Christova P, Kossev A
Bulgarian Academy of Sciences, Institute of Biophysics, Sofia.
Eur J Appl Physiol Occup Physiol. 1998 Mar;77(4):379-87. doi: 10.1007/s004210050348.
Changes accompanying long-lasting intermittent muscle contractions (30%-50% of the maximal) were investigated by tracing the activity of 38 motor units (MU) of the human biceps brachii muscle recorded from fine-wire branched electrodes. The motor task was a continuous repetition of ramp-and-hold cycles of isometric flexion contractions. During ramp-up phases a significant decline in recruitment thresholds was found with no changes in the discharge pattern. During ramp-down phases the unchanged mean value of derecruitment thresholds during the task was accompanied by increased duration of the last two interspike intervals (ISI). These findings would suggest that during fatigue development the main compensatory mechanism during ramp-up contractions is space coding while for ramp-down contractions it is rate coding. During the steady-state phases the mean value of ISI, as well as the firing variability. had increased by the end of the task in most of the MU investigated . In addition 17 recruited MU were also investigated. These units revealed a lower initial discharge rate and a faster decrease in the mean discharge rate with the development of fatigue. The gradual reduction of the recruitment threshold of already active MU and the recruitment of new units demonstrated an increased excitability of the motorneuron pool during fatigue. A typical recruitment pattern (a first short ISI followed by a long one) was observed during ramp-up contractions in units active from the very beginning of the task, as well as during sustained contractions at the onset of the stable discharge of the additionally recruited MU.
通过追踪从细钢丝分支电极记录的人类肱二头肌38个运动单位（MU）的活动，研究了伴随长时间间歇性肌肉收缩（最大收缩的30%-50%）的变化。运动任务是等长屈曲收缩的斜坡-保持周期的连续重复。在斜坡上升阶段，发现募集阈值显著下降，而放电模式没有变化。在斜坡下降阶段，任务期间去募集阈值的平均值不变，同时最后两个峰间间隔（ISI）的持续时间增加。这些发现表明，在疲劳发展过程中，斜坡上升收缩期间的主要补偿机制是空间编码，而斜坡下降收缩期间的主要补偿机制是速率编码。在稳态阶段，大多数被研究的运动单位的ISI平均值以及放电变异性在任务结束时增加。此外，还研究了17个被募集的运动单位。这些单位显示出较低的初始放电率，并且随着疲劳的发展，平均放电率下降得更快。已经活跃的运动单位的募集阈值逐渐降低以及新单位的募集表明，在疲劳期间运动神经元池的兴奋性增加。在从任务开始就活跃的单位的斜坡上升收缩期间，以及在额外募集的运动单位稳定放电开始时的持续收缩期间，观察到典型的募集模式（先是一个短的ISI，然后是一个长的ISI）。
