Rhythmic activity of a group of human motoneurones during voluntary contraction of a muscle.

Abstract MU potentials of m.rectus femoris were led off with the help of needle electrodes under isometric contraction (up to 50% of the maximal strength). As many as 10 MUs, could be identified simultaneously. The experiments showed that under conditions of a fixed posture and a stable movement task, the order of recruitment and switch-off of motoneurones was usually stable. “Group behaviour” was typical when the strength of contraction changed the frequencies of different motoneurones changing in one direction. Such a pattern of behaviour of motoneurones agrees with the ideas of diffuse distribution of excitation in a motoneurone pool and of the organization of its activity on the basis of the “size principle” described by Henneman and co-workers. However, occasional independent changes in frequency of individual motoneurones and sub-groups of motoneurones were observed even under stable conditions of muscle contraction. Independent behaviour of motoneurones was particularly frequent in the case of a subject in a free posture or when his movement task was changed. In such cases, the recruitment order of motoneurones was not stable, discharge frequencies quite often changing in opposite directions and substitution of motoneurones occurring. This pattern of motoneurone behaviour can be explained only by the use of independent inputs. The data obtained give us reason to believe that the organization of activity of a motoneurone pool on the basis of general diffuse input can provide only standard forms of motor activity. Under natural conditions of movement of higher animals and man, in addition to differences in thresholds another more flexible mechanism of selective activation of motoneurones is employed. This mechanism secures a more subtle regulation of mechanical effect and prevents fatigue during prolonged contraction of a muscle by causing substitution of MUs through changes in posture.