Patent Publication Number: US-11375944-B2

Title: Apparatus for assessing human balance capability

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
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     CROSS REFERENCE TO RELATED APPLICATION 
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     BACKGROUND OF THE INVENTION 
     The present invention relates to medical and therapeutic apparatuses and in particular to an apparatus for assessing an individual&#39;s balancing capabilities. 
     The ability of humans to remain upright while standing and walking requires a coordination of many different muscles, a process collectively termed balance. Loss of balance can disrupt daily life and increase the risk of falls, injury, and death. An ability to accurately assess an individual&#39;s ability to balance could provide insight into other medical conditions and inform intervention to prevent falling. 
     While there are many ways of measuring an individual&#39;s ability to maintain balance, for example, using the Berg Balance Test, many such methods may require a trained individual to conduct a test or can be relatively imprecise revealing changes in balance only after there has been significant loss of balance. The complex interaction of multiple neurological and musculoskeletal systems necessary for balance makes establishing a simple quantitative measurement of balance difficult. 
     SUMMARY OF THE INVENTION 
     The present invention provides an apparatus that can assess an individual&#39;s balance capability through measurements of the individual in a standing posture on a stationary rigid force-sensing platform. The assessment quantifies a relationship between a center-of-pressure and angle of force exerted by the individual over a period of time as measured by the force plate. This measurement may be distilled to a single “intersection point” value that can be compared to the individual&#39;s center-of-mass to assess balance or displayed for the purpose of balance rehabilitation. That intersection point summarizes the final common output of the complex neural-muscular-skeletal system in a manner that is directly relatable to the success or failure of meeting the mechanical demands of the balancing task. 
     Specifically, in one embodiment, the invention provides an instrument for assessing balance in an individual having at least one platform sized to receive an individual&#39;s foot applying a force against the platform with natural ankle freedom. A set of sensors communicates with the platform to provide a set of measurements determining a center-of-pressure of the force on the platform in a measurement plane of the foot and corresponding angle of the force on the platform within the measurement plane of the foot, and an analysis circuit receives input from the sensors to determine a functional relationship between the center-of-pressure and angle of force of the set of measurements. This functional relationship is output to provide an assessment of individual balance based on the functional relationship. 
     It is thus a feature of at least one embodiment of the invention to provide a simple and rapid assessment of individual&#39;s balance capabilities without the need for expert intervention. 
     The functional relationship may be a slope in the change in center-of-pressure versus a change in angle of force association. 
     It is thus a feature of at least one embodiment of the invention to combine force-angle and center-of-pressure, readily obtained with the force plate, to produce a simple value qualifying complex neuro-muscular-skeletal interactions in the individual. 
     The analysis circuit may apply a bandpass filtering to the input from the sensors passing spectral energy from 1-5 Hz. 
     It is thus a feature of at least one embodiment of the invention to isolate narrowband frequency relationships between force-angle and center of contact such as to provide improved insight into the? complex control phenomenon of balance. 
     The output may provide a comparison between the functional relationship and a measurement of the body of the individual. 
     It is thus a feature of at least one embodiment of the invention to develop a series of common reference points (each associated with a specific frequency) applicable longitudinally among individuals (normalized to the body size of the individual) to assist in the development of normal and abnormal balance capability in a population. 
     The measurement of the body of the individual may be an estimate of the height of the center-of-mass of the individual. 
     It is thus a feature of at least one embodiment of the invention to provide a standard against which the balance output can be assessed related to what appears to be the mechanism of generating posture-restoring forces and torques in the relationship between center-of-pressure and force-angle. 
     The output may be a function of an intersection point derived from an intersection of force lines-of-action passing through the center-of-pressure at the force-angle for each of the series of measurements. 
     It is thus a feature of at least one embodiment of the invention to provide a simple representation of balance capability in the location of the intersection point of lines of force in the individual, and particularly with respect to the individual center-of-mass. 
     The intersection point height may be calculated as an average value of IP z  according to the following formula:
 
 IP   z   =CP   x /( F   x   /F   z )
 
where CP x  is a horizontal displacement of the center-of-pressure on the platform for a given measurement, F x  is a horizontal force on the platform for a given measurement and F z  is a normal force on the platform for a given measurement.
 
     It is thus a feature of at least one embodiment of the invention to provide a simple calculation for determining intersection point height from quantities readily determined with the force plate. 
     The instrument may include at least two independent platforms each positioned to receive a different of corresponding left and right foot of the individual and each providing an independent set of measures of a center-of-pressure of the force on the platform in the measurement plane of the foot and corresponding angle of the force on the platform within the measurement plane of the foot for each foot and providing separate outputs for each foot. 
