Patent Publication Number: US-2013245462-A1

Title: Apparatus, methods, and articles of manufacture for determining and using heart rate variability

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. provisional patent application Ser. No. 61/531,095, entitled HEALTH MARKERS FOR MOBILE SYSTEMS, filed on Sep. 6, 2011, which is hereby incorporated by reference in its entirety as if fully set forth herein, including text, figures, claims, tables, and computer program listing appendices (if present), and all other matter in the United States provisional patent application. 
    
    
     FIELD OF THE INVENTION 
     This application relates to estimating and using heart rate variability health markers. 
     BACKGROUND 
     Heart rate variability or “HRV” refers the variation in the heart beat-to-heart beat interval. It is also known as R-R variability and heart period variability. Heart rate variability may be represented by various statistical measures derived from the raw R-R period data, such as variance and standard deviation. 
     Typical methods for measuring HRV include direct electrical (using electrocardiography) measurements of R-R periods of the QRS complex, and ultrasound (echocardiography) techniques. These techniques, however, generally require specialized, expensive, and bulky medical equipment, as well as trained personnel. It would be useful to provide methods for measuring or estimating HRV that do not require such equipment or personnel, and that can be administered in venues where people commonly find themselves in the course of the day. 
     However measured or represented, HRV can be used to estimate fitness level of an individual. This is described, for example, in Kinnunen et al., U.S. Patent Application Publication Number 2012/0010478; and in Riftine, U.S. Patent Application Publication Number 2012/0108916. Each of these patent document publications is incorporated by reference in its entirety. A need exists in the art to improve the precision of fitness estimates. Another need exists in the art to use HRV to derive or create other information regarding individuals based on HRV. 
     SUMMARY 
     Embodiments described in this document are directed to methods, apparatus, and articles of manufacture that may satisfy one or more of the above described and other needs. 
     In embodiments, a computer-implemented method includes obtaining a plurality of electronic images of at least one region of interest (ROI) of face of the evaluated person. The method also includes decomposing the plurality of electronic images of the at least one ROI into a plurality of color channels, each channel of the plurality of color channels corresponding to a different color. The method additionally includes performing at least one operation on the plurality of color channels to obtain one or more processed color channel signals, each processed color channel signal of the one or more processed color channel signals corresponding to a different color channel of the plurality of color channels. The method further includes detecting peaks in at least one processed color channel of the one or more processed color channels. The method further include computing one or more heart rate variability (HRV) statistics based on the peaks in the at least one processed color channel. The method further includes using the one or more HRV statistics. Using the one or more HRV statistics may include storing the one or more HRV statistics, displaying the one or more HRV statistics, transmitting the one or more HRV statistics over a network, presenting to the evaluated person an exercise regimen selected based on the one or more HRV statistics, presenting to the evaluated person a fitness index of the evaluated person computed based on the HRV statistics, and/or indicating to the evaluated person a specific person selected from a set of reference persons based on the one or more HRV statistics. 
     In embodiments, a computing apparatus is configured to obtain a plurality of electronic images of at least one region of interest (ROI) of face of an evaluated person. The apparatus is also configured to decompose the plurality of electronic images of the at least one ROI into a plurality of color channels, each channel of the plurality of color channels corresponding to a different color. The apparatus is additionally configured to perform at least one operation on the plurality of color channels to obtain one or more processed color channel signals, each processed color channel signal of the one or more processed color channel signals corresponding to one color channel of the plurality of color channels. The apparatus is further configured to detect peaks in the at least one processed color channel. The apparatus is further configured to compute one or more heart rate variability (HRV) statistics based on the peaks in the at least one processed color channel. The apparatus is further configured to perform at least one of the following: store the one or more HRV statistics, display the one or more HRV statistics, transmit the one or more HRV statistics over a network, present to the evaluated person an exercise regimen selected based on the one or more HRV statistics, present to the evaluated person a fitness index of the evaluated person computed based on the HRV statistics, and indicate to the evaluated person a specific person selected from a set of reference persons based on the one or more HRV statistics. 
     In embodiments, a computer-implemented method uses heart rate variability statistics of an evaluated person. The method includes maintaining a database storing heart rate variability (HRV) statistics for a plurality of reference persons, a plurality of HRV statistics for each of the reference persons in the database, the plurality of HRV statistics for said each person comprising a vector of HRV statistics for said each person. The method also includes obtaining a plurality of HRV statistics of the evaluated person, the plurality of HRV statistics for the evaluated person comprising a vector x of HRV statistics corresponding to the evaluated person. The method additionally includes computing a fitness index y of the evaluated person. The method further includes using the fitness index, the step of using comprising at least one of storing the fitness index, displaying the fitness index, transmitting the fitness index over a network, and presenting to the evaluated person an exercise regimen selected based on the fitness index. 
     According to the method, the fitness index is computed according to the following: 
     
