Programmed electromyographic activity and negative incremental muscle stiffness in monkeys jumping downward

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.