     It is thus a feature of at least one embodiment of the invention to permit separate assessments of the left and right leg balance mechanisms, for example, useful for assessing neurological-deficit-induced balance problems. 
     The output may also indicate weight of individual. 
     It is thus a feature of at least one embodiment of the invention to provide a multipurpose instrument that can, for example, be used for routine assessment of balance in a doctor&#39;s office or the like as well as providing weight measures. 
     These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of the present invention incorporated into a floor scale form factor having a remote readout to be visible to an individual standing on a scale platform; 
         FIG. 2  is an exploded diagram of a force plate forming half of the platform of  FIG. 1  such as may communicate with an electronic computer for making the balance measurements of the present invention; 
         FIG. 3  is a data flow diagram showing the collection of a series of center-of-pressure and force-angle measurements, filtration, and analysis of the relationship between center-of-pressure and force-angle measurements underlying the series; 
         FIG. 4  is a simplified side elevational view of an individual bisected by a sagittal plane showing the analysis of center-of-pressure and force-angle in a series of measurements to define an intersection point above or below an individual&#39;s center-of-mass; 
         FIG. 5  is a flowchart depicting the processing of  FIGS. 3 and 4  by an electronic computer of  FIG. 2 ; and 
         FIGS. 6 a  and 6 b    are figures similar to  FIG. 4  showing an analysis of torque produced about an individual center-of-mass by the individual&#39;s balance response when the intersection point of that balance response is above or below the center-of-mass respectively. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 1 , in one embodiment, a balance assessment apparatus  10  may provide for a floor unit  12  having an upper exposed surface providing a first and second horizontally extending force plate  14   a  and  14   b  sized to receive a left and right foot of a standing individual. The floor unit  12  may be conveniently supported on the floor with the force plates  14   a  and  14   b  elevated slightly so that a user may step up onto the floor unit  12  without inconvenience. As so positioned on the floor unit  12 , the individual&#39;s mid-sagittal plane  16  separates the force plates  14   a  and  14   b  extending along an anterior-posterior axis  18 . 
     The floor unit  12  may be associated with an electronic display  20 , for example, that may be mounted on a stand or wall to be viewed by the individual when the individual is standing on the floor unit  12 . The electronic display  20  may provide for alphanumeric or graphic display as will be discussed below. 
     Generally, each of the force plates  14   a  and  14   b  are instrumented to independently measure a center-of-pressure exerted by the feet of the individual on the force plates  14   a  and  14   b  and the angle of force applied against those plates by the user&#39;s feet. In this regard, and referring to  FIG. 2 , each force plate  14  may be rectangular and sized to receive within those rectangular perimeters soles of the user&#39;s feet. The force plates  14  are ideally a light-weight but stiff material such as an aluminum plate. 
     The corners of the force plate are supported from beneath by upwardly oriented force-sensors  22   a - d  each measuring downward force exerted by the force plate  14  transmitted through the force-sensors  22  to lower support surface  24  within the floor unit  12  generally contacting in parallel to the floor. Each of the force-sensors  22  may be, for example, a load cell measuring force along a vertical load cell axis  26  and may be separated from the support surface  24  by a ball bearing and race assembly  28  providing free translative movement (horizontal) in response to any horizontal component of the force exerted on force-sensor  22  thereby ensuring that only a vertical component of any force is measured by the force-sensors  22 . Force-sensors  22  suitable for use with the present invention are described generally, for example, in US patent application 2014/0013862 published Jan. 16, 2014, and hereby incorporated by reference in its entirety. 
     Force plate  14  may be constrained to move only along anterior-posterior axis  18  with respect to the support surface  24  by means of linear bearings  30  on the support surface  24  engaging with downwardly extending guide pins  34  from the bottom of the force plate  14 . Ideally this constraint is close to frictionless. 
     An additional force-sensor  22   e  may be oriented horizontally along the anterior-posterior axis  18  to be supported by a support bracket attached to the support surface  24  on an exterior face (not shown) and on an interior face to abut a tab  32  extending downwardly from the lower surface of the force plate  14 . In this way the force-sensor  22   e  may measure forces on the force plate  14  in the horizontal plane directed along the anterior-posterior axis  18 . A spring  36  extending horizontally between an upwardly extending bracket on the support surface  24  and a similar downwardly extending tab  38  may provide bias of the force plate  14  against the force-sensor  22   e  to ensure contact therebetween. Each of the force-sensors  22  may communicate with an internal microcontroller  40  having a processor unit  42  and a memory  44  holding a stored program  46 . The microcontroller  40  may also communicate with the display  20  to provide for the display of alphanumerical or graphic data as will be discussed. 
