Extrinsic muscles of the hand signal fingertip location more precisely than they signal the angles of individual finger joints

Studies have demonstrated that muscle spindle organs provide the majority of the proprioceptive information available to the nervous system about limb position. Other studies suggest that a sense of position may be lacking in the fingers, as subjects were unaware of rather large excursions of finger joints if the excursions were made slowly enough. We sought to investigate the basis for this unexpected finding with a biomechanical model of the human long finger and the forearm muscles which actuate it, in order to study potential contributions of spindle organs in the extrinsic muscles of the hand to a sense of position of the finger. The model, based on cadaver data, allowed us to determine how precisely estimates of the lengths of the extrinsic finger muscles can be transformed into estimates of: (1) the flexion/extension angles of the individual finger joints, and (2) the location of the fingertip in the flexion/extension plane. We found that, for some finger positions, length information from all three extrinsic muscles was not sufficient to precisely estimate the flexion angles of all finger joints. Precision of joint angle estimates could be as poor as +/-18% of joint range of motion. However, length information from just two of the extrinsic muscles taken together could always provide information sufficient to estimate the location of the fingertip relative to the metacarpophalangeal joint within a reasonably small tolerance (+/-one-half thickness of the fingertip). Furthermore, it was possible to make this estimate without determining any of the finger joint angles. These results suggest that spindles in the extrinsic muscles alone can signal fingertip location, even though they may not provide sufficient information to estimate the individual joint angles that set the position of the fingertip. Thus, an absence of position sense for individual joints (the sense many studies have tried to measure) may say little about a sense of location of the tip of the finger.