MOVEMENT DETECTION AT THE DISTAL JOINT OF THE HUMAN THUMB AND FINGERS

To determine whether proprioceptive acuity is the same at all digits, particularly when postured as in a 'grasp', we imposed 10 degrees move ments at the distal joint of the thumb, index and ring finger, at thre e velocities; 1.25 degrees/s, 2.5 degrees degrees/s and 5 degrees/s. T he test joint was initially flexed by 25 degrees and the joints proxim al to the test joint were maintained in a standard posture for each st udy. When in a grasp posture that disengaged the extensor muscles at t he distal joint of the finger, movement detection at the thumb was sup erior to that at the fingers for all velocities. However, when the fin gers were positioned so that all proprioceptive inputs were able to co ntribute (i.e. cutaneous, joint and both flexor and extensor muscle af ferents), proprioceptive acuity was similar for the three digits. Loss of local cutaneous (and joint) inputs by digital anaesthesia signific antly impaired performance at all digits, suggesting a critical role f or cutaneous input in normal proprioceptive sensibility at all distal joints of the digits. Anaesthesia of the extensor muscle afferents inn ervating the thumb did not affect its proprioceptive acuity. Thus, for the thumb, the extensor muscle afferents do not provide critical info rmation. The greater change in muscle fascicle length for the thumb's long flexor muscle (3% per 10 degrees) compared with that in the finge r flexor muscles (e.g. 0.1% per 10 degrees) could contribute to the th umb's performance. There appears to be less redundancy of muscle and n on-muscle signals for the fingers than for the thumb, because a reduct ion in either cutaneous or muscle input significantly impaired acuity at the fingers. Overall, when the hand is in a grasping posture, irres pective of the contribution of local cutaneous inputs, the long flexor acting on the thumb may contribute more to its proprioceptive acuity than the long finger flexors contribute to acuity at the fingers.