     Referring now to  FIGS. 2, 3 and 5 , an individual  50  may stand on the force plates  14  for a predetermined period of time, as instructed, for example, by instructions received through the display  20 . During these measurements, the individual&#39;s ankle, knee, and hip joints are generally unrestrained so as to permit the natural muscular forces required for balancing to be applied on these joints. During that period of time, as indicated by process block  52  a center-of-pressure  54  and force-angle  55  (in the sagittal plane of a foot aligned with each force plate  14   a  and  14   b ) may be collected at multiple points in time during the predetermined interval. 
     More specifically, the relative force on the force sensors  22   a - 22   d  may be used to determine the center-of-pressure  54 . Because the center-of-pressure  54  is only required along a single dimension of the anterior-posterior axes, it will be appreciated that as few as two force sensors  22  may be used for this purpose. The sum of the forces from the sensors  22   a - 22   d  may also be used to deduce a downward force F z  (being equal to the weight of the individual plus any accelerative forces). Similarly the force measured by force-sensor  22   e  may be used to deduce a horizontal force along the anterior-posterior axis  18  of F x  (being equal to an accelerative force of balance exerted by the individual). These values, in turn, may be used to determine a force-angle  55 , for example, determined as the arctangent of F x /F z . 
     The force-angle  55  and the center-of-pressure  54  find the location and direction of a ground reaction force vector  56  being a force vector which if applied to the force plate  14  (at the center-of-pressure  54  and having the force-angle  55 ) would produce the identical readings on each of the force-sensors  22  as provided by the distributed forces applied to the force plate by the individual&#39;s foot. 
     Force-sensors  22  produce a stream of data  60  representing different force vectors  56  at different sample points in time. If the relationship between center-of-pressure  54  and force-angle  55  is analyzed (for example, as shown in plot line  62 ), it portrays a complex relationship that is practically opaque to direct analysis. Accordingly, in the present invention, the stream of data  60  is filtered into multiple different frequency bins  64 , for example, each having a one hertz passband  65  and arranged from 0 to 10 hertz (only three shown for clarity) as represented by process block  67 . This filtration, for example, may make use of discrete frequency filters or may be performed using the fast Fourier transform generally known in the art. 
     Within each passband  65 , the relationship between center-of-pressure  54  and force-angle  55  reveals a simpler relationship characterizable as different linear functions  66   a - 66   c  associated with a different passband  65 , for example when discrete bandpass filters are implemented. Each of these linear functions  66  may have a different slope  68   a - 68 , determined, for example, by the slope of a line fit to the data by linear regression or other similar technique. 
     Generally the slopes  68  of this data for different frequency passbands  65  may be plotted as shown by graph lines  80   a  and  80   b  as will be discussed below. Preferably, however, the relationship between center-of-pressure  54  and force-angle  55  expressed by these linear slopes  68  are used to define an “intersection point” within the individual  50 . Referring now also to  FIG. 4 , the intersection  70  is generally the point of intersection of each of the force vectors  56  associated with the stream of data  60 . When the linear relationship between force-angle  55  and center-of-pressure  54  is associated with a relatively low slope  68  (e.g., shown by force vectors  56   a ), a high intersection point  70   a  will be identified resulting from the relatively low angular differences between the force vectors  56   a  for a given center-of-pressure displacement. Conversely when the linear relationship between force-angle  55  and center-of-pressure  54  is associated with a relatively high slope  68  (e.g., shown by force vectors  56   b ), a low intersection point  70   b  will be identified resulting from the relatively higher angular differences between the force vectors  56   b  for a given center-of-pressure displacement. 
     Generally this height may be calculated as IP z =CP x /(F x /F z ), per process block  75 , where IP z  is the height of the intersection point and CP x  is lateral displacement of the center-of-pressure. 
     The height of the calculated intersection point  70  may be viewed with respect to a height of the individual&#39;s center-of-mass  72  to provide an intuitive understanding of the intersection point. In this regard, it is useful to analyze the height of the intersection point  70  with respect to the individual&#39;s center-of-mass  72 . The center-of-mass  72  may be estimated based on the individual&#39;s height and similarities in human anatomy among individuals to be, for example, at a fixed percentage of the individual&#39;s height. Measurements producing force vectors  56   a  related to a relatively low slope  68  may identify an intersection point  70  being above the height of the center-of-mass  72 . Conversely, measurements producing force vectors  56   b  related to a relatively high slope  68  may identify an intersection point  70   b  below the height of the center-of-mass  72 . 
