Van Cutsem Michaël, Duchateau Jacques
Laboratory of Applied Biology, Université Libre de Bruxelles, 28 avenue P. Héger, CP 168, 1000 Brussels, Belgium.
J Physiol. 2005 Jan 15;562(Pt 2):635-44. doi: 10.1113/jphysiol.2004.074567. Epub 2004 Nov 11.
To investigate the effect of initial conditions on the modulation of motor unit discharge during fast voluntary contractions, we compared ballistic isometric contractions of the ankle dorsiflexor muscles that were produced from either a resting state or superimposed on a sustained contraction. The torque of the dorsiflexors and the surface and intramuscular EMGs from the tibialis anterior were recorded. The results showed that the performance of a ballistic contraction from a sustained contraction ( approximately 25% maximal voluntary contraction (MVC)) had a negative effect on the maximal rate of torque development. Although the electromechanical delay was shortened, the EMG activity during the ballistic contraction was less synchronized. These observations were associated with a significant decline in the average discharge rate of single motor units (89.8 +/- 3.8 versus 115 +/- 5.8 Hz) and in the percentage of units (6.2 versus 15.5% of the whole sample) that exhibited double discharges at brief intervals (= 5 ms). High-threshold units that were not recruited during the sustained contraction displayed the same activation pattern, which indicates that the mechanisms responsible for the decline in discharge rate were not restricted to previously activated units, but appear to influence the entire motor unit pool. When a premotor silent period (SP) was observed at the transition from the sustained muscular activity to the ballistic contraction (19% of the trials), these adjustments in motor unit activity were not present, and the ballistic contractions were similar to those performed from a resting state. Together, these results indicate that initial conditions can influence the capacity for motor unit discharge rate and hence the performance of a fast voluntary contraction.
为了研究初始条件对快速随意收缩过程中运动单位放电调制的影响，我们比较了由静息状态产生的或叠加在持续收缩上的踝背屈肌的爆发式等长收缩。记录了背屈肌的扭矩以及胫骨前肌的表面肌电图和肌内肌电图。结果表明，从持续收缩（约25%最大随意收缩（MVC））开始的爆发式收缩对最大扭矩发展速率有负面影响。尽管机电延迟缩短，但爆发式收缩期间的肌电图活动同步性较差。这些观察结果与单个运动单位的平均放电率显著下降（89.8±3.8对115±5.8Hz）以及在短时间间隔（=5ms）内出现双放电的单位百分比下降（整个样本的6.2%对15.5%）有关。在持续收缩期间未被募集的高阈值单位表现出相同的激活模式，这表明导致放电率下降的机制不仅限于先前激活的单位，而是似乎影响整个运动单位池。当在从持续肌肉活动向爆发式收缩转变时观察到运动前静息期（SP）（19%的试验）时，运动单位活动的这些调整不存在，并且爆发式收缩与从静息状态进行的收缩相似。总之，这些结果表明初始条件可以影响运动单位放电率的能力，从而影响快速随意收缩的表现。
