Articulated ankle-foot-orthosis improves inter-limb propulsion symmetry during walking adaptability task post-stroke.

BACKGROUND: Community ambulation involves complex walking adaptability tasks such as stepping over obstacles or taking long steps, which require adequate propulsion generation by the trailing leg. Individuals post-stroke often have an increased reliance on their trailing nonparetic leg and favor leading with their paretic leg, which can limit mobility. Ankle-foot-orthoses are prescribed to address common deficits post-stroke such as foot drop and ankle instability. However, it is not clear if walking with an ankle-foot-orthosis improves inter-limb propulsion symmetry during adaptability tasks. This study sought to examine this hypothesis. METHODS: Individuals post-stroke (n = 9) that were previously prescribed a custom fabricated plantarflexion-stop articulated ankle-foot-orthosis participated. Participants performed steady-state walking and adaptability tasks overground with and without their orthosis. The adaptability tasks included obstacle crossing and long-step tasks, leading with both their paretic and nonparetic leg. Inter-limb propulsion symmetry was calculated using trailing limb ground-reaction-forces. FINDINGS: During the obstacle crossing task, ankle-foot-orthosis use resulted in a significant improvement in inter-limb propulsion symmetry. The orthosis also improved ankle dorsiflexion during stance, reduced knee hyperextension, increased gastrocnemius muscle activity, and increased peak paretic leg ankle plantarflexor moment. In contrast, there were no differences in propulsion symmetry during steady-state walking and taking a long-step when using the orthosis. INTERPRETATION: Plantarflexion-stop articulated ankle-foot-orthoses can improve propulsion symmetry during obstacle crossing tasks in individuals post-stroke, promoting paretic leg use and reduced reliance on the nonparetic leg.

Influence of Walking Over Unexpected Uneven Terrain on Joint Loading for Individuals With Transtibial Amputation.

Individuals with transtibial amputation (TTA) experience asymmetric lower-limb loading which can lead to joint pain and injuries. However, it is unclear how walking over unexpected uneven terrain affects their loading patterns. This study sought to use modeling and simulation to determine how peak joint contact forces and impulses change for individuals with unilateral TTA during an uneven step and subsequent recovery step and how those patterns compare to able-bodied individuals. We expected residual limb loading during the uneven step and intact limb loading during the recovery step would increase relative to flush walking. Further, individuals with TTA would experience larger loading increases compared to able-bodied individuals. Simulations of individuals with TTA showed during the uneven step, changes in joint loading occurred at all joints except the prosthetic ankle relative to flush walking. During the recovery step, intact limb joint loading increased in early stance relative to flush walking. Simulations of able-bodied individuals showed large increases in ankle joint loading for both surface conditions. Overall, increases in early stance knee joint loading were larger for those with TTA compared to able-bodied individuals during both steps. These results suggest that individuals with TTA experience altered joint loading patterns when stepping on uneven terrain. Future work should investigate whether an adapting ankle-foot prosthesis can mitigate these changes to reduce injury risk.

Differences in Glenohumeral Joint Contact Forces Between Recovery Hand Patterns During Wheelchair Propulsion With and Without Shoulder Muscle Weakness: A Simulation Study.

The majority of manual wheelchair users (MWCU) develop shoulder pain or injuries, which is often caused by impingement. Because propulsion mechanics are influenced by the recovery hand pattern used, the pattern may affect shoulder loading and susceptibility to injury. Shoulder muscle weakness is also correlated with shoulder pain, but how shoulder loading changes with specific muscle group weakness is unknown. Musculoskeletal modeling and simulation were used to compare glenohumeral joint contact forces (GJCFs) across hand patterns and determine how GJCFs vary when primary shoulder muscle groups are weakened. Experimental data were analyzed to classify individuals into four hand pattern groups. A representative musculoskeletal model was then developed for each group and simulations generated to portray baseline strength and six muscle weakness conditions. Three-dimensional GJCF peaks and impulses were compared across hand patterns and muscle weakness conditions. The semicircular pattern consistently had lower shear (anterior-posterior and superior-inferior) GJCFs compared to other patterns. The double-loop pattern had the highest superior GJCFs, while the single-loop pattern had the highest anterior and posterior GJCFs. These results suggest that using the semicircular pattern may be less susceptible to shoulder injuries such as subacromial impingement. Weakening the internal rotators and external rotators resulted in the greatest increases in shear GJCFs and decreases in compressive GJCF, likely due to decreased force from rotator cuff muscles. These findings suggest that strengthening specific muscle groups, especially the rotator cuff, is critical for decreasing the risk of shoulder overuse injuries.

