Compliant characteristics of single joints: preservation of equifinality with phasic reactions

Electromyograms (EMGs) of elbow flexors and extensors, elbow joint angle and torque were recorded in healthy subjects in the "do not react voluntarily" paradigm. Random loading and unloading torque perturbations of different rise times (Tr) were applied. Perturbations with Tr less than 300 ms evoke a phasic increase (in case of loading) or decrease (in case of unloading) in the flexor EMGs. The shortest EMG latency of about 30 ms was observed with very rapid perturbations (Tr less or equal to 20 ms). Smooth torque changes (Tr of 500-800 ms) were not accompanied by noticeable phasic reactions. Two types of the compliant characteristics were recorded: those with a zone of relatively high stiffness at the starting point ("S-shaped" curves) and those with a relatively constant stiffness. The presence or absence of the S-shape does not depend on Tr and therefore, does not seem to depend on phasic reflex or triggered reactions. In some of the subjects who were tested twice with an intervals of 2 months, the form of the compliant characteristics changed. This finding suggests that presence or absence of the S-shape can depend upon the subject's ability to follow the "do not react voluntarily" instruction. The invariance of the compliant characteristics in the presence or absence of reflex reactions can be regarded as an example of equifinality in a case of transient perturbations generated within the motor control system.