function of the acceleration signal, then, in a second step, to detect the maximum of the function auto-correlation. In certain cases, in order to increase the resolution of the detection of the maximum and the precision of the result, one can carry out, inside the sub-block 201, a polynomial approximation of the function of autocorrelation around the maximum.

Of course, any other mathematical method for determining the fundamental frequency f₀ movement is usable. Mention will be made, for example, of transformation methods such as the Fourier transformation (FFT), the cosine transformation (DCT), or the parametric methods. However, these methods are not limited to the calculation of the useful parameter, the fundamental frequency f₀, but, on the contrary, relate to a large number of parameters. They are heavy in nature, and require considerable computing power. This is why, the calculation of the fundamental frequency f₀ by the autocorrelation function is to be preferred.

Following the sub-block 201 for determining the fundamental frequency f₀, a second sub-block 202 relates to the calculation of the harmonics f₁, f₂, ... f_i. The harmonics are obtained by simple multiplication of the fundamental frequency f₀ by natural numbers (1, 2, 3, ...).

A third sub-block 203 follows sub-block 202 for calculating harmonics. This sub-block 203 relates to the development of a comb filter as illustrated in FIG. 4. Such a filter conventionally comprises wide passbands 50 and narrow rejection bands 52 centered on the fundamental frequency f₀ of movement and its harmonics f₁, f₂, ... f_i. The width of the rejection bands is non-zero and fixed by a set of parameters (β₀, β₁, β₂, ... β_i). The parameters (β₀, β₁, β₂, ... β_i) of the comb filter allow to take into account the small variations of the fundamental frequency and the harmonic frequencies due to the imperfection of the real global movement. Advantageously, the set of parameters β_i is determined for a given activity. For more details on the β parameters_i, please refer to the joint request entitled 'Integrated heartbeat measurement method and device' filed on October 6, 2006 in the name of ETA SA Manufacture Horlogère Suisse.

Finally, a sub-block 204 for filtering the optical signal is connected on the one hand to the block 203 for developing the comb filter, and on the other hand, to the sub-block 101 for pre-processing the optical signal. It corresponds to the optical signal filtering operation using the comb filter developed from the frequencies f_i movement. After the filtering operation carried out in 204, the movement-related artifacts are eliminated from the optical signal.

-vs. Estimated heartbeat 300

The enhanced optical signal is filtered, using an adaptive bandpass filter, or a PCA type noise reduction algorithm as described in the patent. <patcit id="pcit0003" dnum="US7018338B"><text>US 7,018,338</text></patcit>. Then, the heartbeat is estimated from the filtered enhanced optical signal. For this, we use, for example, the zero crossing method in English terminology, that is to say the zero crossing method, well known to those skilled in the art.

We have thus described a device and a method for measuring a heartbeat whose performance is increased by a method of enhancing the optical signal based on an in-depth analysis of the periodic sports movement.

Of course, the method and the device according to the invention are not limited to the embodiment which has just been described and various modifications and simple variants can be envisaged by those skilled in the art without departing from the scope of the invention as as defined by the appended claims.