Baudry Stéphane, Rudroff Thorsten, Pierpoint Lauren A, Enoka Roger M
Department of Integrative Physiology, University of Colorado, Boulder, CO 80309-0354, USA.
J Neurophysiol. 2009 Sep;102(3):1725-35. doi: 10.1152/jn.00382.2009. Epub 2009 Jul 22.
Twenty subjects participated in four experiments designed to compare time to task failure and motor-unit recruitment threshold during contractions sustained at 15% of maximum as the elbow flexor muscles either supported an inertial load (position task) or exerted an equivalent constant torque against a rigid restraint (force task). Subcutaneous branched bipolar electrodes were used to record single motor unit activity from the biceps brachii muscle during ramp contractions performed before and at 50 and 90% of the time to failure for the position task during both fatiguing contractions. The time to task failure was briefer for the position task than for the force task (P=0.0002). Thirty and 29 motor units were isolated during the force and position tasks, respectively. The recruitment threshold declined by 48 and 30% (P=0.0001) during the position task for motor units with an initial recruitment threshold below and above the target force, respectively, whereas no significant change in recruitment threshold was observed during the force task. Changes in recruitment threshold were associated with a decrease in the mean discharge rate (-16%), an increase in discharge rate variability (+40%), and a prolongation of the first two interspike intervals (+29 and +13%). These data indicate that there were faster changes in motor unit recruitment and rate coding during the position task than the force task despite a similar net muscle torque during both tasks. Moreover, the results suggest that the differential synaptic input observed during the position task influences most of the motor unit pool.
20名受试者参与了四项实验，这些实验旨在比较在以最大力量的15%持续收缩时，当肘部屈肌支撑惯性负荷（位置任务）或对抗刚性约束施加等效恒定扭矩（力任务）时，达到任务失败的时间和运动单位募集阈值。在疲劳收缩期间，使用皮下分支双极电极记录肱二头肌在位置任务达到失败时间的50%和90%时以及之前进行的斜坡收缩过程中的单个运动单位活动。位置任务达到任务失败的时间比力任务更短（P = 0.0002）。在力任务和位置任务期间分别分离出30个和29个运动单位。对于初始募集阈值低于和高于目标力的运动单位，在位置任务期间募集阈值分别下降了48%和30%（P = 0.0001），而在力任务期间未观察到募集阈值有显著变化。募集阈值的变化与平均放电率降低（-16%）、放电率变异性增加（+40%）以及前两个峰间间隔延长（+29%和+13%）有关。这些数据表明，尽管两个任务中的净肌肉扭矩相似，但在位置任务期间运动单位募集和速率编码的变化比力任务更快。此外，结果表明在位置任务期间观察到的不同突触输入影响了大多数运动单位池。
