Patent Publication Number: US-11048776-B2

Title: Methods and systems for control of human locomotion

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 13/808,886 having a 35 U.S.C. 371 date of 7 Jan. 2013, which is a national phase entry of PCT application No. PCT/CA2011/050417 having an international filing date of 7 Jul. 2011, which claims the benefit of the priority under 35 USC § 119(e) of U.S. application No. 61/362,170 filed 7 Jul. 2010. All of the applications referred to in this paragraph are hereby incorporated herein by reference for all purposes. 
    
    
     TECHNICAL FIELD 
     This invention relates to the automatic control of human locomotion (e.g. running and/or walking). Some embodiments provide methods and systems for automatic control of human locomotive speed, position and/or intensity. 
     BACKGROUND 
     There is a general desire to describe and/or control various means of human locomotion. Such description and/or control can assist with navigation, predicting arrival times and the like. For example, the description of the speed of an automobile (e,g. provided by a speedometer) may be used to predict how far the automobile can travel in a particular length of time and/or when the automobile will arrive at a particular destination. Speed control of the automobile (e.g. provided by a cruise control system) can be used to achieve target arrival times, target speeds and the like. 
     There is a similar desire to describe and/or control human locomotion (e.g. locomotion, such as running, walking and/or the like). 
     Like the case of the exemplary automobile discussed above, such control can assist with achieving target navigation parameters, such as arrival times and the like. By way of non-limiting example, description and control of human locomotion can also have application to training (e.g. for athletes, recreational runners, soldiers and the like). Many runners, ranging from world class athletes to recreational runners, set objectives (goals) to cover a given distance in a certain amount of time. To achieve such objectives, such runners have to run the distance at a particular speed or with a particular speed profile. 
     Various systems and techniques are known in the prior art to estimate running/walking speed and/or position. Such prior art systems include:
         The “Nike+”™ sportsband developed by Nike, Inc and the “Rock and Run”™ system developed by Apple Inc. in conjunction with Nike, Inc. use an in-shoe sensor and a handheld or band-mounted user interface to estimate time, distance and speed and to provide such information to the shoe wearer—(see http://nikerunning.nike.com/nikeos/p/nikeplus/en_EMEA/sportband and http://www.apple.com/ipod/nike/run.html).   The “Forerunner”™ series of wrist-worn devices sold by Garmin Ltd. which use global positioning system (GPS) technology to estimate position, speed and time and to provide such information to the user—(See https://buy.garmin.com/shop/shop.do?cID=141&amp;fKeys=FILTER_SERIES_FORE RUNNER).   The “Polar S3 Stride Sensor W.I.N.D.”™ sensor sold by Polar Electro Oy which mounts to the user&#39;s shoe, measures the acceleration of a user&#39;s foot and uses this acceleration information to estimate ground speed and/or distance—(http://www.polarusa.com/us-en/products/accessories/s3_Stride_Sensor_WIND).   The “Speedmax”™ technology developed by Dynastream Innovations Inc. which uses inertial sensors to detect running/walking speed and distance.       

     Other than for providing the user with information about their speed, however, these systems and techniques do not appear to permit automatic control of human running/walking speed and/or position. Using such systems, a user would have to repetitively monitor the user interface (or repetitively receive output from an output device (e.g headphones)) and then the user would have to determine on their own whether they were meeting their speed objective. Based on their own consideration of whether they were meeting their speed objective, the user would then have to adjust their speed on their own and then recheck the user interface at a later time to determine if their new speed meets the speed objective. For most humans, this speed adjustment is difficult to perform accurately. No information is provided to the user between the time that the user first checks the user interface and the time that the user subsequently rechecks the user interface at the later time. These systems are analogous to the speedometer of an automobile, wherein speed information is provided to the driver, but the driver adjusts the speed on their own (i.e. without automatic cruise control). Such systems do not provide automatic speed control of locomotion in a manner that is analogous to cruise control in an automobile. 
     There is a desire for systems which help a subject to automatically control a speed and/or position of their human locomotion (e.g. locomotion such as running and/or walking). 
     In addition to or in the alternative to controlling locomotive speed and/or position, there is a general desire to control locomotion intensity. Locomotive intensity is usually estimated based on one or more measurable or estimatable or measurable intensity indicators. Such intensity indicators include, by way of non-limiting example, heart rate, metabolic rate, oxygen consumption, perceived exertion, mechanical power and/or the like. 
     Various systems and techniques are known for estimating heart rate. Such systems include:
         Strapped heart rate monitors (for example by Polar Electro Oy—see http://www.polarusa.com/us-en/products/get_active); and   Strapless heart rate monitors (for example by Physi-Cal Enterprises Inc.—see http://mioglobal.com/main_products).
 
Again, as is the case with speed measurement, these heart rate monitors merely provide the user with information about their heart rate and do not appear to permit automatic control of the intensity of human locomotion. Accordingly, these systems suffer from analogous drawbacks to those of the speed and distance measurement systems described above.
       

     There has been some attempt in the art at control of a user&#39;s heart rate. Examples may include the BODIBEAT™ music player marketed by Yamaha—see http://www.yamaha.com/bodibeat/consumer.asp; and the TRIPLEBEAT™ application marketed by the individual Dr. Nuria Oliver—see http://www.nuriaoliver.com/TripleBeat/TripleBeat.htm. 
     SUMMARY 
     The following embodiments and aspects thereof are described and illustrated in conjunction with systems, tools and methods which are meant to be exemplary and illustrative, not limiting in scope. In various embodiments, one or more of the above-described problems have been reduced or eliminated, while other embodiments are directed to other improvements. 
     One aspect of the invention provides a method for the automatic control of locomotion speed in a human or other animal subject. The method comprises: estimating the subject&#39;s actual locomotion speed using one or more sensors to thereby obtain a measured speed; determining an error comprising a difference between a desired speed and the measured speed; and outputting, to the subject, a stimulus frequency signal wherein the stimulus frequency signal is based on the error in such a manner that when the subject ambulates in a manner that matches a frequency of the stimulus frequency signal, the subject&#39;s actual speed controllably tracks the desired speed. 
     Another aspect of the invention provides a method for the automatic control of locomotion position of a human or other animal subject. The comprises: estimating the subject&#39;s actual locomotion position using one or more sensors to thereby obtain a measured position; determining an error comprising a difference between a desired position and the measured position; and outputting, to the subject, a stimulus frequency signal wherein the stimulus frequency signal is based on the error in such a manner that when the subject ambulates in a manner that matches a frequency of the stimulus frequency signal, the subject&#39;s actual position controllably tracks the desired position. 
