This study investigates whether mechanomyography (MMG) produced from contracting muscles as a measure of their performance could be a proxy of muscle fatigue during a sustained functional electrical stimulation (FES)-supported standing-to-failure task. Bilateral FES-evoked contractions of quadriceps and glutei muscles, of four adults with motor-complete spinal cord injury (SCI), were used to maintain upright stance using two different FES frequencies: high frequency (HF – 35 Hz) and low frequency (LF – 20 Hz). The time at 30° knee angle reduction was taken as the point of critical “fatigue failure”, while the generated MMG characteristics were used to track the pattern of force development during stance. Quadriceps fatigue, which was primarily responsible for the knee buckle, was characterized using MMG-root mean square (RMS) amplitude. A double exponential decay model fitted the MMG fatigue data with good accuracy [R ² = 0.85–0.99; root mean square error (RMSE) = 2.12–8.10] implying changes in the mechanical activity performance of the muscle’s motor units. Although the standing duration was generally longer for the LF strategy (31–246 s), except in one participant, when compared to the HF strategy, such differences were not significant (p > 0.05) but suggested a faster muscle fatigue onset during HF stimulation. As MMG could discriminate between different stimulation frequencies, we speculate that this signal can quantify muscle fatigue characteristics during prolonged FES applications.

中文翻译：

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股四头肌力学检查反映了成年人在脊髓损伤的电刺激持续站立过程中的肌肉疲劳-概念证明。

这项研究调查了由收缩肌肉产生的机械能描记术（MMG）是否可以作为在持续功能性电刺激（FES）支持的站立失败过程中肌肉疲劳的指标。FES诱发的四名运动完全性脊髓损伤（SCI）成人的双侧FES引起的股四头肌和臀肌收缩被用于通过两种不同的FES频率来保持直立姿势：高频（HF – 35 Hz）和低频（LF – 20 Hz）。膝关节角度减少30°时的时间被视为严重的“疲劳破坏”点，而生成的MMG特性则用于跟踪姿势期间力量发展的模式。使用MMG均方根（RMS）振幅来表征股四头肌疲劳，这主要是造成膝关节屈曲的原因。R ²  = 0.85–0.99；均方根误差（RMSE）= 2.12–8.10]暗示着肌肉运动单位的机械活动性能发生了变化。尽管通常采用低频策略的站立时间较长（31-246 s），但只有一名参与者除外，与高频策略相比，这种差异并不显着（p> 0.05），但提示高频刺激期间肌肉疲劳发作更快。由于MMG可以区分不同的刺激频率，因此我们推测该信号可以量化长时间FES应用期间的肌肉疲劳特性。