Falla Deborah, Farina Dario
Center for Sensory-Motor Interaction (SMI), Department of Health Science and Technology, Aalborg University, Fredrik Bajers Vej 7, D-3, 9220 Aalborg, Denmark.
Exp Brain Res. 2007 Sep;182(1):99-107. doi: 10.1007/s00221-007-0974-4. Epub 2007 May 23.
This study compared fatigue and the spatial distribution of upper trapezius electromyographic (EMG) amplitude during a 6-min constant force shoulder elevation task at 20% of the maximal voluntary contraction force (MVC) (constant force) and during the same task interrupted by brief (2 s) periodic increases in force to 25% MVC every 30 s (variable force). Surface EMG signals were recorded with a 13 x 5 grid of electrodes from the upper trapezius muscle of nine healthy subjects. The centroid (center of activity) of the EMG root mean square map was computed to assess changes over time in the spatial distribution of EMG amplitude. MVC force decreased by (mean +/- SD) 9.0 +/- 3.9% after the constant force task (P < 0.05) but was unchanged following the variable force contraction. The centroid of EMG amplitude shifted in the cranial direction across the duration of the variable force contraction (P < 0.05) but not during the constant force contraction (shift of 2.9 +/- 2.3 mm and 1.4 +/- 1.1 mm, respectively). The results demonstrate that periodic increases in force during a sustained contraction enhance the modifications in spatial distribution of upper trapezius EMG amplitude and reduce fatigue compared to a constant force contraction performed at a lower average load. The change in spatial distribution of EMG amplitude over time during a sustained contraction may reflect a mechanism to counteract fatigue during prolonged muscle activity.
本研究比较了在一项持续6分钟的肩部上抬任务中，以最大自主收缩力（MVC）的20%进行恒力收缩时（恒力）以及在相同任务中每30秒短暂（2秒）将力增加至MVC的25%（变力）时，上斜方肌肌电图（EMG）振幅的疲劳程度和空间分布。用13×5电极网格记录了9名健康受试者上斜方肌的表面肌电信号。计算EMG均方根图的质心（活动中心），以评估EMG振幅空间分布随时间的变化。恒力任务后MVC力下降了（平均值±标准差）9.0±3.9%（P<0.05），但变力收缩后MVC力没有变化。在变力收缩过程中，EMG振幅的质心向头侧方向移动（P<0.05），而在恒力收缩过程中没有移动（分别移动2.9±2.3毫米和1.4±1.1毫米）。结果表明，与在较低平均负荷下进行的恒力收缩相比，持续收缩过程中周期性增加力可增强上斜方肌EMG振幅空间分布的改变，并减轻疲劳。持续收缩过程中EMG振幅空间分布随时间的变化可能反映了一种在长时间肌肉活动中对抗疲劳的机制。
