rehabilitation includes physical therapy that enables long - term improvements through short - term efforts during daily interventions ; it promotes mobility , improves functional ability , and improves the quality of life . during physical therapy , kinesthesia , that is , motion perception that  i am moving ,  which is generated through the interaction dynamics between motor and sensory systems , that is , the  motor - sensory loop , plays a crucial role in long - term motor learning as well as short - term motion generation . thus , achievement of kinesthesia is essential for the rehabilitation of physical impairments and disabilities . however , sensory impairments caused by neurological or physical disorders hamper kinesthesia , making rehabilitation difficult . for the rehabilitation of sensory impairments , we proposed a novel biofeedback prosthesis   that transforms weak or deficient kinesthetic feedback into an alternative sensory modality . from the viewpoint of system engineering , sensory impairments in humans are considered as input failure in a system , leading to dysfunction of the entire system ; the dysfunction can be corrected through repair or replacement by another input component ( figure 1 ) . in this situation , enhancement of kinesthetic feedback or replacement with another sensory modality allows intervention for the dysfunction and enables reestablishment of motor - sensory loop in patients undergoing rehabilitation . thus , the aim of this study was to verify the short - term effect of the prosthesis in transforming sensory modalities for patients with sensory impairments . in particular , we focused on an auditory feedback prosthesis that transformed plantar sensation in walking rehabilitation   for the following four reasons : plantar sensation , that is , the trajectory of the center of pressure ( cop ) on the plantar region and the magnitude of load , is an essential kinesthesia in walking [ 27 ] ; in stroke patients with hemiparesis , the range of cop trajectories during walking is narrowed on the affected foot through the change of gait ; the time required for the cognitive resolution of auditory signals in the human brain ( approximately 1  ms ) is shorter than that required for the resolution of visual feedback signals ( approximately 50100  ms ) ; and visual feedback systems , for example , a display showing visual feedback signals , constrains the posture of subjects , resulting in limited rehabilitation spaces and approaches . previous studies have proposed auditory feedback systems for walking rehabilitation ; miyake   proposed the walk - mate system that utilizes the  mutual entrainment  of the timing of footsteps of a subject and an agent modeled on a computer system and showed that patients ' as well as healthy subjects ' gait were restored to a stable and natural walking state . schauer and mauritz   verified the timing effect of auditory signals at touchdown during walking rehabilitation for stroke patients . however , no previous studies focused on transforming the spatiotemporal pattern of loading on a foot to auditory feedback signals . here , we applied our prosthesis , called auditory foot , which transforms multipoint cutaneous plantar sensations to auditory feedback signals ( figure 2 ) , to walking rehabilitation in stroke patients with hemiparesis , and demonstrated the short - term effect of the prosthesis on the intervention . we proposed a sensor prosthesis called auditory foot   for transforming sensory modalities for waking rehabilitation . the entire system consists of four components ( figure 2(a ) ) : ( i ) a pressure sensor sheet ( input component ) , ( ii ) a microcomputer ( data processing component ) , ( iii ) wireless communication devices ( data transport component ) , and ( iv ) a pc ( output component ) . the pressure sensor sheet consists of many pressure sensors ( e.g. , five sensors ; figure 2 middle bottom , interlink electronics : fsr402 ) . the sheet structure allowed us to easily modify the distribution of sensor points for individuals and easily attach the sheet to physical or prosthetic feet or insert it into shoes . a microcomputer ( mbed nxp lpc1768 ) converted analog data from pressure sensors to digital data and sent them to a wireless communication device ( xbee , digital international : zb rf module ) via serial communication . using xbee , digital data from the microcomputer the processing software   computes digital data from the xbee device and transformed them to auditory and visual outputs with a speaker and pc monitor , respectively . in the processing software , we designed a transformation protocol from plantar sensation to auditory signal outputs as follows : the position of pressure sensors corresponded to a musical interval ( cf . do , mi , so , do , and mi ) and the magnitude of pressure sensor values corresponded to audio volumes . thus , auditory signals corresponded to the spatiotemporal pattern of loading on a foot , as shown in figure 2(b ) . average age , height , and weight were 55  12 years old , 168  8  cm , and 67  7  kg , respectively . inclusion criteria were as follows : ability