       
         
           
             
               y 
               = 
               
                 f 
                  
                 
                   ( 
                   
                     
                       ∑ 
                       
                         i 
                         = 
                         1 
                       
                       n 
                     
                      
                     
                         
                     
                      
                     
                       
                         α 
                         i 
                       
                        
                       
                         y 
                         
                           [ 
                           i 
                           ] 
                         
                       
                        
                       
                         p 
                          
                         
                           ( 
                           
                             x 
                             , 
                             
                               x 
                               
                                 [ 
                                 i 
                                 ] 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                   ) 
                 
               
             
             , 
             where 
           
         
       
       
         
           
             
               
                 f 
                  
                 
                   ( 
                   u 
                   ) 
                 
               
               = 
               
                 1 
                 
                   1 
                   + 
                   
                      
                     
                       - 
                       u 
                     
                   
                 
               
             
             , 
           
         
       
     
     i refers to ith reference person in the database of reference persons, 
     α [i]  is a predetermined positive scalar value applicable to the ith reference person in the database, 
     y [i]  is a binary term indicating whether the ith reference person has an athletic lifestyle or a sedentary lifestyle, 
     x [i]  is a vector of HRV statistics of the ith reference person, 
     σ is standard deviation of the heart rate variability sequence of the evaluated person, and 
     
       
         
           
             
               
                 p 
                  
                 
                   ( 
                   
                     x 
                     , 
                     
                       x 
                       
                         [ 
                         i 
                         ] 
                       
                     
                   
                   ) 
                 
               
               = 
               
                 exp 
                  
                 
                   ( 
                   
                     
                       - 
                       
                         1 
                         σ 
                       
                     
                      
                     
                       
                         ∑ 
                         
                           j 
                           = 
                           1 
                         
                         n 
                       
                        
                       
                           
                       
                        
                       
                         
                           ( 
                           
                             
                               x 
                               j 
                             
                             - 
                             
                               x 
                               j 
                               
                                 [ 
                                 i 
                                 ] 
                               
                             
                           
                           ) 
                         
                         2 
                       
                     
                   
                   ) 
                 
               
             
             , 
           
         
       
     
     where j is the running variable indicating a particular HRV statistic from the plurality of HRV statistics in a vector of HRV statistics; 
     In embodiments, a computer-implemented method for determining heart rate variability of an evaluated person includes the following steps: obtaining a plurality of electronic images of skin on a body of the evaluated person; compensating for motion in the plurality of electronic images, thereby obtaining compensated images; identifying at least one region of interest (ROI) in the plurality of electronic images; decomposing the at least one ROI into a plurality of color channels, each channel of the plurality of color channels corresponding to a different color; performing at least one operation (e.g., resampling, band filtering, thresholding) on the plurality of color channels to obtain one or more processed color channel signals, each processed color channel signal of the one or more processed color channel signals corresponding to a different color channel of the plurality of color channels; detecting peaks in at least one processed color channel of the one or more processed color channels; computing one or more heart rate variability (HRV) statistics based on the peaks in the at least one processed color channel; and using the one or more HRV statistics, the step of using comprising at least one of: storing the one or more HRV statistics, displaying the one or more HRV statistics, transmitting the one or more HRV statistics over a network, presenting to the evaluated person an exercise regimen selected based on the one or more HRV statistics, presenting to the evaluated person a fitness index of the evaluated person computed based on the HRV statistics, and indicating to the evaluated person a specific person selected from a set of reference persons based on the one or more HRV statistics. 
     These and other features and aspects of the present invention will be better understood with reference to the following description, drawings, and appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  illustrates selected steps of a process for obtaining heart rate variability estimates; 
         FIG. 2  illustrates decomposition of an exemplary video signal into primary color channels; 
         FIG. 3A  illustrates smoothed exemplary signals resulting from processing of the color channels; 
         FIG. 3B  illustrates peak detection in the smoothed exemplary signals; 
         FIG. 4  illustrates an example of a power spectral density curve of heart rate variability; 
         FIG. 5  is a block diagram representation of selected elements of a computer-based system configured to perform selected steps of methods described in this document; and 
         FIG. 6  illustrates selected steps of another process for obtaining heart rate variability estimates. 
     
