Stephen D. Perry

The role of plantar cutaneous mechanoreceptors in the control of compensatory stepping reactions evoked by unpredictable, multi-directional perturbation

The role of plantar pressure sensation in controlling compensatory stepping was explored via hypothermic anesthesia of the foot soles, in 10 healthy young adults. Stepping reactions were evoked by unpredictable platform translation in forward, backward and lateral directions. The findings suggest three specific direction- and phase-dependent roles for the plantar cutaneous afferents: (1) sensing posterior stability limits during initiation of backward steps, (2) sensing and controlling heel-contact and subsequent weight transfer during termination of forward steps, and (3) maintaining stability during the prolonged swing phase of lateral crossover steps.

Evaluation of age-related plantar-surface insensitivity and onset age of advanced insensitivity in older adults using vibratory and touch sensation tests.

Age-related decline in plantar-surface sensitivity has been cited as one of the reasons for balance problems in older adults. This study investigated the level of plantar-surface sensitivity in older adults compared to young adults. Additionally, this study attempted to identify the onset age of advanced insensitivity in older adults and how well monofilament testing was able to predict insensitivity and onset age of advanced insensitivity. Vibration thresholds were assessed at four frequencies (3, 25, 100 and 250 Hz) and four foot sole locations (heel, first metatarsal, fifth metatarsal and great toe). Touch thresholds were evaluated with Semmes-Weinstein monofilaments at the same four locations. Results indicated that older adults showed insensitivity to vibratory (25 and 100 Hz) and touch stimuli when compared to young adults. Vibration testing at 100 Hz indicated that early in the seventh decade (72-73 years old) participants started to show a doubling of their detection threshold as compared to their younger counterparts (65-71 years old). Regression analysis indicated a significant predictive value of the monofilaments to 100 Hz vibration thresholds, but a caution is noted that this type of testing is not as sensitive to the onset of advanced plantar-surface insensitivity. Therefore, older adults have significant plantar-surface insensitivity as compared to young adults and have an onset of advanced insensitivity in the seventh decade of life. Monofilaments are useful to assess the age-related insensitivity but are not when attempting to identify the onset of advanced insensitivity.

Thresholds for step initiation induced by support-surface translation: a dynamic center-of-mass model provides much better prediction than a static model

The need to initiate a step in order to recover balance could, in theory, be predicted by a static model based solely on displacement of the center of mass (COM) with respect to the base of support (BOS), or by a dynamic model based on the interaction between COM displacement and velocity. The purpose of this study was to determine whether the dynamic model provides better prediction than the static model regarding the need to step in response to moving-platform perturbation. The COM phase plane trajectories were determined for 10 healthy young adults for trials where the supporting platform was translated at three different acceleration levels in anterior and posterior directions. These trajectories were compared with the thresholds for step initiation predicted by the static and dynamic COM models. A single-link-plus-foot biomechanical model was employed to mathematically simulate termination of the COM movement, without stepping, using the measured platform acceleration as the input. An optimization routine was used to determine the stability boundaries in COM state space so as to establish the dynamic thresholds where a compensatory step must be initiated in order to recover balance. In the static model, the threshold for step initiation was reached if the COM was displaced beyond the BOS limits. The dynamic model showed substantially better accuracy than the static model in predicting the need to step in order to recover balance: 71% of all stepping responses predicted correctly by the dynamic model versus only 11% by the static model. These results support the proposition that the central nervous system must react to and control dynamic effects, i.e. COM velocity, as well as COM displacement in order to maintain stability with respect to the existing BOS without stepping.

Effects of Shoe Characteristics on Dynamic Stability When Walking on Even and Uneven Surfaces in Young and Older People

OBJECTIVEnTo systematically investigate the effects of various shoe features (sole hardness, heel height, heel collar height, tread pattern) on dynamic balance control and perceptions of comfort and stability in young and older people walking over even and uneven surfaces.nnnDESIGNnA mixed-design 3-way repeated measures with age as a between-subjects factor and surface and shoe conditions as within-subjects factors.nnnSETTINGnGait laboratory.nnnPARTICIPANTSnYoung adults (n=11) and community-dwelling older adults (n=15).nnnINTERVENTIONSnNot applicable.nnnMAIN OUTCOME MEASURESnCenter of mass (COM)-base of support (BOS) margins, vertical and braking loading rates, and subjective ratings of perceived shoe comfort and stability.nnnRESULTSnOverall, compared with the standard shoes, the soft sole shoes led to greater lateral COM-BOS margin (P<.001), whereas the elevated heel shoes caused reductions in posterior COM-BOS margin (P=.001) and in vertical and braking loading rates (both, P<.001). Subjects rated the elevated heel shoes as significantly less comfortable (P<.001) and less stable (P<.001) than the standard shoes. Only the young subjects perceived the soft-sole shoes to be less stable than the standard shoes (P=.003).nnnCONCLUSIONSnBoth young and older subjects adopted a conservative walking pattern in the elevated heel shoes and exhibited impaired mediolateral balance control in the soft-sole shoes. In contrast, increased sole hardness (above that found in a standard shoe), a tread sole, and a raised collar height did not improve walking stability in either group. It is concluded that shoes with elevated heels or soft soles should not be recommended for older people and that a standard laced shoe with a low collar and a sole of standard hardness with or without a tread provides optimal dynamic stability when walking on even and uneven surfaces.

