Superposition of motor programs—I. Rhythmic forearm movements in man

The so-called invariant characteristics of the elbow muscles are described. An invariant characteristic is a curve on a plot of elbow static moment vs angle. An invariant characteristic shows the elbow displacements in response to different amplitudes of load change when the subject is told not to compensate, or not to intervene voluntary to correct the deflection of the arm. If an inclination of the invariant characteristic is known, the degree of central tonic coactivation of antagonistic motoneurones can be estimated. The idea was utilized when analysing rhythmic movements (2–6 Hz) of the forearm that were carried out against a spring resistance (2–10 Nm). When the spring was suddenly switched off, the deflection of forearm rose. The subject was told not to correct the deflection and not to interrupt the swingings. The mean changes of both muscle torque and joint angle were determined in the experiments so that it was possible to estimate the inclination of the invariant characteristic used at a given frequency. Judging by the results, central tonic coactivation of elbow flexors and extensors occurs and increases to maximum as the frequency of movement increases. In addition, the reciprocal activation of antagonistic muscles is well known to occur upon execution of this movement.It can be concluded that both the reciprocal and the unidirectional central commands to antagonistic motoneurones occur simultaneously. The functional significance, as well as a scheme of superposition, of these motor commands are discussed. It is suggested that the commands are universal, i.e. the nervous system operates by using them during the performance of any movement.