    
    
     DETAILED DESCRIPTION 
     In this document, the words “embodiment,” “variant,” “example,” and similar expressions refer to a particular apparatus, process, or article of manufacture, and not necessarily to the same apparatus, process, or article of manufacture. Thus, “one embodiment” (or a similar expression) used in one place or context may refer to a particular apparatus, process, or article of manufacture; the same or a similar expression in a different place or context may refer to a different apparatus, process, or article of manufacture. The expression “alternative embodiment” and similar expressions and phrases are used to indicate one of a number of different possible embodiments. The number of possible embodiments is not necessarily limited to two or any other quantity. Characterization of an item as “exemplary” means that the item is used as an example. Such characterization of an embodiment does not necessarily mean that the embodiment is a preferred embodiment; the embodiment may but need not be a currently preferred embodiment. All embodiments are described for illustration purposes and are not necessarily strictly limiting. 
     The words “couple,” “connect,” and similar expressions with their inflectional morphemes do not necessarily import an immediate or direct connection, but include within their meaning connections through mediate elements. 
     Reference will be made in detail to several embodiments that are illustrated in the accompanying drawings. Same reference numerals are used in the drawings and the description to refer to the same apparatus elements and method steps. The drawings are in a simplified form, not to scale, and omit apparatus elements and method steps that can be added to the described systems and methods, while possibly including certain optional elements and steps. 
     Heart rate variability is an indicator of physical fitness. It is also an indicator and outcome predictor of various diseases and health-related conditions, including sleep apnea; pulmonary diseases such as Chronic Obstructive Pulmonary Disease or COPD; neurological diseases such as epilepsy and fibromyalgia; endocrine and metabolic diseases such as diabetes mellitus and resulting neuropathy; hypertension; and cardiac diseases such as myocardial dysfunction/infarction, and ischemic heart disease. 
       FIG. 1  illustrates selected steps of an operational flow of a process  100  for obtaining HRV estimates of a person by analyzing images of the person&#39;s face, beginning with flow point  101  and ending in flow point  199 . 
     In step  105 , the person is asked one or more pre-selected questions, and the answers to the questions are recorded. The questions may be selected to capture variations in emotional states, mood, physical activity, and perception of the person&#39;s own health state. The questions may also include a survey of medications taken by the person. In variants, the person is asked to respond to a questionnaire with approximately fifteen questions. 
     In step  110 , the person is given instructions regarding the proper behavior during the time interval when the person&#39;s facial images are obtained. For example, a computer device or a mobile device (e.g., a desktop computer, laptop, netbook, tablet, or smartphone) displays or sounds the instructions to the person. In embodiments, the person is presented with a video clip containing the instructions. The person may be instructed to sit in a comfortable position in front of the device, place the device about twenty inches from the person&#39;s face in a stable position so that the person&#39;s face is within the field of view of the device&#39;s camera, turn the device&#39;s camera (e.g., webcam) on and focus it on the face, start the video-recording at a specified resolution (e.g., 640×480), and not to move or change facial expression for the duration of the test (e.g., three minutes). Instead, or in addition to sitting substantially without movement, the person may be instructed to smile for a picture, and/or to talk about his/her daily activity for a period of time (e.g., twenty seconds) in front of the camera. 
     In step  115 , the device captures a plurality of images. For example, the device may take a video of the person&#39;s face. This step may be performed in response to the person activating the camera or starting the video recording, or automatically, for example, after a predetermined period after the instructions are displayed. 
     In step  120 , the face of the person is automatically detected in the captured video, using a face detection module. 
     In step  125 , face motion in the video is compensated using an optical flow algorithm. 
     In step  130 , one or a plurality of face regions are recognized in the captured and compensated video, using automatic facial feature detector(s). For example, the eyes, nose, and mouth can be recognized. 
     In step  135 , the system performing the process  100  identifies one or more regions of interest (“ROIs”) in the face, based on the recognized facial features. A region of interest provides signals useful for determining heart beats in color channels obtained from the images of the region, as is discussed in more detail below. One or more of the ROIs may be near the lips. 
     In step  140 , the video is decomposed into a plurality of channels corresponding to different colors. For example, the channels may correspond to primary colors such as green, red, and blue (“RGB”) channels.  FIG. 2  illustrates decomposition of an exemplary video into a red channel, a green channel, and a blue channel. 
     The channels are then processed, in step  145 , for example, by applying one or more spatial filters developed using canonical correlation analysis (analysis using cross-covariance matrices) or a similar supervised learning approach (such as a neural network). The goal of the filtering may be to predict the timing of the heart beat based on the visual information. This step may be focused on the identified region(s) of interest; in other words, portions of the video not corresponding to the regions of interest may be ignored. Furthermore, this step may be performed for each sub-region of one pixel or a plurality of contiguous pixels. There may be a separate filter for each channel-sub-region pair. 
     The output of the spatial filters is passed through a temporal analysis module, in step  150 . This module is configured to remove artifacts based on prior statistics of human heart rate variability. 
     In step  155 , noise components from the signals processed in the previous step are filtered out using, for example, a Kalman temporal smoother, resulting in smoothed signals. Additionally (or, sometimes, instead), the filtering may include bandpass or lowpass filtering.  FIG. 3A  illustrates the smoothed signals resulting from processing of the channels shown in  FIG. 2 . 
     In step  160 , peaks in the smoothed signals are detected. The time between peaks corresponds to the interval between consecutive heart beats. One of the channels (smoothed signals) may be selected for determining the intervals. For example, the smoothed signal with the best defined peaks may be selected. Alternatively, a plurality or even all of the channels may be analyzed concurrently to arrive at the best decision regarding each particular interval. As shown in  FIG. 3B , the green channel is selected in the illustrated example, based on the superior definition of the peaks in this channel compared to the other two channels. 
     In step  165 , the time series of intervals between heart beats obtained in the preceding step is processed to derive one or more HRV statistics. The HRV statistics may be derived in the time domain, frequency domain, or in both domains. 
     Time domain HRV statistics may include RR mean , SDNN, SDNN index , RMSSD, NN50, pNN50, and TINN. We now briefly explain these time domain HRV statistics: 
     RR mean  is the mean interval, 
     