Reducing fall risk by improving balance control: Development, evaluation and knowledge-translation of new approaches

PROBLEMnFalling is a leading cause of serious injury, loss of independence, and nursing-home admission in older adults. Impaired balance control is a major contributing factor.nnnMETHODSnResults from our balance-control studies have been applied in the development of new and improved interventions and assessment tools. Initiatives to facilitate knowledge-translation of this work include setting up a new network of balance clinics, a research-user network and a research-user advisory board.nnnRESULTSnOur findings support the efficacy of the developed balance-training methods, balance-enhancing footwear, neuro-prosthesis, walker design, handrail-cueing system, and handrail-design recommendations in improving specific aspects of balance control. IMPACT ON KNOWLEDGE USERS: A new balance-assessment tool has been implemented in the first new balance clinic, a new balance-enhancing insole is available through pharmacies and other commercial outlets, and handrail design recommendations have been incorporated into 10 Canadian and American building codes. Work in progress is expected to have further impact.

The efficacy of head immobilization techniques during simulated vehicle motion.

STUDY DESIGNnLaboratory experiment.nnnOBJECTIVEnTo compare the efficacy of different head immobilization techniques during motion simulating ambulance transport.nnnBACKGROUNDnA significant number of neurologic injuries associated with cervical spine fractures arise or are aggravated during emergency extrication or patient transport. Previous studies have not addressed the effect of head immobilization on the passive motion that could occur across the neck during transport.nnnMETHODSnThree different head-immobilization methods were compared in six healthy young adults by using a computer-controlled moving platform to simulate the swaying and jarring movements that can occur during ambulance transport. In all tests, the trunk was secured by means of a commonly used criss-cross strapping technique. Efficacy of head immobilization was evaluated using measures of head motion and neck rotation.nnnRESULTSnNone of the three immobilization techniques was successful in eliminating head motion or neck rotation. Movement of the trunk contributed substantially to the lateral bending that occurred across the neck. A new product involving the placement of wedges underneath the head provided some small, but statistically significant improvements in fixation of the head to the fracture board; however, there was no improvement in terms of the relative motion occurring across the neck.nnnCONCLUSIONSnSomewhat improved fixation of the head to the fracture board can be achieved by placing wedges under the head; however, the benefits of any fixation method, in terms of cervical spine immobilization, are likely to be limited unless the motion of the trunk is also controlled effectively. Future research and development should address techniques to better control head and trunk motion.

Inter-limb centre of pressure symmetry during gait among stroke survivors

The purpose of the present study was to describe the spatial-temporal parameters of the centre of pressure (COP) trajectory during the single-support phase of gait among stroke survivors and relate these parameters to the severity of sensorimotor impairment. Fifty-seven participants were asked to walk at their preferred and fast speed over a pressure sensitive mat. Outcome measures included anterior-posterior (AP) COP displacement, AP-COP velocity, medial-lateral (ML) COP variability and foot region COP time. The results demonstrated an asymmetrical AP-COP displacement in favour of the non-paretic limb for the majority of participants. The inter-limb difference scores for AP-COP displacement and AP-COP velocity were related to the severity of sensorimotor impairment and greater among gait aid users. ML-COP variability was greater under the non-paretic limb, possibly suggesting difficulty with paretic limb swing phase. Reduced or absent forefoot COP time suggests difficulty with forward progression and modified foot function during push-off. The inter-limb difference in COP parameters highlights the asymmetrical nature of post-stroke gait and the challenge of maintaining single limb support. We view this information as potentially important to clinicians as an outcome measure for gait rehabilitation.

