School of Health and Exercise Sciences, Faculty of Health and Social Development, University of British Columbia, Okanagan, British Columbia, Canada.
Department of Human Health and Nutritional Sciences, College of Biological Sciences, University of Guelph, Guelph, Ontario, Canada.
J Neurophysiol. 2020 Jun 1;123(6):2209-2216. doi: 10.1152/jn.00394.2019. Epub 2020 Apr 29.
Following active lengthening, steady-state isometric (ISO) torque is greater than a purely ISO contraction at the same muscle length, this is referred to as residual torque enhancement (rTE). A phenomenon of rTE is activation reduction, characterized by reduced electromyography (EMG) amplitude for a given torque output. We hypothesized that lower motor unit discharge rates would contribute to activation reduction and lessening torque steadiness. Ten young male subjects performed ISO dorsiflexion contractions at 10 and 20% of maximal voluntary contraction (MVC) torque. During rTE trials, the muscle was activated at 10° of plantar flexion, then the ankle was rotated to the ISO position at 40°. Fine wire electrodes recorded motor unit (MU)-discharge rates and variability from the tibialis anterior. Surface EMG quantified activation reduction, and steadiness was determined as the coefficient of variation of torque. The activation reduction was 44 and 24% at 10 and 20% MVC, respectively ( < 0.05). Fewer MUs were recorded in the rTE than ISO condition at 10% (47%) and 20% (36%) MVC ( < 0.05). Discharge rates were 19 and 26% lower in the rTE compared with the ISO condition for 10 and 20% MVC, respectively ( < 0.05), with no difference in variability between conditions ( > 0.05). Steadiness was ~22 and 18% lower for the rTE than ISO condition at 10 and 20% MVC ( < 0.05). Our findings indicate that activation reduction may be attributed to lower MU discharge rate and fewer detectable MUs and that this theoretically contributes to a reduction in steadiness in the rTE condition. Our findings indicate that lower electromyographic activity during the torque enhanced condition following active lengthening compared with a purely isometric contraction arises from fewer active motor units and a lower discharge rate of those that are active. We used an acute condition of increased torque capacity to induce a decrease in net output of the motor neuron pool during a submaximal task to demonstrate, in humans, the impact of motor unit activity on torque steadiness.
主动牵伸后，等长（ISO）状态下的扭矩大于相同肌肉长度下的纯 ISO 收缩，这被称为残余扭矩增强（rTE）。rTE 的一个现象是激活减少，其特征是给定扭矩输出时肌电图（EMG）幅度减小。我们假设较低的运动单位放电率将有助于减少激活并降低扭矩稳定性。10 名年轻男性受试者以最大自主收缩（MVC）的 10%和 20%进行 ISO 背屈收缩。在 rTE 试验中，肌肉在跖屈 10°时被激活，然后踝关节以 40°旋转至 ISO 位置。细电线电极记录胫骨前肌的运动单位（MU）放电率和变异性。表面肌电图量化了激活减少，稳定性由扭矩变异系数确定。rTE 时的激活减少分别为 10% MVC 的 44%和 20% MVC 的 24%（<0.05）。rTE 时记录的 MU 少于 ISO 条件下的 MU，分别为 10% MVC 时的47%和 20% MVC 时的36%（<0.05）。rTE 时的放电率分别比 ISO 条件下的放电率低 19%和 26%，分别用于 10% MVC 和 20% MVC（<0.05），但条件之间的变异性没有差异（>0.05）。rTE 时的稳定性比 ISO 条件低约 22%和 18%，分别用于 10% MVC 和 20% MVC（<0.05）。我们的发现表明，激活减少可能归因于较低的 MU 放电率和较少可检测的 MU，这理论上导致 rTE 条件下的稳定性降低。我们的发现表明，与纯 ISO 收缩相比，主动牵伸后增强扭矩状态下的肌电图活动减少，原因是主动运动单位减少，且活跃运动单位的放电率降低。我们使用增加扭矩能力的急性条件，在亚最大任务期间减少运动神经元池的净输出，从而在人类中证明了运动单位活动对扭矩稳定性的影响。