to walk without walking prosthetics , ability to understand instructions by physical therapists , and a first - time stroke due to either ischemic or hemorrhagic causes . subjects with hemiplegia were excluded if they had any of the following : brainstem or cerebellar lesions , abnormal circulatory and respiratory status , abnormal mental status , or higher brain dysfunction . the brunnstrom stage and stroke impairment assessment set ( sias )   of the patients are shown in table 1 . patients gave written informed consent before participating in the study , which was approved by the local ethics committee of tohoku university hospital , japan . for assessment of the effect of our prosthesis on gait in patients , we compared four conditions with the use of two pressure sensors at the heel and fifth metatarsal ( figure 3 ) : ( i ) without auditory feedback but with prosthesis , called  without  condition ; ( ii ) auditory feedback from only the heel sensor , called  heel condition ; ( iii ) auditory feedback from only the fifth metatarsal sensor , called fifth  condition ; and ( iv ) auditory feedback from both the heel and fifth metatarsal sensors , called  all  condition . we asked participants to walk for 7  m and two to eight trials were conducted for each condition , including more than three walking periods ( from touchdown of the affected foot to the next touchdown ) . to investigate unconscious effects , we did not instruct patients , for example ,  apply load on sounding places .  in all measurements , the coordinates of each reflective marker were measured by the mac 3d system ( 120  hz ) ( motion analysis corporation , santa rosa , ca , usa ) . forty - one reflective markers were placed using adhesive tape on 12 segments according to the anatomical positions ( table 2 ) . ground reaction force ( grf ) data were acquired at a 1200  hz sampling rate using four 90  cm  60  cm force plates ( anima corporation , chofu - shi , tokyo , japan ) . the three - dimensional coordinates and grf data were smoothed using a bidirectional forth - order butterworth low - pass filter with cut - off frequencies of 20 and 200  hz , respectively . a twelve - segment model based on the anthropometric data suggested by dumas et al . was comprised of the feet , shanks , thighs , pelvis , thorax , upper arms , and forearms . the kinematic data at each joint in the lower extremities were calculated using the joint coordinate system . moreover , lower extremity joint kinetics was estimated using inverse dynamics . the representative parameter for gait in patients with hemiparesis was extracted from kinematic and kinetic data , referring to the previous study by kinsella and moran . the parameters were calculated with custom software in matlab ( the mathworks incorporated , natick , ma ) . in this study , we used dynamic joint stiffness measurement , which has been proposed for the assessment of the resistance of a joint during human walking . the dynamic joint stiffness of a joint ki is defined as the ratio of the variation of its moment i to the variation of the corresponding joint angle i , described by the following equation:(1)ki=ii.we choose dynamic joint stiffness at the ankle joint as this measure can systematically evaluate the contribution of ankle function to walking gait when using our prosthesis . evaluation of joint stiffness to demonstrate gait recovery has been reported in previous studies [ 18 , 19 ] . an increased period of stiffness at the end of the stance phase implied effective pushing off the ground , and a decreased period of stiffness at the beginning of the stance phase implied shock absorption during gait . repeated measures anovas followed by dunnett post hoc tests were used to reveal differences in gait parameters between the without ( i ) and the with auditory feedback conditions ( ii)(iv ) . statistical analyses were performed using a statistical software package ( spss ver.22 , spss inc . , chicago , il , usa ) . gait speed [ cm / s ] , stride length [ cm ] , step length [ cm ] of the affected side , maximum hip extensor angle [ degree ] , and maximum ankle plantar flexor moment [ nm / kg ] of the affected side for the four conditions are presented in table 3 ( 7 patients ) . differences were observed in the gait speed , stride length , and step length , although these differences were not significant ( n.s . ) . a significant difference in the maximum hip extension angle of the affected side during the stance phase was observed between ( i )  without  and ( iv )  all  conditions , as shown in figure 4 . we also found a significant difference in the maximum ankle plantar flexor moment of the affected side between ( i )  without  and ( iv )  all  conditions ( figure 5 ) . moreover , we thoroughly analyzed data for the patient  e  with the sias score for sensory function touch / position = 0 , as shown in table 1 . the sias score is widely used for the evaluation of residual function in the field of stroke rehabilitation . moreover , a previous study reported moderate interclass reliability in sensory function touch / position , with scores ranging from 0.47 to 0.84 . 