     Another aspect of the invention provides a method for the automatic control of locomotion intensity in a human or other animal subject. The method comprises: estimating the subject&#39;s actual locomotion intensity using one or more sensors to thereby obtain a measured intensity; and determining an intensity error comprising a difference between a desired intensity and the measured intensity. If an absolute value of the intensity error is outside of a threshold region around the desired intensity, then the method involves: estimating the subject&#39;s actual locomotion speed using one or more sensors to thereby obtain a measured speed; converting the desired intensity to a desired speed; determining a speed error comprising a difference between the desired speed and the measured speed; and outputting, to the subject, a speed-based stimulus frequency signal wherein the speed-based stimulus frequency signal is based on the speed error in such a manner that when the subject ambulates in a manner that matches a frequency of the speed-based stimulus frequency signal, the subject&#39;s actual intensity controllably tracks the desired intensity. If the absolute value of the intensity error is within the threshold region around the desired intensity, then the method involves outputting, to the subject, an intensity-based stimulus frequency signal wherein the intensity-based stimulus frequency signal is based on the intensity error in such a manner that when the subject ambulates in a manner that matches a frequency of the intensity-based stimulus frequency signal, the subject&#39;s actual intensity controllably tracks the desired intensity. 
     Another aspect of the invention provides a system for automatically controlling a locomotion speed of a human or other animal subject. The system comprises: one or more sensors for sensing one or more corresponding parameters of the locomotion movement of the subject and for generating therefrom a measured speed which represents an estimate of the subject&#39;s actual locomotion speed; a controller configured to: determine an error comprising a difference between a desired speed and the measured speed and output, to the subject, a stimulus frequency signal; wherein the stimulus frequency signal is based on the error in such a manner that when the subject ambulates in a manner that matches a frequency of the stimulus frequency signal, the subject&#39;s actual speed controllably tracks the desired speed. 
     Another aspect of the invention provides a system for automatically controlling a locomotion position of a human or other animal subject. The system comprises: one or more sensors for sensing one or more corresponding parameters of the locomotion movement of the subject and for generating therefrom a measured position which represents an estimate of the subject&#39;s locomotion position; a controller configured to: determine an error comprising a difference between a desired position and the measured position and output, to the subject, a stimulus frequency signal; wherein the stimulus frequency signal is based on the error in such a manner that when the subject ambulates in a manner that matches a frequency of the stimulus frequency signal, the subject&#39;s actual position controllably tracks the desired position. 
     Another aspect of the invention provides a system for automatically controlling a locomotion intensity of a human or other animal subject. The system comprises: one or more sensors for sensing one or more corresponding parameters of the locomotion movement of the subject and for generating therefrom a measured speed which represents an estimate of the subject&#39;s actual locomotion speed; one or more sensors for sensing one or more corresponding parameters correlated with an intensity indicator of the subject and for generating therefrom a measured intensity which represents an estimate of the subject&#39;s actual locomotion intensity; and a controller configured to: determine an intensity error comprising a difference between a desired intensity and the measured intensity; and if an absolute value of the intensity error is outside of a threshold region around the desired intensity: convert the desired intensity to a desired speed; determine a speed error comprising a difference between the desired speed and the measured speed; and output, to the subject, a speed-based stimulus frequency signal wherein the speed-based stimulus frequency signal is based on the speed error in such a manner that when the subject ambulates in a manner that matches a frequency of the speed-based stimulus frequency signal, the subject&#39;s actual intensity controllably tracks the desired intensity; and if the absolute value of the intensity error is within the threshold region around the desired intensity: output, to the subject, an intensity-based stimulus frequency signal wherein the intensity-based stimulus frequency signal is based on the intensity error in such a manner that when the subject ambulates in a manner that matches a frequency of the intensity-based stimulus frequency signal, the subject&#39;s actual intensity controllably tracks the desired intensity. 
     In addition to the exemplary aspects and embodiments described above, further aspects and embodiments will become apparent by reference to the drawings and by study of the following detailed descriptions. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In drawings, which illustrate non-limiting embodiments of the invention: 
         FIG. 1A  is a graphical depiction of plots which show experimentally determined correlation between stimulus frequency (which is output to a subject via auditory tones and which the subject is instructed to match) and estimated running speed; 
         FIG. 1B  is a schematic block diagram depiction of the experimental setup used to obtain the  FIG. 1A  plots; 
         FIG. 2  is a schematic block diagram depiction of a control system for automatically controlling human/animal running/walking speed according to a particular embodiment of the invention; 
         FIG. 3  is a schematic block diagram depiction of a controller of the  FIG. 2  control system according to a particular embodiment of the invention; 
         FIG. 4  is a schematic block diagram depiction of a control system for automatically controlling human running/walking position according to a particular embodiment of the invention; 
         FIG. 5  is a schematic depiction of a number of reference speed profiles that could be generated by the  FIG. 2  reference speed generator in response to user input; 
         FIG. 6  depicts one particular implementation of the  FIG. 2  control system according to a particular embodiment; 
         FIG. 7  is a graphical depiction of plots which show the operation of the  FIG. 6  implementation; 
         FIG. 8  is a schematic block diagram depiction of a control system for automatically controlling human running/walking intensity according to a particular embodiment of the invention; and 
         FIG. 9  is a graphical depiction of plots which show the operation of the  FIG. 8  system for the control of locomotion intensity. 
     
    
    
     DESCRIPTION 
     Before the embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of the operative components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is understood that the phraseology and terminology used herein are for the purpose of description and should not be regarded as limiting. The use herein of “including” and “comprising”, and variations thereof, is meant to encompass the items listed thereafter and equivalents thereof. Unless otherwise specifically stated, it is to be understood that steps in the methods described herein can be performed in varying sequences. 
     One may define the frequency of locomotion (e.g. running or walking) as the number of steps taken in a unit of time. Locomotion frequency may be measured in units of s −1  or Hz. When a human is running and/or walking, the human exhibits a high degree of correlation (e.g. a one-to-one mapping) between their locomotion frequency and speed—i.e. when instructed or otherwise caused or motivated to run at a particular frequency, humans and other animals automatically adjust their speed accordingly. When instructed or otherwise caused or motivated to run at a higher frequency, humans will tend to run faster. When instructed or otherwise caused or motivated to run at a lower frequency, humans will tend to run slower. 
     Particular embodiments of the invention provide methods and systems for automatic control of the locomotion (e.g. running or walking) speed of a human or other animal subject. The methods and systems involve estimating the subject&#39;s locomotion speed using one or more sensors, determining a difference (referred to as an error) between a desired speed and the estimated speed, and outputting (to the subject) a stimulus frequency wherein the output stimulus frequency is based on the error in such a manner that when the subject runs in a manner that matches the output stimulus frequency, the subject&#39;s actual speed tracks or matches the desired speed or otherwise tends to minimize the error. Other embodiments provide automatic control of human locomotion position (rather than speed). Systems and methods of particular embodiments, help the subject&#39;s locomotion speed and/or position automatically converge to, and stay at, desired speed and position parameters (e.g. speed and/or positions profiles). 
