Torres-Oviedo, G. | bioRxiv Science

bioRxiv2018

The adaptation of muscle activity during split-belt walking reveals age-dependent decline of motor learning

Recent studies suggest that planned and corrective actions are recalibrated during some forms of motor adaptation. However, corrective (a.k.a., reactive) movements in human locomotion are thought to simply reflect sudden environmental changes independently from sensorimotor recalibration. Thus, we asked if corrective responses can indicate the motor systems adapted state following prolonged exposure to a novel walking situation inducing sensorimotor adaptation. We recorded electromyographic signals bilaterally on 15 leg muscles before, during, and after split-belts walking (i.e., novel walking situation), in which the legs move at different speeds. We exploited the rapid temporal dynamics of corrective responses upon introduction or removal of an unexpected speed transition to isolate them from the overall motor output. We found that corrective muscle activity was structurally different following short vs. long exposures to split-belts walking. Only after a long exposure, removal of the novel environment elicited corrective muscle patterns that matched those expected in response to a perturbation opposite to the one originally experienced. This indicated that individuals who recalibrated their motor system adopted split-belts environment as their new "normal" and transitioning back to the original walking environment causes subjects to react as if it was novel to them. Interestingly, this learning declined with age, but steady state modulation of muscle activity during split-belts walking did not, suggesting potentially different neural mechanisms underlying these motor patterns. Taken together, our results show that corrective motor commands reflect the adapted state of the motor system, which is less flexible as we age. Significance statementWe showed that corrective muscle activity elicited by sudden environmental transitions is revealing of the underlying recalibration process during sensorimotor adaptation. This allowed us to identify age-related decline in motor learning that was not discernible from kinematic measures conventionally used in motor adaptation studies. These findings suggest that older populations may have limited potential to correct their movements through error-based protocols simply given their age. Moreover, we describe the distinct motor patterns recruited during and after the split condition, informing our understanding of the therapeutic effect of this task. Therefore, our detailed EMG characterization provides valuable normative data of muscle activity that could be reinforced with repeated exposure of split-belts walking.

neuroscience↗

bioRxiv2018

Large propulsion demands increase locomotor learning at the expense of step length asymmetry

There is a clinical interest in increasing the extent of locomotor learning induced by split-belt treadmills that move each leg at different speeds. However, factors facilitating locomotor learning are poorly understood. We hypothesized that augmenting the braking forces, rather than propulsion forces, experienced at the feet would increase locomotor adaptation and learning. To test this, forces were modulated during split-belt walking with distinct slopes: inclined (larger propulsion than braking), declined (larger braking than propulsion), and flat (similar propulsion and braking). These groups were compared using step length asymmetry, which is a clinically relevant measure robustly adapted on split-belt treadmills. Unexpectedly, the group with larger propulsion demands (i.e., the incline group) adapted faster and more, and had larger after-effects when the split-belt perturbation was removed. We also found that subjects who propelled more during baseline and experienced larger disruptions of propulsion forces in early adaptation exhibited greater after-effects, which further highlights the catalytic role of propulsion on locomotor learning. The relevance of mechanical demands on shaping our movements was also indicated by the steady state split-belt behavior, during which each group recovered their baseline leg orientation to meet leg-specific force demands at the expense of step length symmetry. Notably, the flat group was nearly symmetric, whereas the incline and decline group overshot and undershot symmetry, respectively. Taken together, our results indicate that forces propelling the body facilitate gait adaptation during split-belt walking. Therefore, interventions that increase propulsion demands may be a viable strategy for augmenting locomotor learning in individuals with hemiparesis.\n\nKey Points SummaryO_LISplit-belt walking (i.e., legs moving at different speeds) can induce locomotor learning and even improve hemiparetic gait, but little is known about how to facilitate this process.\nC_LIO_LIWe investigated the effect of braking and propulsion forces on locomotor learning by testing young unimpaired subjects on the split-belt condition at different slopes (i.e., flat, decline, and incline), which distinctively modified these forces.\nC_LIO_LIPropulsion forces facilitated locomotor learning indicated by 1) greater adaptation and after-effects following split-belt walking of the inclined group, which experienced larger propulsion demands and 2) a positive correlation between individual after-effects and subject-specific propulsion during regular walking and initial steps in the split condition.\nC_LIO_LIInterestingly, incline and decline groups self-selected asymmetric step lengths at steady state in the split condition, challenging the prominent view that step length asymmetry is a biomarker for inefficient gait.\nC_LIO_LIOur results suggest that interventions augmenting propulsion demands could correct hemiparetic gait more effectively.\nC_LI

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