       
         
           
             
               
                 1 
                 N 
               
                
               
                 
                   ∑ 
                   
                     i 
                     = 
                     1 
                   
                   N 
                 
                  
                 
                     
                 
                  
                 
                   RR 
                    
                   
                     ( 
                     i 
                     ) 
                   
                 
               
             
             ; 
           
         
       
     
     SDNN is the standard deviation (e.g., in seconds) of the interval, √{square root over ((Σ i=1   N (RR(i)−RR mean ) 2 /(N−1);)}{square root over ((Σ i=1   N (RR(i)−RR mean ) 2 /(N−1);)} 
     RMSDD is the square root of the mean of the sum of the squared difference between successive (adjacent) intervals (e.g., in milliseconds), √{square root over ((Σ i&lt;1   N-1 (DRR(i) 2 )/(N−2),)}{square root over ((Σ i&lt;1   N-1 (DRR(i) 2 )/(N−2),)} where DRR(i)=RR(i+1)−RR(i)∀iε{1, N−1}; 
     SDNN index =1/L Σ i=1   L σ RR |5 minutes ; 
     NN50 is the number of pairs of successive (adjacent) intervals that differ by more than 50 ms; 
     pNN50 is the proportion of NN50 divided by total number of intervals; and 
     TINN is the integral of sample density distribution of RR intervals divided by the maximum of the density distribution (RR triangular index) and baseline width of the minimum square difference triangular interpolation of the maximum of the sample density distribution of RR intervals (e.g., in seconds). 
     Frequency domain HRV statistics may include: 
     HF, high-frequency components spectral power, between about 0.15 and 0.4 Hz; 
     LF, low-frequency components spectral power, between about 0.04 and 0.15 Hz; 
     VLF, very low frequency components spectral power, between about 0 and 0.04 Hz; 
     LF/HF ratio; 
     HFnu, percentage of HF power in normalized units (100*HF/(LF+HF−VLF)); 
     LFnu, percentage of LF power in normalized units (100*LF/(LF+HF−VLF)); and 
     VLFnu, percentage of VLF power in normalized units (100*VLF/(LF+HF−VLF)). 
     The HRV statistics may include other statistical measures. 
       FIG. 4  illustrates an example of a power spectral density curve  400  of heart rate variability. 
     In step  170 , the HRV statistics (or a subset of the HRV statistics) are processed with an estimation unit configured to estimate the fitness level or other physical abilities of the person, resulting in one or more physical ability estimates (indices) of the person. The estimation unit may employ a Support Vector Machine (SVM) or another neural network using learning algorithms that analyze data and recognize patterns. 
     One group of fitness indices was developed using machine learning methods applied to reference databases storing information regarding people with known fitness levels. In an exemplary specific implementation, a dataset of professional athletes and of people with sedentary lifestyles was used. (This example can be modified to store information regarding people spanning the entire spectrum of physical experience and/or ability, or any portion of this spectrum.) A Support Vector Machine was then trained to predict the lifestyle of the people in the dataset based on the aforementioned HRV statistic. The result of training is a formula that estimates a fitness index, y, of a person being evaluated and helps answer the question of whether the person has an athletic or a sedentary lifestyle: 
     