     Other aspects of the invention make use of the aforementioned methods and systems for automatic locomotive speed control to assist with automatic control of the intensity of locomotion (e.g. running or walking) of a human or other animal subject. In particular embodiments, speed control is used to control the subject&#39;s locomotion speed to cause the subject&#39;s locomotion intensity to move toward a desired intensity until the subject&#39;s locomotive intensity is within a threshold range around the desired intensity. Once the subject&#39;s locomotive intensity is within the threshold range around the desired intensity, the methods and systems switch to direct automatic intensity control. The subject&#39;s locomotive intensity is estimated using one or more intensity indicators, which may be measured or otherwise determined using one or more corresponding sensors. Within the threshold range around the desired intensity, direct automatic intensity control may be effected by: determining a difference (referred to as an intensity error) between the desired intensity and the estimated intensity, and outputting (to the subject) a stimulus frequency wherein the output stimulus frequency is based on the intensity error in such a manner that when the subject runs in a manner that matches the output stimulus frequency, the subject&#39;s actual intensity tracks or matches the desired intensity or otherwise tends to minimize the intensity error. Systems and methods of particular embodiments, help the subject&#39;s locomotion intensity automatically converge to, and stay at, desired intensity parameters (e.g. intensity profiles). 
     A basic and well understood principle that underlies our scientific understanding of neural control of human locomotion (e.g. running and walking) is that humans use a distinct step frequency for each speed. This relationship can also be inverted—i.e. when a human is instructed or otherwise caused or motivated to match locomotion frequency to a reference frequency, a distinct speed is selected, resulting in a high degree of correlation (e.g. a one-to-one relationship) between step frequency and locomotion speed. 
       FIG. 1A  shows a pair of plots taken in a laboratory experiment which demonstrate the high degree of correlation in the relationship between the frequency at which a human is instructed to run (plot  10 ) and their resultant speed (plot  12 ).  FIG. 1B  is a schematic block diagram showing the experimental apparatus  20  giving rise to the  FIG. 1A  plots. As shown in  FIG. 1B , a human subject  26  was instructed (instructions  24 ) to run in a manner which matched their step frequency to an auditory frequency stimulus  30  output (by a frequency generator  22 , to subject  26 ) via a pair of headphones (not explicitly shown). Subject  26  ran on a 400 meter outdoor track and was free to choose their running speed (actual running speed  32 ). The actual running speed  32  of subject  26  was measured by a speed measurement device  28  to obtain estimated running speed  34 . Estimated speed  34  sensed or otherwise detected by speed measurement device  28  may also be referred to herein as measured speed  34 . In the particular case of the experiment giving rise to the plots of  FIG. 1A , speed measurement device  28  involved using gyroscopic sensors  28 A,  28 B coupled to the subject&#39;s feet, as discussed in more detail below (see  FIG. 6 ). 
     For the exemplary plots of  FIG. 1A , frequency generator  22  was programmed to output a frequency stimulus signal  30  which included a series of n=4 constant reference frequencies for t=2 minute each. The frequency output stimulus  30  of frequency generator  22  is shown in  FIG. 1A  as frequency plot  10  and the estimated speed  34  of subject  26  is shown in  FIG. 1A  as speed plot  12 . It can be seen from the  FIG. 1A  plots, that whenever a change in frequency  10  occurred, the runner automatically adjusted their speed  12 , even though they were only instructed to match the frequency and not specifically instructed to adjust their speed. In addition, the adjustments to the speed  12  occurred within a few seconds after each corresponding change in frequency  10 . 
       FIG. 2  is a schematic block diagram of a human running/walking speed control system  50  according to a particular embodiment. Like experimental system  20  of  FIG. 1B , control system  50  comprises a frequency generator  22  for outputting a stimulus frequency  30  and a speed measurement device  28  for measuring the actual running/walking speed  32  of subject  26  and outputting a measured/estimated speed  34 . In particular embodiments, frequency generator  22  outputs an auditory frequency stimulus signal  30  which may be provided to subject  26  via a pair of headphones/ear buds or the like. It is envisaged, however, that in other embodiments, frequency generator  22  may provide the subject with additional or alternative forms of frequency stimulus  30  (e.g. optical and/or tactile frequency stimulus). In one currently implemented embodiment, speed measurement device  28  comprises gyroscopic sensors  28 A,  28 B coupled to the subject&#39;s feet, as discussed in more detail below (see  FIG. 6 ), but it is envisaged that system  50  could make use of any suitable speed measurement device, such as any of those described herein. 
     Control system  50  incorporates a controller  52  which may be used to control measured speed  34  to track a desired speed (also referred to as a reference speed)  62 . Controller  52  may be implemented on or by one or more suitably configured data processors, personal computers, programmable logic devices and/or the like. Controller  52  may be implemented via one or more embedded data processors or micro-electronic devices to permit system  50  to be carried with subject  26  when they are running or walking. In the illustrated embodiment, reference speed  62  is generated by a reference speed generator  54  in response to user input  56 . Reference speed generator  54  may also be implemented on or by one or more suitably configured data processors, personal computers, programmable logic devices and/or the like which may be programmed with suitable user interface and speed generator software. 
     In the illustrated embodiment, reference speed generator  54  and controller  52  are implemented by the same hardware (e.g. one or more suitably programmed data processors) which is shown in dashed lines as control hardware  58 . Control hardware  58  may perform instructions in the form of suitably programmed software. In some embodiments, control hardware  58  may be implemented in the form of one or more embedded processors that can perform substantially all of the functionality of controller  52  and reference speed generator  54 . In some embodiments, control hardware  58  may interface with (e.g. plug into or wirelessly interface with) a suitably programmed computer to accept user input  56  and then the remaining functions of controller  50  and/or reference speed generator  54 ) may be implemented by a suitably programmed embedded processor. In still other embodiments, controller  52  and reference speed generator  54  can be implemented using separate hardware. 
     In some embodiments (although not specifically shown in  FIG. 2 ), some of the functionality of speed measurement device  28  may also be implemented by control hardware  58 . For example, control hardware  58  may be configured to receive information from one or more sensors (e.g. gyroscopes, GPS sensors or the like) and may process or otherwise interpret this information to determine an estimated speed  34 . By way of a specific example, control hardware  58  may determine measured speed  34  by receiving two different position measurements from a position sensor (e.g. a GPS sensor) and dividing the two position measurements by an intervening time to obtain measured speed  34 . In some embodiments (although not specifically shown in  FIG. 2 ), control hardware  58  may perform some (or even all) of the functionality of frequency generator  22 . For example, control hardware  58  could implement a portion of frequency generator  22  in the form a “count-down register” which outputs a pulse when it counts down from a specified period. This pulse could then be amplified and output to subject  26  via a pair of headphones or some other output device. 
