Andersen Birgit, Westlund Barbro, Krarup Christian
Department of Clinical Neurophysiology 3063, Rigshospitalet, Copenhagen University Hospital, Blegdamsvej 9, DK-2100 Copenhagen, Denmark.
J Physiol. 2003 Aug 15;551(Pt 1):345-56. doi: 10.1113/jphysiol.2003.043562. Epub 2003 Jun 24.
During a sustained maximal effort a progressive decline in the ability to drive motoneurones (MNs) develops. We used the recently developed triple stimulation technique (TST) to study corticospinal conduction after fatiguing exercise in healthy subjects. This method employs a collision technique to estimate the proportion of motor units activated by a transcranial magnetic stimulus. Following a sustained contraction of the abductor digiti minimi muscle at 50 % maximal force maintained to exhaustion there was an immediate reduction of the TST response from > 95 % to about 60 %. This effect recovered to control levels within 1 min and implies that a decreased number of spinal MNs were excited. Additional TST experiments after maximal and submaximal efforts showed that the decrease in size of the TST response was related to duration and strength of exercise. Motor evoked potentials (MEPs) after conventional transcranial magnetic stimulation (TMS) and responses to peripheral nerve stimulation were recorded following the same fatigue protocol. The size of both the MEPs and the peripheral responses increased after the contraction and were in direct contrast to the decrease in size of the TST response. This points to increased probability of repetitive spinal MN activation during fatigue even if some MNs in the pool failed to discharge. Silent period duration following cortical stimulation lengthened by an average of 55 ms after the contraction and recovered within a time course similar to that of the TST response depression. Overall, the results suggest that the outflow from the motor cortex could become insufficient to drive all spinal MNs to discharge when the muscle is fatigued and that complex interactions between failure of activation and compensatory mechanisms to maintain motor unit activation occur during sustained voluntary activity. When inability to maintain force occurs during submaximal effort, failure of activation of motor units is predominant.
在持续的最大努力过程中，驱动运动神经元（MNs）的能力会逐渐下降。我们使用最近开发的三重刺激技术（TST）来研究健康受试者疲劳运动后的皮质脊髓传导。该方法采用碰撞技术来估计经颅磁刺激激活的运动单位比例。在小指展肌以50%最大力量持续收缩至疲劳后，TST反应立即从>95%降至约60%。这种效应在1分钟内恢复到对照水平，这意味着兴奋的脊髓运动神经元数量减少。最大努力和次最大努力后的额外TST实验表明，TST反应大小的降低与运动的持续时间和强度有关。按照相同的疲劳方案，记录了传统经颅磁刺激（TMS）后的运动诱发电位（MEPs）和对周围神经刺激的反应。收缩后，MEPs和周围反应的大小均增加，这与TST反应大小的降低形成直接对比。这表明即使运动神经元池中的一些运动神经元未能放电，疲劳期间脊髓运动神经元重复激活的可能性也会增加。皮质刺激后的静息期持续时间在收缩后平均延长55毫秒，并在与TST反应抑制相似的时间进程内恢复。总体而言，结果表明，当肌肉疲劳时，运动皮层的输出可能不足以驱动所有脊髓运动神经元放电，并且在持续的自愿活动期间，激活失败和维持运动单位激活的补偿机制之间会发生复杂的相互作用。当在次最大努力期间无法维持力量时，运动单位激活失败占主导地位。
