Mathiassen S E, Aminoff T
Department of Ergonomics, National Institute for Working Life, Solna, Sweden.
Eur J Appl Physiol Occup Physiol. 1997;76(5):434-44. doi: 10.1007/s004210050273.
Ten females (25-50 years of age) performed isometric shoulder flexions, holding the right arm straight and in a horizontal position. The subjects were able to see the rectified surface electromyogram (EMG) from either one of two electrode pairs above the upper trapezius muscle and were instructed to keep its amplitude constant for 15 min while gradually unloading the arm against a support. The EMG electrodes were placed at positions representing a "cranial" and a "caudal" region of the muscle suggested previously to possess different functional properties. During the two contractions, recordings were made of: (1) EMG root mean square-amplitude and zero crossing (ZC) frequency from both electrode pairs on the trapezius as well as from the anterior part of the deltoideus, (2) supportive force, (3) heart rate (HR) and mean arterial blood pressure (MAP), and (4) perceived fatigue. The median responses during the cranial isoelectric contraction were small as compared to those reported previously in the literature: changes in exerted glenohumeral torque and ZC rate of the isoelectric EMG signal of -2.81% x min(-1) (P = 0.003) and 0.03% x min(-1) (P = 0.54), respectively, and increases in HR and MAP of 0.14 beats x min(-2) (P = 0.10) and 0.06 mmHg x min(-1) (P = 0.33), respectively. During the contraction with constant caudal EMG amplitude, the corresponding median responses were -2.51% x min(-1) (torque), 0.01% x min(-1) (ZC rate), 0.31 beats x min(-2) (HR), and 0.93 mmHg x min(-1) (MAP); P = 0.001, 0.69, 0.005, and 0.003, respectively. Considerable deviations from the "isoelectric" target amplitude were common for both contractions. Individuals differed markedly in response, and three distinct subgroups of subjects were identified using cluster analysis. These groups are suggested to represent different motor control scenarios, including differential engagement of subdivisions of the upper trapezius, alternating motor unit recruitment and, in one group, a gradual transition towards a greater involvement of type II motor units. The results indicate that prolonged low-level contractions of the shoulder muscles may in general be accomplished with a moderate metabolic stress, but also that neuromuscular adaptation strategies differ significantly between individuals. These results may help to explain why occupational shoulder-neck loads of long duration cause musculoskeletal disorders in some subjects but not in others.
10名女性（年龄在25至50岁之间）进行了等长肩部屈曲动作，保持右臂伸直并处于水平位置。受试者能够看到斜方肌上部上方两对电极中任意一对的整流表面肌电图（EMG），并被要求在逐渐将手臂靠在支撑物上卸载的同时，保持其振幅恒定15分钟。EMG电极放置在先前表明具有不同功能特性的肌肉的“颅侧”和“尾侧”区域的位置。在两次收缩过程中，记录了以下数据：（1）斜方肌上两对电极以及三角肌前部的EMG均方根振幅和过零（ZC）频率；（2）支撑力；（3）心率（HR）和平均动脉血压（MAP）；（4）主观疲劳感。与先前文献报道相比，颅侧等电收缩期间的中位数反应较小：等电EMG信号施加的盂肱扭矩和ZC率的变化分别为-2.81%×min⁻¹（P = 0.003）和0.03%×min⁻¹（P = 0.54），HR和MAP的增加分别为0.14次/分钟²（P = 0.10）和0.06 mmHg/分钟¹（P = 0.33）。在尾侧EMG振幅恒定的收缩过程中，相应的中位数反应为-2.51%×min⁻¹（扭矩）、0.01%×min⁻¹（ZC率）、0.31次/分钟²（HR）和0.93 mmHg/分钟¹（MAP）；P值分别为0.001、0.69、0.005和0.003。两次收缩中，与“等电”目标振幅的显著偏差都很常见。个体反应差异明显，使用聚类分析确定了三个不同的受试者亚组。这些组被认为代表不同的运动控制模式，包括斜方肌上部各亚部分的不同参与程度、运动单位交替募集，以及在一组中逐渐向更大程度地参与II型运动单位转变。结果表明，肩部肌肉的长时间低水平收缩通常可以在适度的代谢应激下完成，但个体之间的神经肌肉适应策略也存在显著差异。这些结果可能有助于解释为什么长时间的职业性肩颈负荷会在一些受试者中导致肌肉骨骼疾病，而在另一些受试者中则不会。
