Matching forces: constant errors and differential thresholds.

A contralateral limb-matching procedure was used to study the perception of forces generated by the elbow flexor muscles. Subjects were required to generate forces ranging from 15 to 85% of their maximum voluntary contraction (MVC range: 169-482 N), and after achieving the target force to produce a force of the same perceived intensity with their contralateral arm. Under these conditions, subjects consistently overestimated the amplitude of the two lowest forces, and the most accurate matching of forces occurred in the middle of the response range (around 50% MVC). This pattern of constant errors could be explained in terms of an artifact of the starting position, which has been shown to influence the accuracy of judgements of stimulus magnitude. The Weber fraction for force calculated from the matching data was 0.07, which is within the range reported previously for weight. These thresholds are, however, considerably lower than those described for friction and the moment of inertia, the perception of which also involves the proprioceptive system.