       
         
           
             
               y 
               = 
               
                 f 
                  
                 
                   ( 
                   
                     
                       ∑ 
                       
                         i 
                         = 
                         1 
                       
                       n 
                     
                      
                     
                         
                     
                      
                     
                       
                         α 
                         i 
                       
                        
                       
                         y 
                         
                           [ 
                           i 
                           ] 
                         
                       
                        
                       
                         p 
                          
                         
                           ( 
                           
                             x 
                             , 
                             
                               x 
                               
                                 [ 
                                 i 
                                 ] 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                   ) 
                 
               
             
             , 
             where 
           
         
       
       
         
           
             
               
                 f 
                  
                 
                   ( 
                   u 
                   ) 
                 
               
               = 
               
                 1 
                 
                   1 
                   + 
                   
                      
                     
                       - 
                       u 
                     
                   
                 
               
             
             , 
           
         
       
     
     i refers to the ith person in the database, 
     α [i]  is a positive scalar value (weighting constant) applicable to the ith person in the database, 
     y [i]  is a binary term (+/−1) indicating whether the ith person has an athletic or a sedentary lifestyle (this can be modified to a multi-value or continuous variable designating the fitness level of the ith person, with 1 designating an athlete, zero designating an average person, and −1 designating a typical “couch potato”), 
     x [i]  is a vector of HRV statistics of the ith person and x is the HRV statistics vector corresponding to the person being evaluated, 
     y is the fitness index of the person being evaluated (which may be a number between zero and one), and 
     p is a proximity kernel function that determines how close the observation x is to each of the x [i]  vectors, which can be computed thus: 
     
       
         
           
             
               p 
                
               
                 ( 
                 
                   x 
                   , 
                   
                     x 
                     
                       [ 
                       i 
                       ] 
                     
                   
                 
                 ) 
               
             
             = 
             
               
                 exp 
                  
                 
                   ( 
                   
                     
                       - 
                       
                         1 
                         σ 
                       
                     
                      
                     
                       
                         ∑ 
                         
                           j 
                           = 
                           1 
                         
                         n 
                       
                        
                       
                           
                       
                        
                       
                         
                           ( 
                           
                             
                               x 
                               j 
                             
                             - 
                             
                               x 
                               j 
                               
                                 [ 
                                 i 
                                 ] 
                               
                             
                           
                           ) 
                         
                         2 
                       
                     
                   
                   ) 
                 
               
               . 
             
           
         
       
     