     The operation of system  50  may be controlled by control hardware  58 . Referring to  FIG. 2 , system  50  compares measured locomotion speed  34  with user-defined reference speed  62 . System  50  generates an error signal  64  which comprises a difference between reference speed  62  and measured speed  34 . Based on error signal  64 , controller  52  outputs a control signal  60  which causes frequency generator  22  to change stimulus frequency  30  to minimize the speed error (i.e. error signal  64 ). When measured speed  34  is below reference speed  62  (i.e. error signal  64  is positive), controller  52  will output a control signal  60  which causes frequency generator  22  to increase stimulus frequency  30 . Conversely, when measured speed  34  is above reference speed  62 , controller  52  will output a control signal  60  which causes frequency generator  22  to decrease stimulus frequency  30 . Subject  26  tends to synchronize, or can be instructed to synchronize, their movements to match stimulus frequency  30 . The change in stimulus frequency  30  will lead to a corresponding change in actual locomotion speed  32  because, as discussed above, humans and other animals prefer to use a particular running/walking speed for each specified frequency. The new actual speed  32  is detected by speed measurement device  28  which outputs a new measured speed  34  which is again compared to reference speed  62  to adjust stimulus frequency  30  if desired. Stimulus frequency  30  is continually or periodically changed until measured locomotion speed  34  equals reference speed  62 . System  50  thereby provides a feedback-based control system that controls actual running/walking speed  32  using a speed dependent stimulus frequency  30 . 
       FIG. 3  is a schematic block diagram depiction of controller  52  of the  FIG. 2  control system  50  according to a particular embodiment of the invention. Controller  52  of the illustrated embodiment comprises a proportional-integral-derivative (PID) controller which receives error signal  64  and outputs a control signal  60  according to: 
     
       
         
           
             
               
                 
                   
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     where y(t) represents the control signal  60 , e(t) represents the error signal  64  and k p , k i , k d  respectively represent proportional gain 66, integral gain 68 and derivative gain 70. The integration and differentiation operators of equation (1) are respectively depicted as blocks  72 ,  74  of the  FIG. 3  schematic depiction. Not specifically shown in the  FIG. 3  depiction is a mapping between the output of summing junction  76  and a control signal  60  that is suitable for input to frequency generator  22  (see  FIG. 2 ). In one particular implementation, frequency generator  22  outputs a stimulus frequency that matches the stimulus frequency of control signal  60 . In such embodiments, a mapping may not be required between the output of summing junction  76  and control signal  60 . It will be appreciated that such a mapping will depend on the particular frequency generator  22  used for any given application. 
     The gain parameters k p , k i , k d  (blocks  66 ,  68 ,  70 ) specify the relative contribution of the proportional, integral and derivative controller parts to control signal  60 . These gain parameters k p , k i , k d  (blocks  66 ,  68 ,  70 ) can be adjusted (e.g. calibrated and/or experimentally determined) to optimize the controlled behavior of subject  26 . The gain parameters k p , k i , k d  (blocks  66 ,  68 ,  70 ) may be user-configurable constants or may be functions of other parameters (e.g. time and/or speed). In some embodiments, one or more of the gain parameters k p , k i , k d  (blocks  66 ,  68 ,  70 ) may be set to zero. In some embodiments, gain parameters k p , k i , k d  (blocks  66 ,  68 ,  70 ) can be configured so that the changes in stimulus frequency  30  are not overly noisy or do not exhibit overly large jumps. In other embodiments, other control techniques may be used to obtain similar results. By way of non-limiting example, in addition to or in the alternative to using the first derivative (single differentiator  74 ) and first integral (single integrator  72 ) of error signal  64  as shown in  FIG. 3 , some embodiments may involve higher order derivatives and/or integrators of error signal  64  to determine control signal  60 . 
       FIG. 4  is a schematic block diagram of a human running/walking position control system  150  according to another particular embodiment. Position control system  150  is similar in many respects to speed control system  50  of  FIG. 2 , except that position control system  150  uses position (instead of speed) as the control variable. Control system  150  comprises a frequency generator  122  which outputs a stimulus frequency  130  in response to control signal  160 . Frequency generator  122  may be substantially similar to frequency generator  22  of system  50 . Instead of a speed measurement device, position control system  150  comprises a position measurement device  128  which outputs a measured position  134  (also referred to as an estimated position  134 ) of subject  126 . It will be appreciated that in some embodiments, position measurement device  128  of position control system  150  may be implemented by integrating the measured speed output of a speed measurement device (e.g. measured speed output  34  of speed measurement device  28  of speed control system  50 ). Similarly, speed measurement device  28  of speed control system  50  could be implemented by differentiating the measured position output of a position measurement device (e.g. measured position output  134  of position measurement device  150  of position control system  150 ). 
     Position control system  150  comprises controller  152  and reference position generator  154  which may be similar to controller  52  and reference speed generator  54  of speed control system  50 . In particular, controller  152  and reference position generator  154  may be implemented in any of manners discussed above for controller  52  and reference speed generator  54 . In the illustrated embodiment, controller  152  and reference position generator  154  are implemented by control hardware  158 . 
     The operation of system  150  may be controlled by control hardware  158 . Referring to  FIG. 4 , system  150  compares measured locomotion position  134  with user-defined reference position  162 . Reference position  162  may comprise a reference trajectory and/or a desired position  162  for any given time or any other suitable position information. System  150  generates an error signal  164  which comprises a difference between reference position  162  and measured position  134 . Based on error signal  164 , controller  152  outputs a control signal  160  which causes frequency generator  122  to change stimulus frequency  130  to attempt to minimize the position error (i.e. error signal  164 ). When measured position  134  is behind a desired reference position  162  (i.e. error signal  164  is positive), controller  152  will output a control signal  160  which causes frequency generator  122  to increase stimulus frequency  130  with the objective of reducing position error  164  over time. Conversely, when measured position  134  has advanced beyond a desired reference position  162 , controller  152  will output a control signal  160  which causes frequency generator  122  to decrease stimulus frequency  130  with the objective of reducing position error  164  over time. Subject  26  tends to synchronize, or can be instructed to synchronize, their movements to match stimulus frequency  130 . The change in stimulus frequency  130  will lead to a corresponding change in actual locomotion speed (not shown in  FIG. 4 ) because, as discussed above, humans and other animals prefer to use a particular running/walking speed for each specified frequency. After this speed adjustment, a resultant position  132  is detected by position measurement device  128  which outputs a new measured position  134  which is again compared to reference position  162  to adjust stimulus frequency  130  if desired. Stimulus frequency  130  is continually changed until measured locomotion position  134  equals reference position  162 . System  150  thereby provides a feedback system that controls actual running/walking position  132  using a position dependent stimulus frequency  130 . 