A comparison of the effects of mediolateral surface and foot placement perturbations on balance control and response strategies during walking.

BACKGROUND: Balance perturbation studies during walking have improved our understanding of balance control in various destabilizing conditions. However, it is unknown to what extent balance recovery strategies can be generalized across different types of mediolateral balance perturbations. RESEARCH QUESTION: Do similar mediolateral perturbations (foot placement versus surface translation) have similar effects on balance control and corresponding balance response strategies? METHODS: Kinetic and kinematic data were previously collected during two separate studies, each with 15 young, healthy participants walking on an instrumented treadmill. In both studies, medial and lateral balance perturbations were applied at 80% of the gait cycle either by a treadmill surface translation or a pneumatic force applied to the swing foot. Differences in balance control (frontal plane whole body angular momentum) and balance response strategies (hip abduction moment, ankle inversion moment, center of pressure excursion and frontal plane trunk moment) between perturbed and unperturbed gait cycles were evaluated using statistical parametric mapping. RESULTS: Balance disruptions after foot placement perturbations were larger and sustained longer compared to surface translations. Changes in joint moment responses were also larger for the foot placement perturbations compared to the surface translation perturbations. Lateral hip, ankle, and trunk strategies were used to maintain balance after medial foot placement perturbations, while a trunk strategy was primarily used after surface translations. SIGNIFICANCE: Surface and foot placement perturbations influence balance control and corresponding response strategies differently. These results can help inform the development of perturbation-based balance training interventions aimed at reducing fall risk in clinical populations.

Lower-limb joint quasi-stiffness in the frontal and sagittal planes during walking at different step widths.

Quasi-stiffness describes the intersegmental joint moment-angle relationship throughout the progression of a task. Previous work has explored sagittal-plane ankle quasi-stiffness and its application for the development of powered lower-limb assistive devices. However, frontal-plane quasi-stiffness remains largely unexplored but has important implications for the development of exoskeletons since clinical populations often walk with wider steps and rely on frontal-plane balance recovery strategies at the hip and ankle. This study aimed to characterize frontal-plane hip and ankle quasi-stiffness during walking and determine how step width affects quasi-stiffness in both the frontal and sagittal planes. Kinematic and kinetic data were collected and quasi-stiffness values computed for healthy young adults (n = 15) during treadmill walking across a range of step widths. We identified specific subphases of the gait cycle that exhibit linear and quadratic frontal-plane quasi-stiffness approximations for the hip and ankle, respectively. In addition, we found that at wider step widths, sagittal-plane ankle quasi-stiffness increased during early stance (∼12-35% gait cycle), sagittal-plane hip quasi-stiffness decreased in late stance (∼40-55% gait cycle) and frontal-plane hip quasi-stiffness decreased during terminal stance (∼48-65% gait cycle). These results provide a framework for further exploration of frontal-plane quasi-stiffness, lend insight into how quasi-stiffness may relate to balance control at various step widths, and motivate the development of stiffness-modulating assistive devices to improve balance related outcomes.

The Influence of Multiple Pregnancies on Gait Asymmetry: A Case Study.

Gait asymmetry is a predictor of fall risk and may contribute to increased falls during pregnancy. Previous work indicates that pregnant women experience asymmetric joint laxity and pelvic tilt during standing and asymmetric joint moments and angles during walking. How these changes translate to other measures of gait asymmetry remains unclear. Thus, the purpose of this case study was to determine the relationships between pregnancy progression, subsequent pregnancies, and gait asymmetry. Walking data were collected from an individual during 2 consecutive pregnancies during the second and third trimesters and 6 months postpartum of her first pregnancy and the first, second, and third trimesters and 6 months postpartum of her second pregnancy. Existing asymmetries in step length, anterior-posterior (AP) impulses, AP peak ground reaction forces, lateral impulses, and joint work systematically increased as her pregnancy progressed. These changes in asymmetry may be attributed to pelvic asymmetry, leading to asymmetric hip flexor and extensor length, or due to asymmetric plantar flexor strength, as suggested by her ankle work asymmetry. Relative to her first pregnancy, she had greater asymmetry in step length, step width, braking AP impulse, propulsive AP impulse, and peak braking AP ground reaction force during her second pregnancy, which may have resulted from increased joint laxity.