Selective skin sensitivity changes and sensory reweighting following short-duration space flight

Skin sensory input from the foot soles is coupled with vestibular input to facilitate body orientation in a gravitational environment. Anecdotal observations suggest that foot sole skin becomes hypersensitive following space flight. The veritable level of skin sensitivity and its impact on postural disequilibrium observed post space flight have not been documented. Skin sensitivity of astronauts (n = 11) was measured as vibration perception at the great toe, fifth metatarsal and heel. Frequencies targeted four classes of receptors: 3 and 25 Hz for slow-adapting (SA) receptors and 60 and 250 Hz for fast-adapting (FA) receptors. Data were collected pre- and post-space flight. We hypothesized that skin sensitivity would increase post-space flight and correlate to balance measures. Decreased skin sensitivity was found on landing day at 3 and 25 Hz on the great toe. Hypersensitivity was found for a subset of astronauts (n = 6) with significantly increased sensitivity to 250 Hz at the heel. This subset displayed a greater reduction in computerized dynamic posturography (CDP) equilibrium (EQ) scores (-54%) on landing vs. non-hypersensitive participants (-11%). Observed hyposensitivity of SA (pressure) receptors may indicate a strategy to reduce pressure input during periods of unloading. Hypersensitivity of FAs coupled with reduced EQ scores may reflect targeted sensory reweighting. Altered gravito-inertial environments reduce vestibular function in balance control which may trigger increased weighting of FAs (that signal foot contact, slips). Understanding modulations to skin sensitivity has translational implications for mitigating postural disequilibrium following space flight and for on-Earth preventative strategies for imbalance in older adults.

Ankle-foot orthotic management in neuromuscular disorders: recommendations for future research

Purpose: To describe research evidence supporting clinical recommendations for ankle-foot orthotic (AFO) prescription and examine common limitations in current research among individuals with stroke and cerebral palsy. Method: Three databases and one journal website were searched for articles reporting AFO interventions on gait and functional mobility outcome measures in participants with stroke or cerebral palsy. The International Society for Prosthetics and Orthotics (ISPO) best practice recommendations from consensus conferences were reviewed. Data extracted from the articles include participant characteristics, AFO intervention details, evaluation methods, and outcome measures. Results: Sixty articles were included; twenty-seven on stroke and thirty-three on cerebral palsy participants. Many articles reported insufficient detail on severity of lower limb impairment. Type of interventions included nineteen nonarticulating AFO studies, twelve articulating AFO studies and twenty-three studies testing both. Confounding factors, such as compliance, activity level and footwear, need to be considered in longitudinal studies. Conclusions: Most studies demonstrated improvement in walking speed and ankle dorsiflexion, whereas the indirect effect on knee stability remains unclear. Future research needs to provide detailed information on type and severity of lower limb impairment of participants and design features of the AFO intervention. Implications for Rehabilitation Research reports need to provide clinical measures of impairment and function to facilitate understanding the AFO’s mechanical effect during gait. Standardized outcome measures related to the prescription goals and patient’s functional mobility is essential to determine whehter the orthotic effect is beneficial. ISPO recommendations should be consulted for reporting orthotic design characteristics.

Spatial-Temporal Gait Variability Poststroke: Variations in Measurement and Implications for Measuring Change

OBJECTIVEnTo determine the responsiveness to change of spatial-temporal gait parameters among stroke survivors for 3 different variability measures: SD, coefficient of variation (CV), and median absolute deviation (MAD).nnnDESIGNnRetrospective chart review.nnnSETTINGnClinical laboratory in a Canadian hospital.nnnPARTICIPANTSnStroke survivors (N=74) receiving inpatient rehabilitation.nnnINTERVENTIONSnNot applicable.nnnMAIN OUTCOME MEASURESnSpatial-temporal gait variability was calculated for step length, step width, stance time, swing time, and double support time. Responsiveness to change was determined by comparing (1) trials without versus trials with a concurrent cognitive task and (2) admission to discharge from rehabilitation.nnnRESULTSnVariability estimators (SD, CV, and MAD) increased with the addition of a cognitive task and decreased from admission to discharge of rehabilitation. However, these changes were not statistically significant when change in gait velocity was included as a covariate. The effect size values were similar for all variability estimators with a trend toward a greater SD response to temporal parameters. The CV displayed a larger response to change for step length than did the SD and MAD. Although gait variability decreased between admission and discharge, the effect size was larger for the condition without the cognitive task than for the condition with the cognitive task.nnnCONCLUSIONSnOur results show that gait variability estimators demonstrate a similar responsiveness to a concurrent cognitive task and improved walking ability with recovery from stroke. Future work may focus on evaluating the clinical utility of these measures in relation to informing therapy and response to gait-specific training protocols.