Poston Brach, Enoka Joel A, Enoka Roger M
Department of Integrative Physiology, University of Colorado, Boulder, CO, USA.
Exp Brain Res. 2008 May;187(3):373-85. doi: 10.1007/s00221-008-1309-9. Epub 2008 Feb 21.
The minimum variance theory proposes that stronger (larger) muscles produce less variable trajectories compared with weaker (smaller) muscles and thus can accomplish more accurate contractions. The purpose of the study was to determine the influence of muscle size and trajectory variability on the endpoint accuracy of goal-directed isometric contractions. Twelve young (25 +/- 5 years) and 12 old adults (76 +/- 6 years) performed 100 trials with each of two muscles in both hands. Subjects were instructed to match the peak of a force trajectory to a target force by controlling either the abduction (first dorsal interosseus muscle; FDI) or adduction force (second palmar interosseus muscle; SPI) exerted by the index finger of each hand. The time to peak force was 150 ms and the peak force required was 25% of the maximal force that could be achieved in 150 ms. Endpoint accuracy and variability in force and time along with intramuscular EMG activity of the agonist muscle (FDI and SPI) involved in each task were quantified for each set of 100 trials. The MVC force was less for the SPI muscle, and the force endpoint error and variance were greater in the SPI muscle compared with the FDI muscle. Conversely, endpoint measures that included timing were similar for the two muscles. Trajectory variability was greater for the FDI muscle, but did not influence endpoint error for either muscle. The young and old adults had similar strength values, but the old adults were less accurate and more variable than the young subjects. Nonetheless, the accuracy and variability displayed by the old adults for the two muscles was the same as that observed for the young adults. The force accuracy and variability findings are consistent with the predictions of the minimum variance theory that motor-output variability is inversely related to muscle size, strength, and motor unit number.
最小方差理论提出，与较弱（较小）的肌肉相比，较强（较大）的肌肉产生的轨迹变化较小，因此能够完成更精确的收缩。本研究的目的是确定肌肉大小和轨迹变异性对目标导向等长收缩终点准确性的影响。12名年轻人（25±5岁）和12名老年人（76±6岁）双手各两块肌肉分别进行100次试验。受试者被要求通过控制每只手食指施加的外展力（第一背侧骨间肌；FDI）或内收力（第二掌侧骨间肌；SPI），使力轨迹的峰值与目标力相匹配。达到峰值力的时间为150毫秒，所需的峰值力为150毫秒内可达到的最大力的25%。对每组100次试验，量化了终点准确性、力和时间的变异性，以及每项任务中涉及的主动肌（FDI和SPI）的肌内肌电图活动。SPI肌肉的最大自主收缩力较小，与FDI肌肉相比，SPI肌肉的力终点误差和方差更大。相反，包括时间在内的终点测量在两块肌肉中相似。FDI肌肉的轨迹变异性更大，但对两块肌肉的终点误差均无影响。年轻人和老年人的力量值相似，但老年人比年轻受试者的准确性更低、变异性更大。尽管如此，老年人在两块肌肉上表现出的准确性和变异性与年轻人观察到的相同。力准确性和变异性的研究结果与最小方差理论的预测一致，即运动输出变异性与肌肉大小、力量和运动单位数量成反比。