The influence of step width on balance control and response strategies during perturbed walking in healthy young adults.

Individuals with neuromuscular deficits often walk with wider step widths compared to healthy adults. Wider steps have been linked to a higher destabilizing frontal-plane external moment and greater range of frontal-plane whole-body angular momentum (HR), which is an indicator of decreased balance control. The purpose of this study was to experimentally determine 1) how step width alters balance control during steady-state walking, and 2) if step width changes the balance response strategies following mediolateral surface perturbations in healthy adults. Fifteen healthy young adults (7 male, age: 25 ± 4 years) walked on an instrumented treadmill at narrow, self-selected, wide and extra-wide step widths. During perturbed trials, the treadmill provided random mediolateral surface translations to each foot midway through single-leg-stance. Muscle electromyography, biomechanical measures (HR, frontal-plane external moment and joint moments) and deviations (differences in these measures between the perturbed and unperturbed walking trials) were compared across step widths. During steady state walking, wider steps were associated with decreased balance control. Increasing step widths were also associated with increased gluteus medius activity and reduced hip abduction and ankle inversion moments, which suggests healthy subjects rely more on a lateral ankle strategy to maintain balance at increasing step widths. There was no change in the plantarflexion moment. During perturbed walking, lateral, but not medial, surface translations adversely affected balance control. Further, wider steps did not change the balance response strategies following the perturbations, which suggests healthy individuals have the capacity to respond similarly to the perturbations at different step widths.

Biomechanical responses of individuals with transtibial amputation stepping on a coronally uneven and unpredictable surface.

Coronally uneven surfaces are prevalent in natural and man-made terrain, such as holes or bumps in the ground, curbs, sidewalks, and driveways. These surfaces can be challenging to navigate, especially for individuals with lower limb amputations. This study examined the biomechanical response of individuals with unilateral transtibial amputation (TTA) taking a step on a coronally uneven surface while wearing their clinically prescribed prosthesis, compared to individuals without mobility impairments (controls). An instrumented walkway was used with the middle force plate positioned either flush or rotated ± 15˚ in the coronal plane and concealed (blinded). TTAs used greater hip abduction compared to controls across all conditions, but especially during blinded inversion. The recovery step width of TTAs was wider after blinded eversion and narrower after blinded inversion, but unchanged for controls. These results suggest TTAs may have decreased balance control on unexpected, uneven surfaces. Additionally, TTAs generated less positive prosthetic ankle joint work during blinded inversion and eversion, and less negative coronal hip joint work during blinded inversion compared to controls. These biomechanical responses could lead to increased energy expenditure on uneven terrain. Surface condition had no effect on the vertical center of mass for either group of participants. Finally, the TTAs and the control group generated similar vertical GRF impulses, suggesting the TTAs had sufficient body support despite differences in surface conditions. These results are important to consider for future prosthetic foot designs and rehabilitation strategies.

Different aspects of hand grip performance associated with structural connectivity of distinct sensorimotor networks in chronic stroke.

Knowledge regarding the neural origins of distinct upper extremity impairments may guide the choice of interventions to target neural structures responsible for specific impairments. This cross-sectional pilot study investigated whether different brain networks explain distinct aspects of hand grip performance in stroke survivors. In 22 chronic stroke survivors, hand grip performance was characterized as grip strength, reaction, relaxation times, and control of grip force magnitude and direction. In addition, their brain structural connectomes were constructed from diffusion tensor MRI. Prominent networks were identified based on a two-step factor analysis using the number of streamlines among brain regions relevant to sensorimotor function. We used regression models to estimate the predictive value of sensorimotor network connectivity for hand grip performance measures while controlling for stroke lesion volumes. Each hand grip performance measure correlated with the connectivity of distinct brain sensorimotor networks. These results suggest that different brain networks may be responsible for different aspects of hand grip performance, which leads to varying clinical presentations of upper extremity impairment following stroke. Understanding the brain network correlates for different hand grip performances may facilitate the development of personalized rehabilitation interventions to directly target the responsible brain network for specific impairments in individual patients, thus improving outcomes.