     Referring now to  FIGS. 6 a  and 6 b   , an insight into the significance of the intersection point  70  may be gained by considering the individual  50  as a rigid solid responding to torques applied by the individual&#39;s feet by rotating about the individual&#39;s center-of-mass  72 . Referring to  FIG. 6 a   , when the individual  50  is leaning forward (albeit stably with the center-of-mass  72  still behind the center of contact), and when the intersection point ( 70   a ) is above the center-of-mass  72 , the torque about the center-of-mass  72  will be applied at an effective torque arm extending forward from the center-of-mass  72  and tending to restore the individual to upright posture with a counterclockwise torque  76 . Conversely, however, and referring to  FIG. 6 b   , in the same situation but where the intersection point  70   b  is below the center-of-mass  72 , the torque will be applied to the center-of-mass  72  at a point on an effective torque arm extending rearward from the center-of-mass  72  causing a clockwise torque  76  tending to exacerbate the out-of-balance situation. Accordingly a control strategy producing a higher intersection point  70  may tend to be more stable. 
     Referring again to  FIG. 3 , this can be seen in a direct plot of the height of intersection points  70  as a percentage of normalized height of the individual for a non-paretic leg (indicated by plot line  80   a ) as compared to a paretic leg (affected by stroke and indicated by plot line  80   b ), for example, in frequencies centered around two hertz. The data for the non-paretic leg of plot line  80   a  in this frequency range shows an intersection point  70  well above the center-of-mass  72  while the plot line  80   b  of the paretic leg shows intersection point below the center-of-mass  72  generally indicating weaker balance in the paretic leg. 
     Referring again to  FIG. 5 , after calculation of the intersection point  70 , the individual&#39;s weight may also be calculated, for example, by summing the values of the vertically oriented force-sensors  22  as indicated by process block  82 . At process block  84  the balance information and weight information may be output on the display  20  either for assessment of balance capabilities or as part of a rehabilitation training system where the individual attempts to control this value with therapy. This therapy may involve the real-time observation of changes in the intersection point  70  as the individual concentrates on various aspects of his or her balance. 
     The output on the display  20  may be in various forms, for example, displaying any of the plots depicted in figures or displaying numeric values of slope or the like, or providing a graphic representation of the individual showing locations of intersection points and center-of-mass or providing a height measurement of the intersection point or a difference in height between the intersection point and the center-of-mass. 
     While the present invention has been discussed with respect to measuring force-angle and center-of-pressure for a standing individual, there is indication that the same measurements can be made with the individual in a seated position and instructed to press downward on a force plate  14 , the latter tipped, for example, toward the individual to receive the individual&#39;s feet. In this case the intersection point is assessed as if the individual were standing and compared to the individual&#39;s standing center-of-mass. Again, the individual&#39;s ankle, knee joint, and hip joints are unrestrained except as related to their seated posture. 
     Although the above description describes measurements made in the sagittal plane of the individual, it is contemplated that other planes may equally provide comparable balance data and accordingly the invention contemplates measurements in other directions as well as along the sagittal plane axis. 
     Certain terminology is used herein for purposes of reference only, and thus is not intended to be limiting. For example, terms such as “upper”, “lower”, “above”, and “below” refer to directions in the drawings to which reference is made. Terms such as “front”, “back”, “rear”, “bottom” and “side”, describe the orientation of portions of the component within a consistent but arbitrary frame of reference which is made clear by reference to the text and the associated drawings describing the component under discussion. Such terminology may include the words specifically mentioned above, derivatives thereof, and words of similar import. Similarly, the terms “first”, “second” and other such numerical terms referring to structures do not imply a sequence or order unless clearly indicated by the context. 
     When introducing elements or features of the present disclosure and the exemplary embodiments, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of such elements or features. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements or features other than those specifically noted. It is further to be understood that the method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. 
     References to “a microprocessor” and “a processor” or “the microprocessor” and “the processor,” can be understood to include one or more microprocessors that can communicate in a stand-alone and/or a distributed environment(s), and can thus be configured to communicate via wired or wireless communications with other processors, where such one or more processor can be configured to operate on one or more processor-controlled devices that can be similar or different devices. Furthermore, references to memory, unless otherwise specified, can include one or more processor-readable and accessible memory elements and/or components that can be internal to the processor-controlled device, external to the processor-controlled device, and can be accessed via a wired or wireless network. 
     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein and the claims should be understood to include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. All of the publications described herein, including patents and non-patent publications are hereby incorporated herein by reference in their entireties.