While changes in biomechanical variables during slip adaptation have been well analyzed and documented, the underlying neuromuscular control mechanisms that could contribute towards the movement adaptations are still unclear.Īccording to the muscle synergy hypothesis, the CNS simplifies motor control through the flexible combination of several muscle synergies, which are defined as a set of muscles recruited by a single neural command signal 14. It is known that the changes in human movement kinematics and kinetics result from motor programs generated by the CNS to signal the neuromuscular system. The reactive adaptations occur in the form of a better recovery stepping location, increased magnitude of certain joint moments, and earlier onset of muscle activation 4, 13. After experiencing repeated slip perturbations, participants proactively modify their gait pattern (e.g., step length, flat foot landing, and knee flexion at heel strike), resulting in a reduction in the slip intensity 11, 12. These studies have revealed that the rapid adaptation to slips occurs via improvements in both proactive and reactive control. Among the interventions, repeated slip-perturbation training has shown to be efficacious in reducing laboratory-induced falls over longer-term 4, 9, 10. Hence, abundant research is targeted at developing interventions for lowering the likelihood of slip-induced falls in older adults 4, 5, 6, 7, 8. One major cause of injurious falls is slipping, which is responsible for over 40% of outdoor falls among older adults, and nearly one fifth of fractures 3. The annual fall rates for older adults range from 0.3 to 1.6 times per person, with an annual cost of approximately $31 billion 1, 2. Our findings improved the understanding of the key muscle synergies involved in preventing backward balance loss and how neuromuscular responses adapt through repeated slip training, which might be helpful to design synergy-based interventions for fall prevention. During the late-adaptation phase, the redundant synergies generated in the early-adaptation phase get eliminated as the adaptation process progresses with repeated exposure to the slips, which further consolidates the slip adaptation. Our findings indicated that the central nervous system could generate new muscle synergies through fractionating or modifying the pre-existing synergies in the early-adaptation phase, and these synergies produce motor strategies that could effectively assist in recovery from the slip perturbation. In the late-adaptation stage, only 2 out of these 8 new synergies were retained. A few new patterns (n = 8) of muscle synergies presented in the early-adaptation stage to compensate for motor errors due to external perturbation. The participants retained the majority of muscle synergies (5 out of 7) used in novel slips post adaptation. Correspondingly, there was a significant increase in the muscle synergy numbers from no-adaptation slips to the adapted slips. Results showed that participants made significant improvements in their balance outcomes from novel slips to adapted slips. The similarity between the recruited muscle synergies in these different phases was subsequently analyzed. Muscle synergies in no-adaptation (novel slip), early-adaptation (slip 6 to 8), and late-adaptation trials (slip 22 to 24) were extracted. Electromyography signals during 24 repeated slip trials in gait were collected for 30 healthy older adults. This study investigated changes in neuromuscular control across different stages of slip-adaptation by examining muscle synergies during slip training. However, the changes in neuromuscular control contributing to such motor adaptation remain unclear. Individuals can rapidly develop adaptive skills for fall prevention after their exposure to the repeated-slip paradigm.
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