The influence of altered foot placement and cognitive load on balance control during walking in healthy young adults.

BACKGROUND: Clinical populations often walk with altered foot placement, which can adversely affect balance control. However, it is unknown how balance control during walking is influenced when combining a cognitive load with altered foot placement. RESEARCH QUESTION: Is balance control during walking adversely affected by the combination of a more complex motor task, such as walking with altered foot placements, with a cognitive load? METHODS: Fifteen young healthy adults walked on a treadmill with and without a spelling cognitive load during normal walking, with step width targets (self-selected width, narrow, wide and extra wide), or with step length targets (self-selected length, short and long). RESULTS: Cognitive performance, measured by correct spelling response rate, decreased from self-selected (2.407 ± 0.6 letters/s) to the extra wide width (2.011 ± 0.5 letters/s). The addition of the cognitive load caused a decrease in frontal plane balance control across all step lengths (15% change) and at the wider step widths (16% change), but only caused a slight decrease in the sagittal plane for the short step length (6.8% change). SIGNIFICANCE: These results suggest that when combining a cognitive load with walking at non-self-selected widths, a threshold exists at wider steps where attentional resources become insufficient and balance control and cognitive performance decrease. Because decreased balance control increases the risk of falling, these results have implications for clinical populations who often walk with wider steps. Furthermore, the lack of changes to sagittal plane balance during altered step length dual-tasks further supports that frontal plane balance requires more active control.

The relationship between motor pathway damage and flexion-extension patterns of muscle co-excitation during walking.

Background: Mass flexion-extension co-excitation patterns during walking are often seen as a consequence of stroke, but there is limited understanding of the specific contributions of different descending motor pathways toward their control. The corticospinal tract is a major descending motor pathway influencing the production of normal sequential muscle coactivation patterns for skilled movements. However, control of walking is also influenced by non-corticospinal pathways such as the corticoreticulospinal pathway that possibly contribute toward mass flexion-extension co-excitation patterns during walking. The current study sought to investigate the associations between damage to corticospinal (CST) and corticoreticular (CRP) motor pathways following stroke and the presence of mass flexion-extension patterns during walking as evaluated using module analysis. Methods: Seventeen healthy controls and 44 stroke survivors were included in the study. We used non-negative matrix factorization for module analysis of paretic leg electromyographic activity. We typically have observed four modules during walking in healthy individuals. Stroke survivors often have less independently timed modules, for example two-modules presented as mass flexion-extension pattern. We used diffusion tensor imaging-based analysis where streamlines connecting regions of interest between the cortex and brainstem were computed to evaluate CST and CRP integrity. We also used a coarse classification tree analysis to evaluate the relative CST and CRP contribution toward module control. Results: Interhemispheric CST asymmetry was associated with worse lower extremity Fugl-Meyer score (p = 0.023), propulsion symmetry (p = 0.016), and fewer modules (p = 0.028). Interhemispheric CRP asymmetry was associated with worse lower extremity Fugl-Meyer score (p = 0.009), Dynamic gait index (p = 0.035), Six-minute walk test (p = 0.020), Berg balance scale (p = 0.048), self-selected walking speed (p = 0.041), and propulsion symmetry (p = 0.001). The classification tree model reveled that substantial ipsilesional CRP or CST damage leads to a two-module pattern and poor walking ability with a trend toward increased compensatory contralesional CRP based control. Conclusion: Both CST and CRP are involved with control of modules during walking and damage to both may lead to greater reliance on the contralesional CRP, which may contribute to a two-module pattern and be associated with worse walking performance.

Differences in Muscle Demand and Joint Contact Forces Between Running and Skipping.

