Centre of Precision Rehabilitation for Spinal Pain (CPR Spine), School of Sport, Exercise and Rehabilitation Sciences, College of Life and Environmental Sciences, University of Birmingham, Birmingham, UK.
Department of Clinical and Experimental Sciences, Università degli Studi di Brescia, Brescia, Italy.
J Appl Physiol (1985). 2021 Oct 1;131(4):1260-1271. doi: 10.1152/japplphysiol.01011.2020. Epub 2021 Sep 2.
At high forces, the discharge rates of lower- and higher-threshold motor units (MU) are influenced in a different way by muscle pain. These differential effects may be particularly important for performing contractions at different speeds since the proportion of lower- and higher-threshold MUs recruited varies with contraction velocity. We investigated whether MU discharge and recruitment strategies are differentially affected by pain depending on their recruitment threshold (RT), across a range of contraction speeds. Participants performed ankle dorsiflexion sinusoidal-isometric contractions at two frequencies (0.25 and 1 Hz) and two modulation amplitudes [5% and 10% of the maximum voluntary contraction (MVC)] with a mean target torque of 20%MVC. High-density surface electromyography recordings from the tibialis anterior muscle were decomposed and the same MUs were tracked across painful (hypertonic saline injection) and nonpainful conditions. Torque variability, mean discharge rate (MDR), DR variability (DRvar), RT, and the delay between the cumulative spike train and the resultant torque output (neuromechanical delay, NMD) were assessed. The average RT was greater at faster contraction velocities ( = 0.01) but was not affected by pain. At the fastest contraction speed, torque variability and DRvar were reduced ( < 0.05) and MDR was maintained. Conversely, MDR decreased and DRvar and NMD increased significantly during pain at slow contraction speeds ( < 0.05). These results show that reductions in contraction amplitude and increased recruitment of higher-threshold MUs at fast contraction speeds appear to compensate for the inhibitory effect of nociceptive inputs on lower-threshold MUs, allowing the exertion of fast submaximal contractions during pain. Pain induces changes in motor performance, motor unit recruitment, and rate coding behavior that varies across different contraction speeds. Here we show that that pain reduces motor unit discharge rate and prolongs the neuromechanical delay at slow contraction speeds only. This new evidence suggests that there are differential nociceptive inhibitory effects across the motor unit pool, which allows fast submaximal contractions to be exerted despite the presence of pain.
在高力下，较低和较高阈值运动单位 (MU) 的放电率受到肌肉疼痛的不同影响。这些差异效应对于以不同速度进行收缩可能特别重要，因为较低和较高阈值 MU 的募集比例随收缩速度而变化。我们研究了在不同的收缩速度下，MU 放电和募集策略是否因疼痛而产生差异，这取决于它们的募集阈值 (RT)。参与者以两种频率 (0.25 和 1 Hz) 和两种调制幅度 [最大随意收缩 (MVC) 的 5%和 10%] 进行踝关节背屈正弦等长收缩，平均目标扭矩为 20%MVC。从前胫骨肌采集高密度表面肌电图记录，并在疼痛 (高渗盐水注射) 和非疼痛条件下跟踪相同的 MU。评估扭矩变异性、平均放电率 (MDR)、DR 变异性 (DRvar)、RT 和累积尖峰序列与产生的扭矩输出之间的延迟 (神经机械延迟，NMD)。平均 RT 在较快的收缩速度下更大 ( = 0.01)，但不受疼痛影响。在最快的收缩速度下，扭矩变异性和 DRvar 降低 ( < 0.05)，MDR 保持不变。相反，在较慢的收缩速度下，疼痛时 MDR 降低，DRvar 和 NMD 显著增加 ( < 0.05)。这些结果表明，在较快的收缩速度下，收缩幅度的减小和较高阈值 MU 的募集增加似乎补偿了伤害性传入对较低阈值 MU 的抑制作用，从而允许在疼痛下进行快速的次最大收缩。疼痛会引起运动表现、运动单位募集和率编码行为的变化，这些变化在不同的收缩速度下有所不同。在这里，我们表明疼痛仅在较慢的收缩速度下降低运动单位放电率并延长神经机械延迟。这一新证据表明，运动单位池中存在差异的伤害性抑制效应，这使得尽管存在疼痛，仍能进行快速的次最大收缩。
