Department of Bioengineering, Imperial College London, London, UK.
Department of Movement, Human and Health Sciences, University of Rome 'Foro Italico', Rome, Italy.
J Physiol. 2019 Apr;597(7):1873-1887. doi: 10.1113/JP277250. Epub 2019 Feb 6.
Previous studies have indicated that several weeks of strength training is sufficient to elicit significant adaptations in the neural drive sent to the muscles. There are few data, however, on the changes elicited by strength training in the recruitment and rate coding of motor units during voluntary contractions. We show for the first time that the discharge characteristics of motor units in the tibialis anterior muscle tracked across the intervention are changed by 4 weeks of strength training with isometric voluntary contractions. The specific adaptations included significant increases in motor unit discharge rate, decreases in the recruitment-threshold force of motor units and a similar input-output gain of the motor neurons. The findings suggest that the adaptations in motor unit function may be attributable to changes in synaptic input to the motor neuron pool or to adaptations in intrinsic motor neuron properties.
The strength of a muscle typically begins to increase after only a few sessions of strength training. This increase is usually attributed to changes in the neural drive to muscle as a result of adaptations at the cortical or spinal level. We investigated the change in the discharge characteristics of large populations of longitudinally tracked motor units in tibialis anterior before and after 4 weeks of strength training the ankle-dorsiflexor muscles with isometric contractions. The adaptations exhibited by 14 individuals were compared with 14 control subjects. High-density electromyogram grids with 128 electrodes recorded the myoelectric activity during isometric ramp contractions to the target forces of 35%, 50% and 70% of maximal voluntary force. The motor unit recruitment and derecruitment thresholds, discharge rate, interspike intervals and estimates of synaptic inputs to motor neurons were assessed. The normalized recruitment-threshold forces of the motor units were decreased after strength training (P < 0.05). Moreover, discharge rate increased by 3.3 ± 2.5 pps (average across subjects and motor units) during the plateau phase of the submaximal isometric contractions (P < 0.001). Discharge rates at recruitment and derecruitment were not modified by training (P < 0.05). The association between force and motor unit discharge rate during the ramp-phase of the contractions was also not altered by training (P < 0.05). These results demonstrate for the first time that the increase in muscle force after 4 weeks of strength training is the result of an increase in motor neuron output from the spinal cord to the muscle.
先前的研究表明，数周的力量训练足以引起肌肉神经驱动的显著适应。然而，关于力量训练在自愿收缩期间对运动单位募集和率编码引起的变化的数据很少。我们首次表明，胫骨前肌运动单位的放电特征在 4 周的等长自愿收缩力量训练过程中会发生变化。具体的适应包括运动单位放电率的显著增加、运动单位募集阈值力的降低以及运动神经元的类似输入-输出增益。这些发现表明，运动单位功能的适应可能归因于运动神经元池的突触输入的变化或内在运动神经元特性的适应。
肌肉力量通常在进行几次力量训练后才开始增加。这种增加通常归因于皮质或脊髓水平的适应导致的肌肉神经驱动的变化。我们研究了在 4 周的等长踝背屈肌力量训练前后，胫骨前肌中纵向跟踪的大量运动单位放电特征的变化。将 14 名个体的适应与 14 名对照受试者进行了比较。高密度肌电图网格，使用 128 个电极，在等速斜坡收缩期间记录了肌电活动，以达到最大自愿力的 35%、50%和 70%的目标力。评估了运动单位募集和去募集阈值、放电率、肌电间隔和运动神经元的突触输入估计值。力量训练后，运动单位的归一化募集阈值力降低（P < 0.05）。此外，在亚最大等长收缩的平台阶段，放电率平均增加了 3.3 ± 2.5pps（平均跨受试者和运动单位）（P < 0.001）。训练后，募集和去募集时的放电率没有改变（P < 0.05）。训练也没有改变收缩时斜坡阶段力与运动单位放电率之间的关系（P < 0.05）。这些结果首次证明，4 周力量训练后肌肉力量的增加是脊髓向肌肉输出运动神经元增加的结果。
