Electrical stimulation is widely used in rehabilitation to prevent muscle weakness and to assist the functional recovery of neural deficits. Its application is however limited by the rapid development of muscle fatigue due to the non-physiological motor unit (MU) recruitment. This issue can be mitigated by interleaving muscle belly (mStim) and nerve stimulation (nStim) to distribute the temporal recruitment among different MU groups. To be effective, this approach requires the two stimulation modalities to activate minimally-overlapped groups of MUs. In this manuscript, we investigated spatial differences between mStim and nStim MU recruitment through the study of architectural changes of superficial and deep compartments of tibialis anterior (TA). We used ultrasound imaging to measure variations in muscle thickness, pennation angle, and fiber length during mStim, nStim, and voluntary (Vol) contractions at 15% and 25% of the maximal force. For both contraction levels, architectural changes induced by nStim in the deep and superficial compartments were similar to those observed during Vol. Instead, during mStim superficial fascicles underwent a greater change compared to those observed during nStim and Vol, both in absolute magnitude and in their relative differences between compartments. These observations suggest that nStim results in a distributed MU recruitment over the entire muscle volume, similarly to Vol, whereas mStim preferentially activates the superficial muscle layer. The diversity between spatial recruitment of nStim and mStim suggests the involvement of different MU populations, which justifies strategies based on interleaved nerve/muscle stimulation to reduce muscle fatigue during electrically-induced contractions of TA.

中文翻译：

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不同部位电刺激过程中胫腓骨前表浅和深隔室的结构变化

电刺激被广泛用于康复中，以防止肌肉无力并协助神经功能缺损的功能恢复。然而，由于非生理运动单元（MU）募集，肌肉疲劳的迅速发展限制了其应用。可以通过交错插入肌肉腹部（mStim）和神经刺激（nStim）在不同MU组之间分配时间征募来缓解此问题。为了有效，此方法需要两种刺激方式来激活最小重叠的MU组。在这份手稿中，我们通过研究胫前肌（TA）浅表和深部隔室的结构变化，研究了mStim和nStim MU募集之间的空间差异。我们使用了超声成像技术来测量肌肉厚度，下垂角度，和mStim，nStim和自愿（Vol）收缩过程中的纤维长度分别为最大力的15％和25％。对于两种收缩水平，nStim诱导的深层和浅层隔室的结构变化与Vol.2期观察到的相似。相反，在mStim期间，与在nStim和Vol期间所观察到的相比，浅层束的绝对值和它们之间的相对差异都发生了较大的变化。这些观察结果表明，与Vol相似，nStim导致整个肌肉体积上的MU募集分布，而mStim优先激活浅层肌肉层。nStim和mStim的空间募集之间的差异表明，不同的MU人群参与其中，