Age-related changes in muscle coordination patterns of stepping responses to recover from loss of balance.

INTRODUCTION: Reactive stepping capacity to recover from a loss of balance declines with aging, which increases the risk of falling. To gain insight into the underlying mechanisms, we investigated whether muscle coordination patterns of reactive stepping differed between healthy young and older individuals. METHODS: We performed a cross-sectional study between 15 healthy young and 14 healthy older adults. They recovered from 200 multidirectional platform translations that evoked reactive stepping responses. We determined spatiotemporal step variables and used muscle synergy analysis to characterize stance- and swing-leg muscle coordination patterns from the start of perturbation until foot landing. RESULTS: We observed delayed step onsets in older individuals, without further spatiotemporal differences. Muscle synergy structure was not different between young and older individuals, but age-related differences were observed in the time-varying synergy activation patterns. In anterior-posterior directions, the older individuals demonstrated significantly enhanced early swing-leg synergy activation consistent with non-stepping behavior. In addition, around step onset they demonstrated increased levels of synergy coactivation (mainly around the ankle) in lateral and anterior directions, which did not appear to hamper foot clearance. CONCLUSION: Although synergy structure was not affected by age, the delayed step onsets and the enhanced early synergy recruitment point at a relative bias towards non-stepping behavior in older adults. They may need more time for accumulating information on the direction of perturbation and making the corresponding sensorimotor transformations before initiating the step. Future work may investigate whether perturbation-based training improves these age-related deficits.

Improvements in spatiotemporal outcomes, but not in recruitment of automatic postural responses, are correlated with improved step quality following perturbation-based balance training in chronic stroke.

Introduction: People with stroke often exhibit balance impairments, even in the chronic phase. Perturbation-based balance training (PBT) is a therapy that has yielded promising results in healthy elderly and several patient populations. Here, we present a threefold approach showing changes in people with chronic stroke after PBT on the level of recruitment of automatic postural responses (APR), step parameters and step quality. In addition, we provide insight into possible correlations across these outcomes and their changes after PBT. Methods: We performed a complementary analysis of a recent PBT study. Participants received a 5-week PBT on the Radboud Fall simulator. During pre- and post-intervention assessments participants were exposed to platform translations in forward and backward directions. We performed electromyography of lower leg muscles to identify changes in APR recruitment. In addition, 3D kinematic data of stepping behavior was collected. We determined pre-post changes in muscle onset, magnitude and modulation of recruitment, step characteristics, and step quality. Subsequently, we determined whether improvements in step or muscle characteristics were correlated with improved step quality. Results: We observed a faster gastrocnemius muscle onset in the stance and stepping leg during backward stepping. During forward stepping we found a trend toward a faster tibialis anterior muscle onset in the stepping leg. We observed no changes in modulation or magnitude of muscle recruitment. Leg angles improved by 2.3° in forward stepping and 2.5° in backward stepping. The improvement in leg angle during forward stepping was accompanied by a -4.1°change in trunk angle, indicating a more upright position. Step length, duration and velocity improved in both directions. Changes in spatiotemporal characteristics were strongly correlated with improvements in leg angle, but no significant correlations were observed of muscle onset or recruitment with leg or trunk angle. Conclusion: PBT leads to a multi-factorial improvement in onset of APR, spatiotemporal characteristics of stepping, and reactive step quality in people with chronic stroke. However, current changes in APR onset were not correlated with improvement in step quality. Therefore, we suggest that, in addition to spatiotemporal outcomes, other characteristics of muscle recruitment or behavioral substitution may induce step quality improvement after PBT.

Offline effects of transcranial direct current stimulation on reaction times of lower extremity movements in people after stroke: a pilot cross-over study.

BACKGROUND: Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique that has shown promise for rehabilitation after stroke. Ipsilesional anodal tDCS (a-tDCS) over the motor cortex increases corticospinal excitability, while contralesional cathodal tDCS (c-tDCS) restores interhemispheric balance, both resulting in offline improved reaction times of delayed voluntary upper-extremity movements. We aimed to investigate whether tDCS would also have a beneficial effect on delayed leg motor responses after stroke. In addition, we identified whether variability in tDCS effects was associated with the level of leg motor function. METHODS: In a cross-over design, 13 people with chronic stroke completed three 15-min sessions of anodal, cathodal and sham stimulation over the primary motor cortex on separate days in an order balanced across participants. Directly after stimulation, participants performed a comprehensive set of lower-extremity tasks involving the paretic tibialis anterior (TA): voluntary ankle-dorsiflexion, gait initiation, and backward balance perturbation. For all tasks, TA onset latencies were determined. In addition, leg motor function was determined by the Fugl-Meyer Assessment - leg score (FMA-L). Repeated measures ANOVA was used to reveal tDCS effects on reaction times. Pearson correlation coefficients were used to establish the relation between tDCS effects and leg motor function. RESULTS: For all tasks, TA reaction times did not differ across tDCS sessions. For gait initiation and backward balance perturbation, differences between sham and active stimulation (a-tDCS or c-tDCS) did not correlate with leg motor function. Yet, for ankle dorsiflexion, individual reaction time differences between c-tDCS and sham were strongly associated with FMA-L, with more severely impaired patients exhibiting slower paretic reaction times following c-tDCS. CONCLUSION: We found no evidence for offline tDCS-induced benefits. Interestingly, we found that c-tDCS may have unfavorable effects on voluntary control of the paretic leg in severely impaired patients with chronic stroke. This finding points at potential vicarious control from the unaffected hemisphere to the paretic leg. The absence of tDCS-induced effects on gait and balance, two functionally relevant tasks, shows that such motor behavior is inadequately stimulated by currently used tDCS applications. TRIAL REGISTRATION: The study is registered in the Netherlands Trial Register (NL5684; April 13th, 2016).