     Controller  152  of system  150  may also be implemented by a PID control scheme similar to that shown schematically in  FIG. 3 , except that error signal  164  represents a position error in the case of controller  152  (rather than a speed error, as is the case in controller  52  of  FIGS. 2 and 3 ). 
       FIG. 8  is a schematic block diagram of a human running/walking intensity control system  250  according to another particular embodiment. As mentioned above, locomotive intensity is typically estimated using one or more estimatable or measurable intensity indicators which may include, by way of non-limiting example, heart rate, metabolic rate, oxygen consumption, perceived exertion, mechanical power and/or the like. In the illustrated embodiment, control system  250  uses the heart rate of subject  226  as an intensity indicator, but this is not necessary. In other embodiments, other additional or alternative intensity indicators could be used. Intensity control system  250  is similar in some respects to speed control system  50  of  FIG. 2 , except that intensity control system  250  uses both speed and intensity (as reflected in the heart rate of subject  226  which is used as an intensity indicator) as control variables. As described in more detail below, intensity control system  250  uses speed control to achieve a number of advantages over intensity control alone. 
     Control system  250  comprises a frequency generator  222  which outputs a stimulus frequency  230  in response to control signal  260 . Frequency generator  222  may be substantially similar to frequency generator  22  of system  50 . Control system comprises a speed measurement device  228  which may be substantially similar to speed measurement device  28  of system  50  and which senses actual speed  232  of subject  226  and outputs a measured speed  234  (also referred to as an estimated speed  234 ) of subject  226 . In addition to speed measurement device, system  250  comprises a heart rate measurement device  288  which senses actual heart rate  290  of subject  226  and outputs a measured heart rate  284  (also referred to as an estimated heart rate) of subject  226 . 
     Intensity control system  250  also comprises a reference heart rate generator  254  which may be similar to reference speed generator  54  of speed control system  50 . In particular, reference heart rate generator  254  may be implemented in any of manners discussed above for reference speed generator  54 . In the illustrated embodiment, reference heart rate generator  254  is implemented by control hardware  258 . Reference heart rate generator  254  outputs a reference heart rate  262  and intensity control system  250  attempts to cause the actual heart rate  290  of subject  226  to track the reference heart rate  262 . Reference heart rate generator  254  may output reference heart rate  262  in response to user input  256 . 
     Intensity control system  250  comprises a controller  252  which may be similar to controller  52  of speed control system  50 . In the illustrated embodiment, controller  252  is implemented by the same control hardware  258  as reference heart rate generator  254 . For the purposes of the schematic illustration of  FIG. 8 , controller  252  is shown to comprise a speed controller  252 A, a heart rate controller  252 B and a control region switch  286 . As will be discussed in more detail below, speed controller  252 A effects speed control in a manner similar to that discussed above for speed control system  50 , heart rate controller  252 B effects heart rate control and control region switch  286  switches system  250  between heart rate control and speed control. It will be appreciated, especially in view of the description to follow, that in practice, speed controller  252 A, heart rate controller  252 B and control region switch  286  may be implemented by the same logic (e.g. a suitably programmed processor or the like). 
     Intensity control system  250  also comprises a reference speed predictor  280  which receives, as input, reference heart rate signal  262  and outputs a corresponding reference speed  281 . Reference speed predictor  280  may be implemented on or by one or more suitably configured data processors, personal computers, programmable logic devices and/or the like which may be programmed with suitable user interface and speed generator software. In the illustrated embodiment, reference speed predictor  280  is implemented by the same control hardware  258  as reference heart rate generator  254  and controller  252 . 
     In converting an input reference heart rate signal  262  into an output reference speed signal  281 , reference speed predictor  280  may be configured to implement a model which maps human (or animal) heart rate to locomotive speed. Such models are well known in the art and include, by way of non-limiting example, the model proposed by Hermansen L &amp; Saltin B (1969). Oxygen uptake during maximal treadmill and bicycle exercise. Journal of Applied Physiology, 26: 31-37 which is hereby incorporated herein by reference. Reference speed predictor  280  may incorporate or consider subject specific data (e.g. calibration data). Such subject specific data may be incorporated into the heart rate to locomotive speed mapping model implemented by reference speed predictor  280  or may otherwise be incorporated into the heart rate to locomotive speed conversion algorithms of reference speed generator  280 . Such subject specific calibration data may comprise one or more simultaneous measurements of heart rate and locomotive speed for subject  226 —for example, subject  226  may run on a track and their locomotive speed and heart rate may be simultaneously measured at one or more times. 
     In one particular embodiment, subject specific calibration data may be used in the following manner. Once one or more simultaneous measurements of heart rate and locomotive speed are obtained for subject  226 , as described above, the heart rate to locomotive speed mapping model is used to calculate a model-predicted locomotive speed at the heart rates measured during calibration. These model-predicted speeds may be compared to the measured speeds to generate corresponding model errors. Some sort of average may be taken of these model errors and this average model error may be used by reference speed generator  280  to predict an output reference speed signal  281  from reference heart rate signal  262 . More particularly, the result of the heart rate to locomotive speed mapping model may be offset by the average model error to obtain output reference speed  281 . 
     In another particular embodiment, the heart rate to locomotive speed mapping model may itself be calibrated with subject specific calibration data. For example, subject  226  may go on a specific calibration run, which may guide subject  226  through a series of speeds while measuring the corresponding heart rate at each speed. Still another alternative involves using historical data from previous work-outs (e.g. from previous uses of system  250 ) to find instances when the heart rate of subject  226  is in a steady state and to record the corresponding locomotive speeds. Such use of historical data may be able to work without pre-calibration and may be constantly updated based on the present fitness status of subject  226 . If enough user specific calibration data is collected, then reference speed generator  280  may use this user specific calibration data without having to rely on a heart rate to locomotive speed mapping model. 
     In practice, either or both of the heart rate to locomotive speed mapping model and the user specific calibration data used by reference speed generator  280  may be stored in a look up table or the like in accessible memory (not shown) which may be part of control hardware  258 . 
     In operation, intensity control system  250  controls the locomotive intensity of subject  226  (as indicated, in the illustrated embodiment, by the heart rate of subject  226  which represents one or many possible intensity indicators which could be used by system  250 ). Although locomotion speed and intensity are highly correlated, external disturbances like wind and/or terrain changes, and internal disturbances such as fatigue, influence the relationship between locomotion speed and intensity. Locomotion intensity control system  250  leverage speed control (as implemented by speed control portion  250 A) to assist heart rate control portion  250 B to accurately control locomotive intensity (heart rate). 