 figure 6(a ) shows the dynamic joint stiffness of the patient the results indicated that dynamic joint stiffness decreases in the beginning of the stance phase in ( ii )  heel  and ( iv )  all  conditions , whereas stiffness increases in the late stance phase in ( ii )  heel ,  ( iii ) figure 6(b ) represents the effects on the range of cop trajectory on the affected side . these results indicated that the posterior position of cop in ( ii ) and ( iv ) moves backward at the beginning of the stance phase , whereas the anterior position in ( iii ) and ( iv ) moves forward at the end of the stance phase . past studies in humans and animals have shown that cutaneous receptors in the foot play an essential role in the control of gait [ 2 , 3 , 2029 ] and posture [ 30 , 31 ] . in stroke patients with hemiparesis , the range of cop trajectories during walking is narrowed on the affected foot owing to the change in gait . in our prosthesis , we used auditory feedback from the heel and/or fifth metatarsal sensors in conditions ( ii ) , ( iii ) , and ( iv ) . the effects of the prosthesis are summarized as follows : the auditory signal from the heel sensor allowed patients to apply on their heel , and the auditory signal from the fifth metatarsal sensor allowed patients to generate effective pushing off the ground at the end of the stance phase . the first effect was due to the decreasing dynamic joint stiffness at the beginning of the stance phase , whereas the second effect was due to the increasing stiffness at the end of the stance phase , as shown in figure 6(a ) . for the first effect , as in the forefoot ice condition , cop at the beginning of the stance phase tended to move backward in conditions ( ii ) and ( iv ) , as shown in figure 6(b ) . this would have resulted in the effective knee extension because knee rotation center is located anterior to cop , which , in turn , would have resulted in an increase in the stiffness at the end of stance phase owing to the activation of the gastrocnemius muscle . the two effects contributed to the significant difference in the maximum ankle plantar flexor moment between trials with and without the auditory feedback , resulting in the enhancement of the maximum hip extensor angle of the affected side during the stance phase . the effects of reduced plantar sensation using an ice immersion technique [ 57 ] suggested that plantar sensation plays a crucial role in the modification of gait . reduced plantar sensation resulted in modification of the pressure distribution , with significant reduction of the peak pressure on the heel and toe because of a more cautious touchdown to and push - off from the ground . furthermore , the hip extension angle at the end of the stance phase is significantly reduced because of similar cautiousness . these results of the reduced plantar sensation corroborate the results of this study , where auditory feedback that enhanced the plantar sensory feedback also enhanced the maximum hip extensor angles of the affected side . recently , proposed systems or devices for walking rehabilitation have been mainly classified into three categories : training systems for gait recovery ; assistive systems that actively compensate human motor functions ; and assistive devices that passively support human motor abilities . lokomat ( hocoma ag , switzerland ) , autoambulator ( healthsouth , uk ) , and haptic walker ( fraunhofer ipk , germany ) , in the first category , are mounted - systems that mostly consist of a weight supporting belt , treadmill , and walking assistive device . a representative device in the second category is hal ( hybrid assistive limb )   ( cyberdyne inc . , japan ) , which can support and improve human motor function with a robotic system . japan ) [ 36 , 37 ] and acsive ( nambu co. , ltd . , however , few past reports have confirmed the effective short- and long - term effects of such devices . as with noninvasive rehabilitation approaches , for example , transcranial magnetic stimulation ( tms ) or transcranial direct current stimulation ( tdcs ) the wireless communication system enables expansion of the range of applications to various rehabilitation environments . in the future , we would expect that such smart , wireless , portable , and inexpensive systems may enable widespread use for daily rehabilitation in patients ' homes . we focused on the kinematic and kinetic effects of our prosthesis on patient gait , but we did not consider the effects on muscle activation patterns , which are coordinated by the central nervous system ( cns ) using sensory information . recently , muscle synergy patterns , which consist of motor modules as groups of muscles activated at the same time , have been considered as a biomechanical marker associated with pathological conditions [ 38 , 39 ] . therefore , it is important to evaluate gait plasticity through the intervention with the proposed prosthesis on the basis of muscle synergy analysis . the relationship between neurological mechanism and biomechanics seems to be of general