Accuracy of voluntary movements at the thumb and elbow joints.

Subjects made simultaneously movements from a common rest position and attempted to align corresponding joints (elbow joints, or distal joints of thumb), on opposite sides of the body. When misalignments were expressed in angular terms, variability of performance within and between subjects was greater for thumb than for elbow joints. When the misalignments were expressed in terms of linear misalignment at the end of the moved lever arms, variability of performance within and between subjects was less for thumb than elbow joints. However, when the misalignments were expressed in terms of mean proportional changes in the lengths of fascicles in muscles operating at the joints, variabilities of performances at both joints were similar. In another test, subjects made small unloaded movements at either the elbow joint or the distal thumb joint to guide a cursor along a narrow path. When the movement task was made similar for the elbow and thumb joints in terms of either the angular excursion required, or the required linear excursion of the moved lever tip, accuracy of performances at the two joints varied greatly. Only when the tasks were similar in terms of the mean proportional changes of length in fascicles of muscles operating at the joints, were performances at the two joints of similar accuracy. The results suggest that proportional change in muscle fascicle length is a significant variable for the CNS in proprioception and the control of voluntary movement.