     In step  175 , the estimates are stored (locally at the end-user communication device or remotely at the server or another device), displayed (locally at the end-user communication device or remotely at the server or another device), transmitted (to the end-user communication device, to the server, or to another device), used to select an exercise or training regimen (locally at the end-user communication device or remotely at the server or another device, at any time), or used otherwise. 
       FIG. 6  illustrates selected steps of an operational flow of a process  600  for obtaining HRV estimates of a person by analyzing images of the person&#39;s skin with internal blood flow, beginning with flow point  601  and ending in flow point  699 . The process  600  does not require images/video of the face of the person being evaluated; it can use images/video of other portions of the person&#39;s skin. 
     Steps  605  and  610  may be identical or analogous to the steps  105  and  110  described above. For example, the person may be asked one or more pre-selected questions, and the answers to the questions may be recorded. The person may also be given instructions regarding the proper behavior during the time interval when the person&#39;s facial images are obtained. 
     In step  615 , the device captures a plurality of images. For example, the device may take a video of some part of the person&#39;s body, for example, face, neck, arm, hand. This step is analogous to the step  115  described above. 
     In step  625 , motion in the video is compensated (for example, using a standard local optic flow algorithm), and, in step  635 , one or more regions of interest (“ROIs”) within the images are identified. For example, the images may be filtered using a bandpass spatiotemporal filter (such as a spatiotemporal Gabor filter). The temporal bandwidth of the filter may be set in the 0.4 to 10 Hz range. The spatial bandwidth may be set in the range of 0 to 0.1 cycles per pixel (this may change depending on the spatial resolution of the video sensor). For each pixel and each color channel, the correlation with neighboring pixels and color channels may be computed. One way to compute this correlation is to treat the temporal power spectrum of each pixel/color channel as a vector and compute the inner product between this vector and the vectors of neighboring pixels. Other algorithms that provide similar results also exist (e.g., canonical correlation, cross power spectrum analysis). For each pixel, the likelihood that the pixel is skin color is computed. This can be achieved, for example, using standard probabilistic filter based on frequencies of color values of human skin. A standard computer vision object recognizer may be used to provide for each pixel the likelihood that the pixel renders a target part of the human body (e.g., face detector, hand detector, arm detector). The information from the correlation filter, the color skin filter, and the object recognition filters may be combined using standard sensor fusion approaches (e.g., Bayesian inference, neural network). A result of these sub-steps may be an image with the probability for each pixel that the pixel provides relevant blood flow information. 
     A statistical clustering algorithm may be applied to identify regions of interest in the images. A region of interest is a set of continuous pictures that provide a high degree of information about blood flow. 
     In step  640 , the video is decomposed into a plurality of channels corresponding to different colors. For example, the channels may correspond to primary colors such as green, red, and blue (“RGB”) channels. This step may be identical or analogous to the step  140  discussed above. 
     In step  645 , the channels are resampled to improve the temporal resolution. For example, the channels are resampled at a rate between 30 Hz and 1 kHz. 
     In step  650 , the channels are bandpass filtered, using, for example, a second order 1-3 Hz Butterworth bandpass filter. 
     In step  655 , the channels are compared to a fixed or a variable threshold. For example, the threshold for each channel may be set to 0.8 times the channel&#39;s standard deviation. 
     In step  660 , the local maxima in the resulting channel signals are detected. This provides time estimates of the heart beat positions. The RR series is then obtained by differentiation of the estimates of the heart beat positions. This step may be identical or similar to the step  160  described above. 
     In step  665 , the time series of intervals between heart beats (the RR intervals) obtained in the preceding step is processed to derive one or more HRV statistics. This may be done as described above in relation to the step  165 . 
     In step  670 , the HRV statistics (or a subset of the HRV statistics) are processed with an estimation unit configured to estimate the fitness level or other physical abilities of the person, resulting in one or more physical ability estimates (indices) of the person. This step may be identical or analogous to the step  170  described above. 
     In step  675 , the estimates are stored (locally at the end-user communication device or remotely at the server or another device), displayed (locally at the end-user communication device or remotely at the server or another device), transmitted (to the end-user communication device, to the server, or to another device), used to select an exercise or training regimen (locally at the end-user communication device or remotely at the server or another device, at any time), or used otherwise. 
     In another example, sport-specific fitness indices for the person are obtained. Here, the HRV statistics previously obtained (whether using images of a person&#39;s face, images of another body part of the person, an inertial sensor, or otherwise) serve to predict the fitness level of the user with respect to one or more specific sports. The same formula may be used, but the y [i]  term now relates to the sport-specific ability of the ith person, and y is now the sport-specific fitness index of the person being evaluated. Thus, these specific sport indices are developed using machine learning methods applied to databases of people known to practice specific sports. In an example, datasets (including HRV statistics) of professional basketball players and soccer players were collected. A Support Vector Machine was then trained on the datasets. By using datasets from different sports, it is possible to ascertain to what extent new users match the HRV statistics preferred for these specific sports. 
