Lower limb vertical stiffness and frontal plane angular impulse during perturbation-induced single limb stance and their associations with gait in individuals post-stroke.

After stroke, deficits in paretic single limb stance (SLS) are commonly observed and affect walking performance. During SLS, the hip abductor musculature is critical in providing vertical support and regulating balance. Although disrupted paretic hip abduction torque production has been identified in individuals post-stroke, interpretation of previous results is limited due to the discrepancies in weight-bearing conditions. Using a novel perturbation-based assessment that could induce SLS by removing the support surface underneath one limb, we aim to investigate whether deficits in hip abduction torque production, vertical body support, and balance regulation remain detectable during SLS when controlling for weight-bearing, and whether these measures are associated with gait performance. Our results showed that during the perturbation-induced SLS, individuals post-stroke had lower hip abduction torque, less vertical stiffness, and increased frontal plane angular impulse at the paretic limb compared to the non-paretic limb, while no differences were found between the paretic limb and healthy controls. In addition, vertical stiffness during perturbation-induced SLS was positively correlated with single support duration during gait at the paretic limb and predicted self-selected and fast walking speeds in individuals post-stroke. The findings indicate that reduced paretic hip abduction torque during SLS likely affects vertical support and balance control. Enhancing SLS hip abduction torque production could be an important rehabilitation target to improve walking function for individuals post-stroke.

Leading limb biomechanical response following compelled forward and descending body shift in old versus young adults.

BACKGROUND: Falls pose a significant health risk in older adults, with stair descent falls carrying particularly severe consequences. Reduced balance control and limb support due to aging-related physiological and neuromuscular decline are critical components in increased falling risk in older adults. Understanding the age-associated abnormalities in balance control and limb support strategies during sudden forward and downward body shift could reveal potential biomechanical deficits responsible for increased falling risks in older adults. This study investigates balance regulatory responses following first-time exposure to compelled forward and downward body shift in young and older adults. METHODS: Thirteen healthy old and thirteen healthy young adults participated in this study. Participants stood on two adjacent perturbation platforms in modified tandem stance. The leading limb support surface dropped 3 in. vertically at an unknown time. The anterior margin of stability and center of mass velocity, peak vertical ground reaction forces, and leading limb ankle and knee joint angular displacement, torque, and power during the initial response phase were compared between age groups. FINDINGS: Compared to young adults, older adults showed higher center of mass velocity, lower margin of stability, peak vertical ground reaction force, peak ankle and knee joint power, and peak knee joint torque during the initial response phase. INTERPRETATIONS: The abnormalities potentially identified in our study, particularly in dynamic stability regulation, limb support force generation, and shock absorption may affect the ability to arrest the body's forward and downward motion. These deficits may contribute to an increased risk of forward falls in aging.

Biomechanical and neuromuscular control characteristics of sit-to-stand transfer in young and older adults: A systematic review with implications for balance regulation mechanisms.

BACKGROUND: Falls are major health concerns in older adults. Sit-to-stand transfer is an important functional movement that can predict falling risk in older adults. Aging-associated declines in neuromechanical control of movement may negatively impact sit-to-stand performance. This systematic review aims to summarize differences in neuromechanical characteristics of younger vs. older adults that likely affect balance regulation during sit-to-stand. METHODS: Five databases (Academic search complete, MEDLINE, APA PsycInfo, Pubmed, and SPORTDiscus) were systematically searched from January 1985 through March 2023. Three reviewers assessed the quality of methodology, study design, results, and risk of bias using the Appraisal tool for Cross-Sectional Studies. Studies reported neuromuscular and biomechanical characteristics during sit-to-stand in young versus older adults were included. FINDINGS: Seventeen studies (343 older and 225 younger adults) were included. Compared to younger adults, older adults showed slower sit-to-stand time, higher trunk flexion, postural sway, agonist-antagonist muscle co-activation of the ankle and knee muscles, and lower ankle dorsiflexion torque. Lower magnitude and rate of vertical ground reaction force development and lower vertical momentum during rising were observed with aging during fast-speed sit-to-stand. There was heterogeneity among studies on sit-to-stand speed, foot position, use of arms, and seat height adjustability. INTERPRETATIONS: Higher trunk angular displacement and velocity accompanied by higher anterior momentum, likely to compensate for knee extensor muscle weaknesses, may lead to higher postural sway upon standing and therefore require higher knee and ankle muscle co-activation to maintain balance stability. Thus, additional attention to trunk control strategies is needed during clinical evaluations.

Lower Limb Vertical Stiffness and Frontal Plane Angular Impulse during Perturbation-Induced Single Limb Stance and Their Associations with Gait in Individuals Post-Stroke.

Background: After stroke, deficits in paretic single limb stance (SLS) are commonly observed and affect walking performance. During SLS, the hip abductor musculature is critical in providing vertical support and regulating balance. Although disrupted paretic hip abduction torque production has been identified in individuals post-stroke, interpretation of previous results is limited due to the discrepancies in weight-bearing conditions. Objective: To investigate whether deficits in hip abduction torque production, vertical body support, and balance regulation remain during SLS when controlling for weight-bearing using a perturbation-based assessment, and whether these measures are associated with gait performance. Methods: We compared hip abduction torque, vertical stiffness, and frontal plane angular impulse between individuals post-stroke and healthy controls when SLS was induced by removing the support surface underneath one limb. We also tested for correlations between vertical stiffness and angular impulse during perturbation-induced SLS and gait parameters during overground walking. Results: During the perturbation-induced SLS, lower hip abduction torque, less vertical stiffness, and increased frontal plane angular impulse were observed at the paretic limb compared to the non-paretic limb, while no differences were found between the paretic limb and healthy controls. Vertical stiffness during perturbation-induced SLS was positively correlated with single support duration during gait at the paretic limb and predicted self-selected and fast walking speeds in individuals post-stroke. Conclusions: Reduced paretic hip abduction torque during SLS likely affects vertical support and balance control. Enhancing SLS hip abduction torque production could be an important rehabilitation target to improve walking function for individuals post-stroke.

Neuromechanical control of impact absorption during induced lower limb loading in individuals post-stroke.

Decreased loading of the paretic lower limb and impaired weight transfer between limbs negatively impact balance control and forward progression during gait in individuals post-stroke. However, the biomechanical and neuromuscular control mechanisms underlying such impaired limb loading remain unclear, partly due to their tendency of avoiding bearing weight on the paretic limb during voluntary movement. Thus, an approach that forces individuals to more fully and rapidly load the paretic limb has been developed. The primary purpose of this study was to compare the neuromechanical responses at the ankle and knee during externally induced limb loading in people with chronic stroke versus able-bodied controls, and determine whether energy absorption capacity, measured during induced limb loading of the paretic limb, was associated with walking characteristics in individuals post-stroke. Results revealed reduced rate of energy absorption and dorsiflexion velocity at the ankle joint during induced limb loading in both the paretic and non-paretic side in individuals post-stroke compared to healthy controls. The co-contraction index was higher in the paretic ankle and knee joints compared to the non-paretic side. In addition, the rate of energy absorption at the paretic ankle joint during the induced limb loading was positively correlated with maximum walking speed and negatively correlated with double limb support duration. These findings demonstrated that deficits in ankle dorsiflexion velocity may limit the mechanical energy absorption capacity of the joint and thereby affect the lower limb loading process during gait following stroke.