THE SENSE OF EFFORT AND 2 MODELS OF SINGLE-JOINT MOTOR CONTROL

Two sets of experiments were carried out. In the first set, human subj ects were asked to make the same effort with the elbow flexors at diff erent joint angles under isometric conditions. In some experiments, th e subjects were standing with the arm in a vertical (parasagittal) pla ne; in others, they were seated with the arm in a horizontal (transver se) plane. When muscular torque at a given effort level (ordinate) was plotted as a function of elbow joint angle (abscissa), the resulting isoeffort torque-angle profiles tended to be flat or negatively slopin g over a range from 45 degrees to 135 degrees, and they were often non monotonic. Increases in effort up to near-maximal levels caused the is oeffort torque-angle profiles to shift upward with little alteration i n shape. In the second set of experiments, seated subjects with the ar m horizontal resisted baseline torques produced by a motor that acted to extend the elbow joint. Unexpected increases and decreases in torqu e were superimposed on the baseline torque. The subjects either were i nstructed to intervene and return the elbow to the initial (90 degrees ) position, or were told, ''Do not intervene voluntarily; let the moto r move your arm.'' Effort was reported both under baseline conditions and after the changes in torque. It was found that changes in effort w ere a function of the changes in torque opposed by the elbow flexors, and were similar whether the subject had repositioned the arm or allow ed it to be moved by the motor. In the latter case, the arm came to re st after displacements that were a function of the size and direction of the torque change. For individual subjects, the largest angular dis placements ranged from +/- 10 degrees to +/- 20 degrees for changes in torque of +/- 10 N . m. There was no evidence for any angular depende nce of the effort judgments at a given torque over this angular range. Depending on whether effort is primarily an efferent perception propo rtional to voluntary motor activity or also has a significant afferent (involuntary) component, different models of motor control are suppor ted by these data.