The dependence of discharge rate of spindle afferent units on the size of the load during isotonic position holding in man.

The impulse discharge in single afferent units from muscle spindles was recorded in waking human subjects, using tungsten needle electrodes which were percutaneously inserted in the radial nerve. Eighteen spindle afferents from the finger extensor muscles were studied, 11 primaries and 7 secondaries. The subjects performed standardized trapezoidal finger movements in a visual tracking task which included periods of position holding. A load of constant torque opposed finger extension. Its size was varied with successive tests and covered up to 29% of the maximum voluntary contraction force (MVF). The mean discharge of the afferent unit during holding was measured and related to the size of the load. The mean discharge rate of both primary and secondary units increased with the size of the load. There was considerable scatter of the data points, but generally a linear relationship was an acceptable describing function. For the pooled data of the total sample of primaries the slope was 0.47 impulses/s/%MVF, whereas the corresponding figure for the secondaries was 0.66 impulses/s/%MVF after an appropriate correction for a sampling bias. The rates of discharge of the present sample of units were generally low in relation to those reported for afferents from, for instance, ankle muscles in the cat. The rise of spindle discharge with increasing size of the load indicated that a broad parallelism of skeleto- and fusimotor activity was present during accurate position holding. However, there were indications that a rigid linkage between total skeletomotor activity and the fusimotor drive to individual spindles was not present.