interest and will also be studied in further investigations . the variation was considered to result from the variation in residual function among stroke patients , for example , the difference in sensory function touch / position . nonetheless , we found significant differences in maximum hip extension angle and plantar flexor moment in stance between the control and all condition , using anovas followed by dunnett post hoc tests . this indicates the short - term effect of our prosthesis on walking in stroke patients . furthermore , we confirmed that the effect of the prosthesis was much larger in patient e  who has a severe sensory disorder , that is , sias score for sensory function touch / position = 0 , as shown in figures 6(a ) and 6(b ) . we did not investigate differences in walking performance arising from the location of the sounding source . suggested that spatial modulation of audiotactile interactions might occur for complex auditory stimuli originating from the region close to the back of the head . thus , we would especially need to consider the location of the auditory feedback source . furthermore , the coupling or conflict between sensory modalities , for example , visual , vestibular , and somatosensory information , which would significantly affect gait patterns , should also be discussed . our findings can not be generalized to patients with hemiparesis who use canes and/or orthoses . in the present investigation , the short - term effect of the prosthesis on walking in stroke patients was successfully confirmed . rehabilitation is a long - term improvement of dysfunction through the short - term effects of daily interventions . thus , the long - term effect of our prosthesis will be studied in future . a recent report   about a rhythmic motor learning task with visual and auditory signals indicated that the visual feedback ( fb ) group became dependent on the fb for their performance after the practice , whereas the auditory fb group performed equally well with or without fb after practice . this finding suggests that our auditory feedback prosthesis might eventually allow patients to be less reliant on auditory fb for walking performance after the rehabilitation . most studies have reported the effect of impaired plantar sensation on gait plasticity due to aging   or diseases , such as diabetes mellitus [ 4347 ] or congenital insensitivity to pain with anhidrosis ( cipa ) [ 48 , 49 ] . decreased tactile sensation with aging impaired sensory function of limbs and led to falls in the elderly . in patients with diabetic neuropathy , the reduction in the peak of vertical and horizontal ground reaction force has been reported in patients with diabetic neuropathy [ 4446 ] . reported that loss of toe function resulted in load shifting from toe to metatarsal head in patients with early diabetic neuropathy . zhang et al .   reported that younger patients with cipa walked faster , with a longer stride and higher heel contact angular velocity , than controls , owing to sensory impairments . moreover , modifications of gait and posture have been reported in ischemic blocking [ 50 , 51 ] . overall , plantar sensations influence posture and gait in patients with sensory impairments and ischemic block as well as ice immersion [ 57 ] . our plan is to apply further rehabilitation efforts in the patients with the following conditions : cipa , parkinson 's disease ( pd ) , and diabetic neuropathy [ 4347 ] . moreover , we plan to apply the system to experiments involving a prosthetic leg in healthy subjects in a pilot study . finally , we will verify the long - term effects of the proposed rehabilitation system , as discussed above , leading to brain plasticity of the body representation . the present investigation showed the short - term effect of the proposed prosthesis on walking in stroke patients with hemiparesis . significant differences were found in the maximum hip extensor angle and maximum ankle plantar flexor moment with and without auditory feedback from plantar sensations .
sensory impairments caused by neurological or physical disorders hamper kinesthesia , making rehabilitation difficult . in order to overcome this problem 
 , we proposed and developed a novel biofeedback prosthesis called auditory foot for transforming sensory modalities , in which the sensor prosthesis transforms plantar sensations to auditory feedback signals . 
 this study investigated the short - term effect of the auditory feedback prosthesis on walking in stroke patients with hemiparesis . to evaluate the effect , we compared four conditions of auditory feedback from plantar sensors at the heel and fifth metatarsal . 
 we found significant differences in the maximum hip extension angle and ankle plantar flexor moment on the affected side during the stance phase , between conditions with and without auditory feedback signals . 
 these results indicate that our sensory prosthesis could enhance walking performance in stroke patients with hemiparesis , resulting in effective short - term rehabilitation .