Stephens M J, Yang J F
Division of Neuroscience, University of Alberta, Edmonton, Canada.
Exp Brain Res. 1999 Feb;124(3):363-70. doi: 10.1007/s002210050633.
Prior work from mammals suggests that load experienced by extensor muscles of the hindlimbs (i.e. Duysens and Pearson 1980; Pearson and Collins 1993; Fouad and Pearson 1997) or cutaneous afferents from the plantar surface of the foot (Duysens and Pearson 1976; Guertin et al. 1995) enhances activity in extensor muscles during the stance phase, and delays the onset of flexor activity associated with the swing phase. The presumed functional significance of this phenomenon is that extensor activity of the supporting limb during walking can: (a) reinforce the supporting function in proportion to the load experienced, and (b) prolong the stance phase until unloading of the limb has occurred. Whether a similar functional role exists for load-sensitive afferents during walking in the human is unknown. In this study, the effect of adding or removing a substantial load (30% of body weight) at the centre of mass was studied in healthy adult human subjects. Loads were applied near the centre of mass to avoid the need for postural adjustments which might confound the interpretation of the results. Subjects walked on a treadmill with either: (a) a sustained increase or decrease in load, or (b) a sudden unexpected increase or decrease in load. In general, subjects responded to the changes in load by changing the amplitude of the extensor electromyographic (EMG) bursts. For example, with sudden unexpected additions in load, the average increase in amplitude was 40% for the soleus across the stance phase, and 134% for the quadriceps during the early part of the stance phase. Extensor EMGs increased with both sustained and sudden increases in load. Extensor EMG durations also increased (average increase in duration of 4% for soleus with sudden loading, and 7% for sustained loading). Cycle duration hardly changed (average increase of 0.5% with both sudden and sustained loading). These results differ from those of infants subjected to a similar perturbation during supported walking. A large change in timing (i.e. an increase in the duration of the stance phase by 30% and the step cycle by 28%) was seen in the infants, with no change in the amplitude of the EMG burst (Yang et al. 1998). These results suggest that the central nervous system can control the timing and amplitude of extensor EMG activity in response to loading independently. Maturation of the two components most likely occurs independently. In the adult, independent control of the two components may provide greater flexibility of the response.
哺乳动物先前的研究表明，后肢伸肌所承受的负荷（即杜伊森斯和皮尔逊，1980年；皮尔逊和柯林斯，1993年；福阿德和皮尔逊，1997年）或足底皮肤传入神经（杜伊森斯和皮尔逊，1976年；盖尔廷等人，1995年）会增强站立期伸肌的活动，并延迟与摆动期相关的屈肌活动的开始。这种现象的假定功能意义在于，行走过程中支撑肢体的伸肌活动可以：（a）根据所承受的负荷按比例增强支撑功能，以及（b）延长站立期，直到肢体卸载负荷。在人类行走过程中，负荷敏感传入神经是否存在类似的功能作用尚不清楚。在本研究中，对健康成年人类受试者在质心处增加或去除相当大的负荷（体重的30%）的效果进行了研究。在质心附近施加负荷，以避免可能混淆结果解释的姿势调整。受试者在跑步机上行走时，采用以下两种方式之一：（a）负荷持续增加或减少，或（b）负荷突然意外增加或减少。一般来说，受试者通过改变伸肌肌电图（EMG）爆发的幅度来应对负荷变化。例如，在负荷突然意外增加时，比目鱼肌在整个站立期的幅度平均增加40%，股四头肌在站立期早期的幅度平均增加134%。伸肌EMG随着负荷的持续增加和突然增加而增加。伸肌EMG持续时间也增加（突然加载时比目鱼肌持续时间平均增加4%，持续加载时增加7%）。周期持续时间几乎没有变化（突然加载和持续加载时平均增加0.5%）。这些结果与在有支撑行走过程中受到类似扰动的婴儿的结果不同。婴儿出现了时间上的大幅变化（即站立期持续时间增加30%，步周期增加28%），而EMG爆发的幅度没有变化（杨等人，1998年）。这些结果表明，中枢神经系统可以独立地控制伸肌EMG活动的时间和幅度以应对负荷。这两个组成部分的成熟很可能是独立发生的。在成年人中，对这两个组成部分的独立控制可能会提供更大的反应灵活性。