     In yet another example, the cardiovascular parameters of a user, as measured by HRV statistics as well as the previously described fitness index and the sport-specific fitness indices may be used to discover people with similar cardiovascular values. The system may have a database of HRV statistics and sport-specific fitness indices, for well-known sports (and other) celebrities. The system may then find which celebrity in the database best matches the cardiovascular parameters of the person being evaluated. One way to do so is to measure the “distance” between the person being evaluated and other people in a space where each of selected statistics corresponds to a different dimension. The “distance” may be a computed as a square root of the sum of the squares of the differences in all the dimensions. A person in the reference database (e.g., a sport celebrity) may then be selected based on the distance from the person being evaluated. For example, the celebrity (or celebrities) nearest the person being evaluated may be selected. 
     The HRV statistics used for various purposes described in this document (e.g., computing general and sport-specific fitness indices, as well as identification of people with similar profiles) need not be limited to the statistics obtained according to the process  100  and analogous processes. The statistics may be simply read into the fitness index or similarity determination formula, or determined using other sensors. A smartphone inertial sensor (such as an accelerometer or a gyro) can be used to obtain the person&#39;s HRV. 
     For example, a Kalman filter may be used to integrate the output of a mobile device&#39;s inertial sensor, arriving at an estimate of the height and orientation of the inertial measurement unit when the mobile device is placed on top of the user&#39;s chest. The output of the Kalman filter may then be processed using a Bank of Gabor energy filters. This bank of filters focuses on the frequency components caused by breathing and heart beat. The parameters of these filters are set using standard machine learning methods (e.g., logistic regression, support vector machines or Gaussian processes). The peaks in the output of these filters are detected to determine the timing of the heart beat. The interval between two peaks represents the period between two heart beats. 
     In the above discussion, we focused on the analysis of HRV statistics. Other variables may be added to the analysis. For example, the HRV statistics of a person being evaluated (or a subset of these statistics) can be combined with other information regarding the person, for example, the person&#39;s responses to the questionnaire and other medical or physiological data. The same information may be stored for the individuals in the reference database. The analysis may then be performed according to the same formula (y=f(Σ i=1   n α i y [i] p(x,x [i] ))), but with the x and x [i]  vectors now including the additional data. Similarly, in calculating the “distances,” dimensions corresponding to variables other than HRV statistics may be added. 
     In embodiments, the heart rate itself (HR) is not used to determine the indices. 
     The methods described in this document may be executed, in whole or in part, for example, on a personal computer or a mobile device. The processing may also be done on a central computer, such as a server configured to perform at least some of the steps. Users of the methods described in this document may submit data obtained (e.g., HRV and questionnaire responses) to a central server, using, for example, portable/mobile devices. The server may be configured to gather information from a wide range of users. Datamining techniques (e.g., independent component analysis, cluster analysis, and similar techniques) may be used to discover patterns that characterize specific classes of users. Various correlations between HRV statistics and physical conditions may be discovered and used advantageously. For example, it may be that users who report having fibromyalgia (or another disease) present a specific set of cardiovascular parameters, as measured via HRV statistics and derivative indices. When a new user is found to have this or sufficiently similar pattern of HRV statistics, the server may send a message alerting the user of this fact. The system may also discover that the health index tends to drop during the winter holidays (or another time frame) and alert the users ahead of time. 
       FIG. 5  is a simplified block diagram of a computer system  500  configured in accordance with selected aspects described throughout this document. As shown in the Figure, a communication network  580  couples the system  500  to personal end-user communication devices  570 . The system  500  may be configured to perform all or some of the steps of the process  100  of  FIG. 1 . It may be built, for example, on a personal computer platform such as a Unix, Wintel PC, or a Mac computer (including desktop and laptop computers), a group of networked computers, a special purpose data processor, a general-purpose computer, or another kind of processing device.  FIG. 1  omits various hardware blocks, software components, and physical and logical connections. 
     The system  500  may include a processor  510 , read only memory (ROM)  520 , random access memory (RAM)  530 , network interface  540 , a mass storage device  550 , and a database  560 . These components may be coupled together by a bus  515 . The processor  510  may be a microprocessor, and the mass storage device  550  may be a magnetic disk drive. The mass storage device  550  and each of the memory modules  520  and  530  are connected to the processor  510  to allow the processor  510  to write data into and read data from these storage and memory devices. The network interface  540  couples the system  500  to the network  580 , for example, the Internet. The nature of the network  580  and of the devices that may be interposed between the system  500  and the network  580  determines the kind of network interface  540  used in the system  500 . In some embodiments, for example, the network interface  540  may be an Ethernet interface that connects the system  500  to a local area network, which, in turn, connects to the Internet. The network  580  may include a cellular interface (not shown) that provides a connection between the network  580  and the personal communication devices  570 . The personal communication devices  570  may include smartphones, iPads®, iPods®, PCs, PDAs, as well as laptop and desktop computers of various kinds. 