Skipping has been proposed as a viable cross-training exercise to running due to its lower knee contact forces and higher whole-body energy expenditure. However, how individual muscle forces, energy expenditure, and joint loading are affected by differences in running and skipping mechanics remains unclear. The purpose of this study was to compare individual muscle forces, energy expenditure, and lower extremity joint contact forces between running and skipping using musculoskeletal modeling and simulations of young adults (n = 5) performing running and skipping at 2.5 m·s-1 on an instrumented treadmill. In agreement with previous work, running had greater knee and patella contact forces than skipping which was accompanied by greater knee extensor energetic demand. Conversely, skipping had greater ankle contact forces and required greater energetic demand from the uniarticular ankle plantarflexors. There were no differences in hip contact forces between gaits. These findings further support skipping as a viable alternative to running if the primary goal is to reduce joint loading at the commonly injured patellofemoral joint. However, for those with ankle injuries, skipping may not be a viable alternative due to the increased ankle loads. These findings may help clinicians prescribe activities most appropriate for a patient's individual training or rehabilitation goals.

Muscle contributions to pre-swing biomechanical tasks influence swing leg mechanics in individuals post-stroke during walking.

BACKGROUND: Successful walking requires the execution of the pre-swing biomechanical tasks of body propulsion and leg swing initiation, which are often impaired post-stroke. While excess rectus femoris activity during swing is often associated with low knee flexion, previous work has suggested that deficits in propulsion and leg swing initiation may also contribute. The purpose of this study was to determine underlying causes of propulsion, leg swing initiation and knee flexion deficits in pre-swing and their link to stiff knee gait in individuals post-stroke. METHODS: Musculoskeletal models and forward dynamic simulations were developed for individuals post-stroke (n = 15) and healthy participants (n = 5). Linear regressions were used to evaluate the relationships between peak knee flexion, braking and propulsion symmetry, and individual muscle contributions to braking, propulsion, knee flexion in pre-swing, and leg swing initiation. RESULTS: Four out of fifteen of individuals post-stroke had higher plantarflexor contributions to propulsion and seven out of fifteen had higher vasti contributions to braking on their paretic leg relative to their nonparetic leg. Higher gastrocnemius contributions to propulsion predicted paretic propulsion symmetry (p = 0.005) while soleus contributions did not. Higher vasti contributions to braking in pre-swing predicted lower knee flexion (p = 0.022). The rectus femoris had minimal contributions to lower knee flexion acceleration in pre-swing compared to contributions from the vasti. However, for some individuals with low knee flexion, during pre-swing the rectus femoris absorbed more power and the iliopsoas contributed less power to the paretic leg. Total musculotendon work done on the paretic leg in pre-swing did not predict knee flexion during swing. CONCLUSIONS: These results emphasize the multiple causes of propulsion asymmetry in individuals post-stroke, including low plantarflexor contributions to propulsion, increased vasti contributions to braking and reliance on compensatory mechanisms. The results also show that the rectus femoris is not a major contributor to knee flexion in pre-swing, but absorbs more power from the paretic leg in pre-swing in some individuals with stiff knee gait. These results highlight the need to identify individual causes of propulsion and knee flexion deficits to design more effective rehabilitation strategies.

Individual muscle responses to mediolateral foot placement perturbations during walking.

Walking requires active control of frontal plane balance through adjustments to mediolateral foot placement and ground reaction forces. Previous work on mediolateral balance perturbations and control of foot placement has often focused on the bilateral gluteus medius muscles. However, additional leg and trunk muscles can influence foot placement by transferring power to the foot and pelvis during swing. Thus, the purpose of this study was to determine individual muscle contributions to balance control following medial and lateral foot placement perturbations. Ten participants performed treadmill walking trials which included perturbations immediately before randomized heel strikes. Muscle contributions to foot placement, ground reaction forces, trunk power and frontal plane external moments during representative perturbed and unperturbed gait cycles were estimated using musculoskeletal modeling and simulation. Net muscle contributions to foot placement were 61 ± 50% more medial during the first recovery step following lateral perturbations and 28 ± 14% less medial in the second recovery step following medial perturbations. Following lateral perturbations, the swing gluteus medius performed 57 ± 50% more lateral work and the stance gluteus medius performed 61 ± 50% more medial work on the foot. Following medial perturbations, the erector spinae performed 39 ± 33% less lateral work on the foot. Changes in net muscle work on the foot were inconsistent with changes in step width, suggesting that changes in step width were not due to active muscle control but rather the mechanical effect of the perturbation. These outcomes provide a foundation for future studies analyzing balance control in populations at risk of falling.

