Multiple features of motor-unit activity influence force fluctuations during isometric contractions
Multiple features of motor-unit activity influence force fluctuations during isometric contractions
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DOI：
10.1152/jn.00056.2003
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发表时间：
2003-08-01
影响因子：
2.5
通讯作者：
Enoka, RM
中科院分区：
文献类型：
--
作者：
Taylor, AM;Christou, EA;Enoka, RM
为了确定造成自愿收缩期间力波动的机制，将实验测量值与最大自愿收缩强度（MVC）的2%至98%的收缩强度在时间和频域上的模拟力进行了比较。在10名年轻人中测量了由于第一背骨间肌的等距收缩而由食指施加的外展力。用计算机模型模拟了120个运动单元的运动单元招募和速率编码。这些模型改变了运动单元的招募和速率编码特性以及运动单元群的激活模式。主要发现是，在大约30%的MVC下，力的变异系数（CV）的最小值（1.7%）和在更高的力下的平台的实验观察结果不能被任何模型复制。该模型与实验观察到的力CV与平均力之间的关系最接近。此外，同步模型的结果扩展了先前的建模工作，表明同步的影响与放电率变异性无关。模型的力功率谱中的大部分功率都包含在低于5 Hz的频带中。然而，只有在激励中包含低频振荡的模型才能接近实验结果，即频率低于2 Hz时的峰值功率：0.99 Hz和1.37 Hz时分别占总功率的38%和43%。与实验功率谱相比，所有模型谱在更高频率处都包含第二个峰。在同步模型中，由于流量变异性较大，次峰不太突出。这些结果表明，在整个肌肉活动范围内的力波动变化不能用影响运动单元群输出的单一机制来解释。
To identify the mechanisms responsible for the fluctuations in force that occur during voluntary contractions, experimental measurements were compared with simulated forces in the time and frequency domains at contraction intensities that ranged from 2 to 98% of the maximum voluntary contraction (MVC). The abduction force exerted by the index finger due to an isometric contraction of the first dorsal interosseus muscle was measured in 10 young adults. Force was simulated with computer models of motor-unit recruitment and rate coding for a population of 120 motor units. The models varied recruitment and rate-coding properties of the motor units and the activation pattern of the motor-unit population. The main finding was that the experimental observations of a minimum in the coefficient of variation (CV) for force (1.7%) at approximately 30% MVC and a plateau at higher forces could not be replicated by any of the models. The model that increased the level of short-term synchrony with excitatory drive provided the closest fit to the experimentally observed relation between the CV for force and the mean force. In addition, the results for the synchronization model extended previous modeling efforts to show that the effect of synchronization is independent from that of discharge-rate variability. Most of the power in the force power spectra for the models was contained in the frequency bins below 5 Hz. Only a model that included a low-frequency oscillation in excitation, however, could approximate the experimental finding of peak power at a frequency below 2 Hz: 38% of total power at 0.99 Hz and 43% at 1.37 Hz, respectively. In contrast to the experimental power spectra, all model spectra included a second peak at a higher frequency. The secondary peak was less prominent in the synchronization model because of greater variability in discharge rate. These results indicate that the variation in force fluctuations across the entire operating range of the muscle cannot be explained by a single mechanism that influences the output of the motor-unit population.