Adam Alexander, De Luca Carlo J
Department of Biomedical Engineering, Boston University, Boston, Massachusetts 02215, USA.
J Neurophysiol. 2003 Nov;90(5):2919-27. doi: 10.1152/jn.00179.2003.
Motor-unit firing patterns were studied in the vastus lateralis muscle of five healthy young men [21.4 +/- 0.9 (SD) yr] during a series of isometric knee extensions performed to exhaustion. Each contraction was held at a constant torque level, set to 20% of the maximal voluntary contraction at the beginning of the experiment. Electromyographic signals, recorded via a quadrifilar fine wire electrode, were processed with the precision decomposition technique to identify the firing times of individual motor units. In repeat experiments, whole-muscle mechanical properties were measured during the fatigue protocol using electrical stimulation. The main findings were a monotonic decrease in the recruitment threshold of all motor units and the progressive recruitment of new units, all without a change of the recruitment order. Motor units from the same subject showed a similar time course of threshold decline, but this decline varied among subjects (mean threshold decrease ranged from 23 to 73%). The mean threshold decline was linearly correlated (R2 >or= 0.96) with a decline in the elicited peak tetanic torque. In summary, the maintenance of recruitment order during fatigue strongly supports the notion that the observed common recruitment adaptations were a direct consequence of an increased excitatory drive to the motor unit pool. It is suggested that the increased central drive was necessary to compensate for the loss in force output from motor units whose muscle fibers were actively contracting. We therefore conclude that the control scheme of motor-unit recruitment remains invariant during fatigue at least in relatively large muscles performing submaximal isometric contractions.
在五名健康年轻男性（年龄[21.4±0.9（标准差）岁]）的股外侧肌中，研究了一系列等长膝关节伸展直至力竭过程中的运动单位放电模式。每次收缩都保持在恒定的扭矩水平，在实验开始时设定为最大自主收缩的20%。通过四线细钢丝电极记录的肌电信号，采用精确分解技术进行处理，以识别单个运动单位的放电时间。在重复实验中，在疲劳实验过程中使用电刺激测量全肌肉的力学特性。主要发现是所有运动单位的募集阈值单调下降，以及新单位的逐渐募集，且募集顺序均未改变。同一受试者的运动单位显示出相似的阈值下降时间进程，但这种下降在不同受试者之间有所不同（平均阈值下降范围为23%至73%）。平均阈值下降与诱发的强直收缩峰值扭矩下降呈线性相关（R2≥0.96）。总之，疲劳期间募集顺序的维持有力地支持了这样一种观点，即观察到的共同募集适应性是运动单位池兴奋性驱动增加的直接结果。有人认为，增加的中枢驱动对于补偿那些肌肉纤维正在积极收缩的运动单位的力输出损失是必要的。因此，我们得出结论，至少在进行次最大等长收缩的相对较大肌肉中，运动单位募集的控制方案在疲劳期间保持不变。
