Perceptual constancy and the perceived magnitude of muscle forces.

The present study used a force-matching task to measure how accurately subjects could match a force using different muscle groups. Forces ranging in magnitude from 2 to 10 N were produced and matched by three muscle groups: the elbow flexors, the forearm and hand muscles involved in the palmar prehensile grasp, and the index finger flexors. The ability to match forces was considered in terms of precision, that is, how closely the matching forces approximated the reference force produced on the contralateral side, and accuracy, which is the reproducibility of the matching force estimates. The results indicated that the perceived magnitude of forces varied as a function of the muscle group generating the force. Forces produced by the index finger flexors were consistently overestimated in magnitude when matched by elbow flexion forces, and elbow forces were underestimated when matched by flexing the index finger. When evaluated in terms of constant and absolute errors, the index finger flexors were the most precise matching muscle group and the elbow flexors the least precise. These results suggest that forces are perceived relatively, and are scaled with reference to the operating range of the muscles. They also indicate that there is little perceptual constancy in the perceived magnitude of forces generated by different muscle groups.