     In theory, heart rate control portion  250 B could be implemented without the use of additional speed control portion  250 A to effect heart rate control—e.g. heart rate controller  252 B could output a heart rate control signal  285  which would become an input signal  260  to frequency generator  222  and which would cause frequency generator  222  to output a stimulus frequency  230  which, when followed by subject  226 , minimizes the heart rate error  282  between reference heart rate  262  and the measured heart rate  284  of subject  226 . If, for example, measured heart rate  284  is below reference heart rate  262 , heart rate controller  252 B would output a heart rate control signal  285  which would cause frequency generator  222  to increase stimulus frequency  230  to cause a corresponding increase in the speed of subject  226  which in turn would increase the actual and measured heart rate  290 ,  284  of subject  226 . 
     However, heart rate dynamics are slow. Physiological research has determined that after a change in locomotion speed, it may take several minutes for the heart rate to reach a steady state corresponding to the new locomotive speed. As a result of these slow heart rate dynamics, controlling heart rate based purely on the difference between a reference heart rate (e.g. reference heart rate  262 ) and a measured heart rate (e.g. measured heart rate  284 ) can be problematic. For example, if a user&#39;s measured heart rate is below the reference heart rate, the controller will increase the stimulus frequency to minimize the heart rate error. In response to this increased stimulus frequency, the user will increase his or her locomotive speed. However, because it takes time for the user&#39;s heart rate to reach a steady state value corresponding to this new speed, the controller will continue to increase the stimulus frequency. Typically, this will result in overshoot and/or oscillation of the reference heart rate (and corresponding overshoot and/or oscillation of speed) because the user&#39;s speed is increased beyond the speed that would result in the reference heart rate. These issues are the most apparent when there is a large initial error between the reference and measured heart rates. 
     These issues may be overcome to some degree by suitable selection of control parameters, but the resulting control is undesirably slow. These issues may also be overcome to some degree by controlling heart rate relatively loosely—e.g by accepting actual heart rates that are within a large margin of error with respect to the reference heart rate. These potential solutions do not allow for accurate and rapid control of the heart rate. 
     Intensity control system  250  of the illustrated embodiment overcomes this issue by leveraging speed control (implemented by speed control portion  250 A) to bring measured heart rate  284  close to reference heart rate  262  (e.g. within a threshold region around reference heart rate  262 ) and limiting the use of heart rate control (implemented by heart rate control portion  250 B) to provide fine adjustment once measured heart rate  284  of subject  226  is close to reference heart rate  262  (e.g. within the threshold region around reference heart rate  262 ). The threshold region around reference heart rate  262  may be a user-configurable parameter of system  250  or may be a predefined parameter of system  250 . The threshold region around reference heart rate  262  may be defined in a number of different ways. By way of non-limiting example, the threshold region may be specified to be the reference heart rate ±x beats per minute or the reference heart rate ±x % of the reference heart rate, where x may be a user-configurable threshold region parameter. 
     If measured heart rate  284  is outside of the threshold region around reference heart rate  262 , then control system  250  will use speed control portion  250 A which may be considered (in the schematic depiction of  FIG. 8 ) to mean that control region switch  286  is configured to connect speed control signal  287  from speed controller  252 A to input  260  of frequency generator  222 . It will be appreciated by those skilled in the art, that control region switch  286  may not be physically present as a switch and may be implemented (e.g. in software) by controller  252 . Speed control portion  250 A of system  250  attempts to output a stimulus frequency  230  which will cause subject  226  to increase or decrease their locomotive speed so as to move their actual and measured heart rates  290 ,  284  toward reference heart rate  262 . Speed control portion  250 A uses reference speed predictor  280  discussed above to convert reference heart rate  262  into a reference speed  281 . Once this reference speed  281  is obtained, the operation of speed control portion  250 A of system  250  is substantially similar to the operation of speed control system  50  described above, while measured heart rate  284  is outside the threshold region around reference heart rate  262 . 
     Controller  252  may monitor the heart rate error signal  282  (which reflects the difference between measured heart rate  284  and reference heart rate  262 ). Once heart rate error signal  282  is sufficiently small (i.e. measured heart rate  284  is within the threshold region around reference heart rate  262 ), system  250  switches to heart rate control. This may be considered (in the schematic depiction of  FIG. 8 ) to mean that control region switch  286  is switches to connect heart rate control signal  285  from heart rate controller  252 B to input  260  of frequency generator  222 . Thereafter, intensity control system effects control of heart rate. In some embodiments, if the heart rate error  282  goes outside of the threshold region around reference heart rate  262 , then control system  250  may switch back to speed control, but this is not necessary. In some embodiments, control system  250  may also switch from speed control to heart rate control in other circumstances. By way of non-limiting example, control system  250  may switch from speed control to heart rate control if speed control does not bring measured heart rate  284  to within the threshold region around reference heart rate  262  within a threshold period of time. Such a threshold period of time may be a user-configurable parameter. 
     The operation of control system  250  in heart rate control mode (e.g. the operation of heart rate control portion  250 B) may be similar to the various control systems described above. Referring to  FIG. 8 , heart rate control portion  250 B compares measured heart rate  284  with user-defined reference heart rate  262 . Reference heart rate  262  may comprise a reference trajectory and/or a desired heart rate  262  for any given time or any other suitable heart rate information. Heart rate control portion  250 B generates a heart rate error signal  282  which comprises a difference between reference heart rate  262  and measured heart rate  284 . Based on heart rate error signal  282 , heart rate controller  252 B outputs a heart rate control signal  285  which is received by frequency generator  222  as input signal  260  and which causes frequency generator  222  to change stimulus frequency  230  with the objective of minimizing heart rate error  282 . When measured heart rate  284  is below a desired reference heart rate  262  (i.e. heart rate error signal  282  is positive), heart rate controller  252 B will output a heart rate control signal  285  which causes frequency generator  222  to increase stimulus frequency  230  with the objective of reducing heart rate error  282  over time. Conversely, when measured heart rate  284  is greater than a desired reference heart rate  262 , heart rate controller  252 B will output a heart rate control signal  285  which causes frequency generator  222  to decrease stimulus frequency  230  with the objective of reducing heart rate error  282  over time. Subject  226  tends to synchronize, or can be instructed to synchronize, their movements to match stimulus frequency  230 . The change in stimulus frequency  230  will lead to a corresponding change in actual locomotion speed  232  because, as discussed above, humans and other animals prefer to use a particular running/walking speed for each specified frequency. After this speed adjustment, a resultant heart rate is detected by heart rate measurement device  288  which outputs a new measured heart rate  284  which is again compared to reference heart rate  262  to adjust stimulus frequency  230  if desired. Stimulus frequency  230  is continually changed until measured heart rate  284  equals reference heart rate  262 . System  250  thereby provides a feedback system that controls actual heart rate  290  using a heart rate dependent stimulus frequency  230 . 