     The database  560  may be used for organizing and storing data that may be needed or desired in performing the method steps described in this document, including storing the images/video obtained from the personal communication devices  570 . The database  560  may be a physically separate system coupled to the processor  510 . In variants, the processor  510  and the mass storage device  550  are configured to perform the functions of the database  560 . 
     The processor  510  is configured to read and execute program code instructions stored in a machine readable storage device. Under control of the program code, the processor  510  configures the system  500  and/or one or more of the personal communication devices  570  to perform the steps of the methods described throughout this document. (The processor  510  may be able to configure the devices  570  by sending instructions to these devices over the network  580 .) The program code instructions may be embodied in other machine-readable storage media, such as additional hard drives, floppy diskettes, CD-ROMs, DVDs, Flash memories, and similar devices. The program code can also be transmitted over a transmission medium, for example, over electrical wiring or cabling, through optical fiber, wirelessly, or by any other form of physical transmission. The transmission can take place over a dedicated link between telecommunication devices, or through a wide- or local-area network, such as the Internet, an intranet, extranet, or any other kind of public or private network. 
     As noted above, some or all of the process steps may be performed by the system  500  in conjunction with one or more of the personal communication devices  570 . The process steps may be performed by the system  500  alone, in which case the system  500  may be equipped with an image capture device such as a camera, or otherwise have access to images/video of a person. The process steps may also be performed by one or more of the personal communication devices  570 . 
     The analyzed images/video may be obtained by any type of known or available camera, including, but not limited to, webcams built into personal computers or mobile devices, video cameras for taking moving video images, digital cameras capable of taking still pictures and/or capturing continuous video streams, stereo cameras, and/or any other imaging device. 
     Various lenses, filters, and other optical devices, such as zoom lenses, wide angle lenses, minors, prisms, and the like may also be used with the image capture device to assist in capturing the images/video. The image capture devices may be stationary, i.e., fixed in a particular orientation and configuration. The image capture devices (along with any of their accompanying optical devices) may also be fixed on a gimbal and be programmable in orientation and position. The image capture devices can also be capable of moving along one or more directions, such as up, down, left, and right; and/or rotate about one or more axes of rotation. The image capture device may also be capable of moving to follow or track an object, including a person, an animal, or another object in motion. In other words, the image capture device may be capable of moving about an axis of rotation in order to keep a person or object within a viewing range of the device&#39;s lens. 
     Although the process steps and decisions (if decision blocks are present) may be described serially in this document, certain steps and/or decisions may be performed by separate elements in conjunction or in parallel, asynchronously or synchronously, in a pipelined manner, or otherwise. There is no particular requirement that the steps and decisions be performed in the same order in which this description lists them or the Figures show them, except where a specific order is inherently required, explicitly indicated, or is otherwise made clear from the context. Furthermore, not every illustrated step and decision block may be required in every embodiment in accordance with the concepts described in this document, while some steps and decision blocks that have not been specifically illustrated may be desirable or necessary in some embodiments in accordance with the concepts. It should be noted, however, that specific embodiments/variants/examples use the particular order(s) in which the steps and decisions (if applicable) are shown and/or described. 
     In various embodiments, the features, elements, and limitations of apparatus and processes described throughout this document may be present individually, or in any combination or permutation, except where the presence or absence of specific feature(s)/element(s)/limitation(s) is inherently required, explicitly indicated, or otherwise made clear from the context. 
     This document describes in considerable detail the inventive apparatus, methods, and articles of manufacture for determining and using heart rate variability. This was done for illustration purposes only. Neither the specific embodiments nor specific features necessarily limit the general principles underlying the invention. The specific features described herein may be used in some embodiments, but not in others, without departure from the spirit and scope of the invention as set forth herein. Various physical arrangements of components and various step sequences also fall within the intended scope of the invention. Many additional modifications are intended in the foregoing disclosure, and it will be appreciated by those of ordinary skill in the pertinent art that in some instances some features will be employed in the absence of a corresponding use of other features. The illustrative examples therefore do not necessarily define the metes and bounds of the invention and the legal protection afforded the invention, which function is carried out by the claims and their equivalents.