We investigated changes in motor unit (MU) behaviour and vasti-muscle contractile properties during sustained submaximal fatiguing contractions with a new time-domain tracking technique in order to understand the mechanisms responsible for task failure. Sixteen participants performed a non-fatiguing 15s isometric knee-extension at 50% of the maximum voluntary torque (MVC), followed by a 30% MVC sustained contraction until exhaustion. Two grids of 64 surface electromyography electrodes were placed over vastus medialis and lateralis. Signals were decomposed into MU discharge-times and the MUs from the 30% MVC sustained contraction were followed until task failure by overlapping decomposition intervals. These MUs were then tracked between 50% and 30% MVC. During the sustained fatiguing contraction, MUs of the two muscles decreased their discharge rate until ~40% of the endurance time, referred to as the reversal time, and then increased their discharge rate until task failure. This reversal in firing behaviour predicted total endurance time and was matched by opposite changes in twitch force (increase followed by a decrease). Despite the later increase in MU firing rates, peak discharge rates at task failure did not reach the frequency attained during a non-fatiguing 50% MVC contraction. These results show that changes in MU firing properties are influenced by adjustments in contractile properties during the course of the contraction, allowing the identification of two phases. Nevertheless, the contraction cannot be sustained possibly due to progressive motoneuron inhibition/decreased excitability, as the later increase in firing rate saturates at a much lower frequency compared to a higher-force non-fatiguing contraction.

中文翻译：

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无法增加对肌肉的神经驱动与次最大收缩期间的任务失败有关。

我们使用新的时域跟踪技术研究了持续次最大疲劳收缩期间运动单元 (MU) 行为和大肌收缩特性的变化，以了解导致任务失败的机制。16 名参与者以最大自愿扭矩 (MVC) 的 50% 进行非疲劳 15 秒等长膝关节伸展，然后以 30% MVC 持续收缩直至精疲力竭。两个由 64 个表面肌电电极组成的网格放置在股内侧肌和外侧肌上。信号被分解为 MU 放电时间，并跟踪来自 30% MVC 持续收缩的 MU，直到通过重叠分解间隔导致任务失败。然后在 50% 到 30% MVC 之间跟踪这些 MU。在持续的疲劳收缩期间，两块肌肉的 MU 会降低其放电率，直到约 40% 的耐力时间，称为逆转时间，然后增加其放电率，直到任务失败。射击行为的这种逆转预测了总耐力时间，并且与抽搐力的相反变化相匹配（先增加后减少）。尽管后来 MU 发射率增加，但任务失败时的峰值放电率并未达到非疲劳 50% MVC 收缩期间获得的频率。这些结果表明 MU 发射特性的变化受收缩过程中收缩特性调整的影响，从而可以识别两个阶段。然而，由于进行性运动神经元抑制/兴奋性降低，可能无法持续收缩，