Task-prioritization and balance recovery strategies used by young healthy adults during dual-task walking.

BACKGROUND: Maintaining dynamic balance is an essential task during walking, with foot-placement playing a critical role. Dual-task studies analyzing steady-state walking with cognitive loads have found healthy adults prioritize cognitive task performance at the expense of maintaining control of their balance. However, few studies have focused on the influence of cognitive loads on more difficult motor tasks, such as walking with unexpected foot-placement perturbations. Individuals often recover from a loss of balance using an ankle or hip strategy; however, how cognitive loads affect these balance recovery strategies remains unknown. RESEARCH QUESTION: How do individuals prioritize cognitive resources and does the balance recovery strategy used change following mediolateral foot-placement perturbations during steady-state walking when performing cognitive tasks of increasing difficulty? METHODS: Fifteen young healthy adults walked during unperturbed and perturbed conditions with increasing cognitive loads (no cognitive load, attentive listening, spelling short words backwards and spelling long words backwards). No specific task-prioritization instructions were given. Medial and lateral foot-placement perturbations were applied prior to heel-strike during random steps. RESULTS: Cognitive performance decreased between the unperturbed and perturbed conditions. While balance control decreased during perturbed relative to unperturbed walking, the additional cognitive load had little effect on balance control during the perturbations. Lastly, the balance recovery strategy used, as measured by peak joint moments at the ankle and hip, was unaffected by the additional cognitive loads. SIGNIFICANCE: Individuals appear to prioritize their balance control over cognitive performance when experiencing foot-placement perturbations and do not change their balance recovery strategy with the addition of a cognitive load. These results highlight the flexibility of task-prioritization in young adults and provide a foundation for future studies analyzing neurologically impaired populations.

Motion envelopes: unfolding longitudinal rotation data from walking stick-figures.

This work presents Motion Envelopes (ME), a simple method to estimate the missing longitudinal rotations of minimal stick figures, which is based on the spatial-temporal surface traced by line segments that connect contiguous pairs of joints. We validate ME by analyzing the gait patterns of 6 healthy subjects, comprising a total of 18 gait cycles. A strong correlation between experimental and estimated data was obtained for lower limbs and upper arms, indicating that ME can predict their longitudinal orientation in normal gait, hence, ME can be used to complement the kinematic information of stick figures whenever it is incomplete.

Differences in balance control between healthy younger and older adults during steady-state walking.

Each year approximately one third of older adults fall and experience extensive musculoskeletal injuries and functional disabilities. An important element in maintaining dynamic balance is the regulation of whole-body angular momentum, which is achieved by proper foot placement with respect to the body center-of-mass as well as generation of appropriate ground reaction forces. Analyzing these quantities in younger and older adults may provide insight into differences in their underlying mechanics for maintaining dynamic balance. This study examined three-dimensional whole-body angular momentum in 13 healthy older (71.8 ± 8.3 years) and 9 younger (23.2 ± 2.8 years) adults walking at their self-selected and fastest-comfortable speeds. The older adults had a significantly higher range of frontal-plane angular momentum compared to the younger adults at both speeds, suggesting poorer mediolateral balance control. This difference was related to the older adults having a wider foot placement with respect to the body center-of-mass, which when combined with the vertical ground reaction force, created a higher destabilizing external moment during single-limb stance that acts to rotate the body towards the contralateral swing leg. To counteract this destabilizing moment, the older adults generated a higher hip abduction moment. There were no differences in the range of sagittal- and transverse-plane angular momentum between age groups at either speed. These results suggest that control of dynamic balance in the frontal-plane is more challenging than in the sagittal-plane for older adults and highlight the importance of proper weight transfer mechanisms and hip abductor force production for maintaining mediolateral balance during walking.

The influence of cognitive load on balance control during steady-state walking.