     The profile of a reference speed  62  (and the corresponding user input  56  to reference speed generator  154 ), the profile of a reference position  162  (and the corresponding user input  156  to reference position generator  154 ) and/or the profile of a reference heart rate  262  (and the corresponding user input  256  to reference heart rate generator  254 ) may take a variety of forms. By way of non-limiting example, in the case of speed control, a user may specify:
         the total time to cover a certain distance (e.g. 50 min for a 10 km race). The user may also specify that the distance is to be run at a constant speed or that the speed should have some profile (e.g. starting a relatively high speed, stepping down slightly to a middle speed and then increasing for a “kick” at the end of the race).   an interval training regime, which will guide the subject through a series of predetermined or user-configurable speeds (e.g. 5 min at 3 m/s, 2 min at 3.5 m/s, 1 min at 4 m/s etc. or 2 km at 3 m/s, 1 km at 3.5 m/s, 1 km at 4 m/s, etc.).   a training or race profile that increases speed when only a certain amount of time or distance remains.   a completely user-configurable profile for training or racing purpose; and/or   the like.       

     In addition to or in the alternative to a user inputting a training or race profile, such a profile could be input by a real or virtual trainer. The training or race profile can also be changed on the fly by the user or trainer changing reference speed  62  or position  162  or heart rate  262 . It is also possible for a user to download data (e.g. another person&#39;s speed profile data from the other person&#39;s workout at a distant place and/or time). A training or race profile based on this data can then be input so that the user can virtually train with, or race against, this other person. 
       FIG. 5  schematically depicts a number of exemplary and non-limiting speed profiles (i.e. profiles for desired/reference speed  162 ) including constant speed profile  200 , interval speed profile  202  and ramping speed profile  204 . It will be appreciated that position and/or heart rate profiles similar to any of the above-discussed speed profiles could be generated by reference position generator  154  in response to user input  156  and/or by heart rate generator  254  in response to user input  256 . 
     Speed measurement device  28  can be implemented using a variety of different techniques and speed measurement apparatus. A number of technologies capable of measuring running/walking speed are discussed above. Various different sensors may be used, individually or combined with other sensors, to implement such speed measurement apparatus. By way of non-limiting example, signals from accelerometers, GPS, gyroscopes, optical and electromagnetic sensors can be processed to provide locomotion speed and information. Various processing techniques may be used to extract speed and/or position information from such sensors. The particular nature of the processing depends on the type of sensors used. Signals from such sensors may be combined with one another in an attempt to improve the accuracy of estimated speed  34 . Such sensor combination can involve state estimation techniques such as Kalman-filtering, for example. Similarly, position measurement device  128  can be implemented using a variety of different techniques and position measurement apparatus. For some speed or position measurement devices  28 ,  128 , a calibration procedure might be desirable, whereas other speed or position measurement devices  28 ,  128  could provide accurate speed or position estimates  34 ,  134  without user calibration. Heart rate measurement device  288  can similarly be implemented using a variety of techniques known in the art, such as strapped and/or strapless heart rate measurement systems. 
     Stimulus frequency  30 ,  130 ,  230  can be output to subject  26 ,  126 ,  226  in a variety of ways and may target different sensory systems of subject  26 ,  126 ,  226 . One particular embodiment, makes use of an auditory metronome which outputs an auditory frequency stimulus signal  30 ,  130 ,  230  to subject  26 ,  126 ,  226 . Another implementation using auditory signals involves the use of music as frequency stimulus  30 ,  130 ,  230 . For example, the frequency (tempo) of music could be controlled so that either songs with the right frequency are selected, or the frequency of a song is adjusted to better match the intended locomotion frequency. Frequency stimulus  30 ,  130 ,  230  could also be implemented as a tactile stimulus, either by mechanical or electrical stimulation to different body parts (heel, back, arm, wrist etc.). Also, frequency stimulus  30 ,  130 ,  230  could be provided visually, for example by projecting it on the inside of a pair of glasses or in some other location visible to subject  26 ,  126 ,  226 . 
     Control signals  60 ,  160 ,  285 ,  287  (and corresponding stimulus frequency  30 ,  130 ,  230 ) can be updated whenever estimated speed/position/heart rate  34 ,  134 ,  234 ,  284  is updated and may be accomplished, in one particular example, by continually changing the frequency of a metronome or the tempo of a song. Such relatively short control periods may occur, for example, in time periods on the order of tens of milliseconds. In some situations, it might be more comfortable for the subject if control signal  60 ,  160 ,  285 ,  287  (and corresponding stimulus frequency  30 ,  130 ,  230 ) were only updated at longer control intervals. Such longer control periods may be on the order seconds, tens of seconds or even minutes. Such control periods may not be temporally constant—for example when music is used as stimulus frequency  30 ,  130 ,  230  a control period may correspond to the length of a particular song and an update to control signal  60  (and stimulus frequency  30 ) can be provided each time that a new song is selected. 
     In such embodiments, controller  52 ,  152 ,  252  may establish a relationship between stimulation frequency  30 ,  130 ,  230  and subject-specific locomotion speed and/or heart rate. Such a relationship may be used to predict the locomotion speed or heart rate that subject  26 ,  126 ,  226  is likely to adopt when a certain song is played. This relationship between stimulation frequency and locomotion speed or heart rate can be calibrated on a subject specific basis. For example, the relationship between stimulation frequency and locomotion speed or heart rate may be calibrated using a speed interval regime, where subject  26 ,  126 ,  226  is guided through a number of different speeds. Control signals  60 ,  160 ,  285 ,  287  could also only be played when the measured speed, position or heart rate is outside a threshold range (e.g. a user configurable threshold range), in order to return subject  26 ,  126 ,  226  to the reference speed, position or heart rate. Current estimated step frequency may be used as the initial value for stimulus frequency  30 ,  130 ,  230 . This frequency will then be adjusted by the control system to return subject  26 ,  126 ,  226  to the target speed, position or heart rate. 
       FIG. 6  depicts one particular implementation  300  of a control system  50  according to a particular embodiment. In the  FIG. 6  implementation  300 , a suitably programmed tablet personal computer (not shown), which may be carried by subject  26  in a backpack, is used to implement reference speed generator  54 , a speed detection algorithm (not shown) used by speed measurement device  28  and controller  52 . In the  FIG. 6  implementation  300 , controller  52  also performs the function of frequency generator  22  (see  FIG. 2 ). Speed measurement device  28  comprises a pair of gyroscopes  28 A,  28 B attached to the feet of subject  26 . Frequency stimulus  30  is provided to subject  26  via a pair of headphones for auditory stimulation (e.g. as a metronome). 
     The  FIG. 6  implementation uses foot-mounted gyroscopes  28 A,  28 B to sense the running speed of subject  26 . Gyroscopes  28 A,  28 B generate corresponding gyroscope sensor signals  29 A,  29 B. As is known in the art, gyroscope sensor signals  29 A,  29 B exhibit characteristic events that permit robust detection of foot touchdown and lift-off. By processing gyroscope signals  29 A,  29 B and identifying these events, speed measurement device  28  determines an estimate of the amount of time each foot spends on the ground during each step (contact time). This contact time information, in combination with a predetermined relationship between contact time and running speed, provides estimated speed  34 . In some embodiments, estimated speed  34  may be determined as the moving average of the speed estimates over the previous number (e.g. two) steps. Those skilled in the art will recognize that this implementation of speed measurement device  28  represents one particular embodiment and that there are a variety of additional or alternative techniques for generating estimated running/walking speed  34 . 
