Dyhre-Poulsen P, Laursen A M
J Physiol. 1984 May;350:121-36. doi: 10.1113/jphysiol.1984.sp015192.
We trained monkeys to jump down from different heights, and recorded electromyograms (e.m.g.s) in arm muscles, and ground reaction forces. The landing movements were also recorded by high-speed cinematography. The e.m.g. of the triceps began about 80 ms before landing. The initial burst lasted until about 20 ms after ground contact and was succeeded by bursts of gradually declining amplitude. These discharges were not of reflex origin, because when the monkey was deceived by a collapsible platform, they were time-locked to the expected, not to the true landing. The amplitude of the e.m.g. in the triceps increased with the height of the jump, indicating adaptive control. The timing of the e.m.g. pattern was assumed to be programmed before take off, because it was unaffected by extinction of the light during the fall. The vertical ground reaction force produced by the arms had an inflexion on its rising phase which arose from the very rapid stretch of the muscles which control the wrist. Then came a sharp peak produced mainly by stretch of the triceps. The inflexion and the sharp peak were probably produced by short-range stiffness of the muscles of the upper arm. The torque acting on the elbow joint, and the elbow joint stiffness were calculated from the ground reaction forces and the movement of the arm. The torque was high at impact and gradually declined during the landing. The force produced by the triceps increased sharply, then decreased while it continued to lengthen. Thus, the elbow joint showed high initial stiffness, which then decreased, and finally became negative. This dynamic relation between length and tension was very different from the static length-tension characteristic of skeletal muscles. The observed behaviour of the muscles presumably takes advantage of the resistance of the musculo-skeletal system to transient forces. The observed negative stiffness occurs only during submaximal contractions. We propose that the segmented pattern in the e.m.g. produces submaximal contractions in both slow and fast fibres in spite of a high excitatory drive.
我们训练猴子从不同高度跳下，并记录手臂肌肉的肌电图（e.m.g.s）和地面反作用力。着陆动作也通过高速摄影记录下来。三头肌的肌电图在着陆前约80毫秒开始。最初的爆发持续到地面接触后约20毫秒，随后是幅度逐渐下降的爆发。这些放电不是反射起源的，因为当猴子被可折叠平台欺骗时，它们与预期的着陆时间锁定，而不是与实际着陆时间锁定。三头肌中肌电图的幅度随着跳跃高度的增加而增加，表明存在适应性控制。肌电图模式的时间被认为是在起飞前编程的，因为它不受下落过程中光线熄灭的影响。手臂产生的垂直地面反作用力在其上升阶段有一个拐点，这是由控制手腕的肌肉非常快速的拉伸引起的。然后是一个主要由三头肌拉伸产生的尖峰。这个拐点和尖峰可能是由上臂肌肉的短程刚度产生的。根据地面反作用力和手臂的运动计算作用在肘关节上的扭矩和肘关节刚度。扭矩在撞击时很高，在着陆过程中逐渐下降。三头肌产生的力急剧增加，然后在继续伸长时减小。因此，肘关节最初表现出高刚度，然后降低，最后变为负值。这种长度和张力之间的动态关系与骨骼肌的静态长度-张力特性非常不同。观察到的肌肉行为大概利用了肌肉骨骼系统对瞬态力的阻力。观察到的负刚度仅在次最大收缩期间出现。我们提出，尽管有高兴奋性驱动，肌电图中的分段模式在慢纤维和快纤维中都产生次最大收缩。