For an individual to successfully walk, they must maintain control of their dynamic balance. However, situations that require increased cognitive attention may impair an individual's ability to actively control their balance. While dual-task studies have analyzed walking-while-talking conditions, few studies have focused specifically on the influence of cognitive load on balance control. The purpose of this study was to assess how individuals prioritize their cognitive resources and control dynamic balance during dual-task conditions of varying difficulty. Young healthy adults (n = 15) performed two single-task conditions (spelling-while-standing and treadmill walking with no cognitive load) and three dual-task conditions (treadmill walking with increasing cognitive load: attentive listening and spelling short and long words backwards). Cognitive performance did not change between the single- and dual-task as measured by spelling percent error and response rate (p = 0.300). Balance control, assessed using the range of whole-body angular momentum, did not change between the no load and listening conditions, but decreased during the short and long spelling conditions (p < 0.001). These results highlight that in young adults balance control decreases during dual-task treadmill walking with increased cognitive loads, but their cognitive performance does not change. The decrease in balance control suggests that participants prioritized cognitive performance over balance control during these dual-task walking conditions. This work offers additional insight into the automaticity of walking and task-prioritization in healthy young individuals and provides the basis for future studies to determine differences in neurologically impaired populations.

Biomechanical response to mediolateral foot-placement perturbations during walking.

Dynamic balance in the frontal plane requires active control, which is accomplished largely through control of mediolateral foot placement. Individuals without mobility impairments have the ability to compensate for variability in foot-placement to maintain their balance; however, it is unknown how individuals respond to unexpected mediolateral perturbations to their foot placement that alter their balance control. The purpose of this study was to identify the biomechanical responses of individuals without mobility impairments to medial and lateral foot-placement perturbations during walking. Three-dimensional body segment kinematic and ground reaction force data were collected from 15 participants at 1.0 m/s and their self-selected speed on an instrumented treadmill. Dynamic balance was assessed by analyzing whole-body angular momentum in the frontal plane. We hypothesized that participants would respond to the perturbations with a combination of a lateral ankle strategy, hip adduction strategy and/or ankle push-off strategy to restore their balance. Overall, the medial perturbations adversely affected dynamic balance while lateral perturbations had little effect. Individuals responded to medial (lateral) perturbations with an increased (decreased) ankle inversion moment, which correlated to lateral (medial) shifts in their foot center of pressure. In addition, individuals responded to medial (lateral) perturbations with a decreased (slightly decreased) hip abduction moment. Contrary to our hypothesis, we did not observe an ankle push-off moment response but rather, a small response in the opposite direction. These results highlight the response of individuals without mobility impairments to unexpected foot-placement perturbations and provide a basis of comparison for those with impaired balance control.

The relationship between the hand pattern used during fast wheelchair propulsion and shoulder pain development.

Up to 84% of manual wheelchair users (MWCU) with spinal cord injury experience shoulder pain, which is correlated with shoulder adductor weakness in this population. Modeling studies have shown weak shoulder adductors lead to compensations from the deltoid and rotator cuff muscles during propulsion, which may lead to altered propulsion mechanics. However, the role recovery phase hand pattern has in pain development is unclear, as each hand pattern is associated with unique mechanics and different levels of muscle demand. Previous research found no correlation between hand pattern and shoulder pain at self-selected speeds. However, fast propulsion may exacerbate poor mechanics caused by shoulder muscle weakness, which may reveal those at risk for pain development. The present study evaluated whether the hand pattern used during fast wheelchair propulsion is correlated with shoulder pain. We also assessed whether shoulder adductor strength was correlated with hand pattern. Fast propulsion data from two subsets of MWCU were analyzed at three time points (baseline, 18 months, 36 months). All participants were pain-free at baseline. Subset 1 compared individuals who remained pain-free to those who developed shoulder pain. Subset 2 compared individuals with chronic pain at follow-up to those whose pain resolved over time. The hand pattern used was not different between groups in either subset. However, more over-rim patterns were correlated with lower adductor strength in Subset 1. These results suggest that although the hand pattern used during fast propulsion is not correlated with shoulder pain, more over-rim hand patterns may indicate weaker shoulder adductors.