     Controller  52  of the  FIG. 6  implementation  300  makes use of a discrete PID control scheme of the type shown schematically in  FIG. 3  to control the running speed of subject  26 . Estimated running speed  34  is compared to reference speed  62  to find error signal  64 . Error signal  64  is sent to the different branches of controller  52  to implement the control scheme of  FIG. 3  and equation (1). In the current embodiment, the gain parameters k p , k i , k d  (blocks  66 ,  68 ,  70 ) are constant. Controller  52  of the  FIG. 6  implementation  300  incorporates a frequency generator. Consequently, controller  52  outputs an updated stimulus frequency  30  in the form an auditory stimulus which is delivered to subject  26  via the illustrated earphones. In the current embodiment, stimulus frequency  30  is updated at each control step. 
       FIG. 7  is a graphical depiction of plots which show the operation of the  FIG. 6  implementation. More particularly,  FIG. 7  includes plot  314  of desired/reference speed  62  output by reference speed generator  54 , plot  310  of auditory stimulus frequency  30  output by controller  52  and plot  312  of the estimated speed  34  of subject  26  as estimated by speed measurement device  28 . The  FIG. 7  data was once again obtained by having subject  26  run on a 400 meter outdoor track. Subject  26  was instructed to try to match their step frequency to the auditory stimulus frequency  30 , but was free to choose their running speed. Reference speed generator  54  was programmed to guide subject through a speed interval regime incorporating a series of n=4 constant reference speeds  62  for t=2 minute each. Plots  312  and  314  show that estimated speed  34  of subject  26  converges rapidly toward each reference speed  62  and, on average, stayed at that reference speed  62  until the reference speed  62  changed again. 
       FIG. 9  is a graphical depiction of plots which show the operation of the  FIG. 8  intensity control system  250 . More particularly,  FIG. 9  includes plots of desired/reference heart rate  262 , a plot of the auditory stimulus frequency  230  and a plot of measured heart rate  284  of subject  226  as given by heart rate measurement device  288 . The  FIG. 9  data was once again obtained by having subject  226  run on a 400 meter outdoor track. Subject  226  was instructed to try to match their step frequency to auditory stimulus frequency  230 , but was free to choose their running speed. Reference heart rate generator  254  was programmed to keep subject  226  at a constant heart rate of 160 beats per minute (bpm).  FIG. 9  shows that measured heart rate  284  converged to reference heart rate  262  and then stayed at reference heart rate  262 . Under speed control (the grey-colored region of  FIG. 9 ), measured heart rate  284  climbs quickly up to a region of reference heart rate  262  without overshoot (although in some instances there may be some overshoot). Once measured heart rate  284  reaches a region close to reference heart rate  262 , the control switches to intensity control and measured heart rate  284  tracks reasonably close to reference heart rate  262 . 
     Variations and modifications of the foregoing are within the scope of the present invention. It is understood that the invention disclosed and defined herein extends to all the alternative combinations of two or more of the individual features mentioned or evident from the text and/or drawings. All of these different combinations constitute various alternative aspects of the present invention. The embodiments described herein explain the best modes known for practicing the invention. Aspects of the invention are to be construed to include alternative embodiments to the extent permitted by the prior art. For example:
         It will be appreciated that the above-described PID control schemes represent one particular control scheme for implementing speed and/or position control of human walking/running according to one particular embodiment. Other embodiments may incorporate other control schemes. Such other control schemes may also be based on the error between desired speed and/or position and estimated speed and/or position. Such other control schemes may also be based on controlling a stimulus frequency output to the subject.   The control systems described above are representative examples only. Control systems in other embodiments could be modified to be more adaptive. For example, control systems could be designed to adaptively and dynamically adjust reference speed  62  (or reference position  162  or reference heart rate  262 ) in response to feedback information. By way of non-limiting example, such feedback information could comprise current and historical values for estimated speed  34 ,  234  and/or estimated position  134  and/or estimated heart rate  284  and/or derivatives, integrals or other functions of these values. In one example, user input  56 ,  156  could specify that subject  26 ,  126  would like to cover 10 km in 50 minutes. A dynamic speed/position controller could then help to guide subject  26 ,  126  toward the appropriate speed/position to establish this objective by updating reference speed/position  62 ,  162  and minimizing error  64 ,  164  to achieve this objective. If, for some reason, subject  26 ,  126  is unable to keep up with to desired speed/position  62 ,  162 , the controller might detect this and decide to slow down desired speed/position  62 ,  162  temporarily. When subject  26 ,  126  is able to keep up again, the controller could decide to increase the desired speed/position  62 ,  162  again, in order to get closer to the original objective. Additionally or alternatively, control systems could adaptively modify gain parameters of controller  52 ,  152 ,  252  (e.g. k p , k i , k d  (blocks  66 ,  68 ,  70 ) to improve performance of the control system, such as, by way of non-limiting example, by adjusting rise times, adjusting settling times and/or overshoot.   As is known in the art, humans have the tendency to synchronize their movements to external stimuli, even when not explicitly instructed to do so. Consequently, it may not be necessary to instruct or train subject  26 ,  126 ,  226  to match external frequency stimulus  30 ,  130 ,  230 —this entrainment may happen naturally.   Applications of this invention are not limited strictly to walking and running. Various embodiments may be directed toward other locomotion activities (e.g. snowshoeing, cross-country skiing, speed skating, inline skating and/or the like) and/or other activities involving cyclic movements (e.g. swimming, cycling, wheel chair racing and/or the like).   The above description relates to human subjects. However, the invention is not limited to application to humans. Particular embodiments of the invention may have application to other animals, including, for example, horses, dogs and/or other animals used for racing.   Control systems of particular embodiments may be used for rehabilitation of patients with various diseases or injuries affecting locomotion ability, including but not limited to stroke patients, Parkinson&#39;s patients, patients having spinal cord injuries, amputees, etc.   In the description above, intensity control system  250  is described in terms of a particular intensity indicator—i.e. heart rate. Heart rate is one of a variety of possible intensity indicators which may be used alone or in combination to effect intensity control in a manner analogous to that of control system  250 . References to heart rate in the description above should be understood to incorporate the possibility of using other intensity indicator(s). Similarly references to components or features of control system that reference heart rate (e.g. heart rate measurement device  288 , measured heart rate signal  284 , heart rate controller  252 B, etc.) should be understood to include the possibility of other intensity indicator(s)