Neuromuscular mechanisms of motor adaptation to repeated treadmill-slip perturbations during stance in healthy young adults.

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Treadmill-based repeated perturbation training (PBT) induces motor adaptation in reactive balance responses, thus lowering the risk of slip-induced falls. However, little evidence exists regarding intervention-induced changes in neuromuscular control underlying motor adaptation. Examining neuromuscular changes could be an important step in identifying key elements of adaptation and evaluating treadmill training protocols for fall prevention. Moreover, identifying the muscle synergies contributing to motor adaptation in young adults could lay the groundwork for comparison with high fall-risk populations. Thus, we aimed to investigate neuromuscular changes in reactive balance responses during stance slip-PBT. Lower limb electromyography (EMG) signals (4/leg) were recorded during ten repeated forward stance (slip-like) perturbations in twenty-six young adults. Muscle synergies were compared between early-training (slips 1-2) and late-training (slips 9-10) stages. Results showed that 5 different modes of synergies (named on dominant muscles: WTA, WS_VLAT, WR_GAS, WR_VLAT, and WS_GAS) were recruited in both stages. 3 out of 5 synergies (WTA, WR_VLAT, and WS_GAS) showed a high similarity (r>0.97) in structure and activation between stages, whereas WR_GAS and WS_VLAT showed a lower similarity (r<0.83) between the two stages, and the area of activation in WTA, the peak value of activation in WR_VLAT and the activation onset in WR_GAS showed a reduction from early-to late-training stage (p<0.05). These results suggest that a block of stance slip-PBT resulted in modest changes in muscle synergies in young adults, which might explain the smaller changes seen in biomechanical variables. Future studies should examine neuromuscular changes in people at high risk of falls.