Strength measurements of the lumbrical muscles.

This study was designed to measure the strength of the lumbrical muscles in the index and long fingers in patients with ulnar nerve paralysis. A hand-held dynamometer was used. The results show that in ulnar nerve damage the index and long fingers have a mean metacarpophalangeal (MCP) joint flexion strength of 0.8 kg (range 0.3-1.5), compared with 6.4 kg (range 4.6-7.9) in the noninvolved hand. Thus, the damaged fingers have only about 12% of the strength of those of the noninvolved hand. In the hand with ulnar paralysis, the loss of intrinsic strength (dorsal and palmar interosseous muscles) is considerable (almost 90%). The contribution of the interosseous muscles in maintaining the intrinsic position is considerably greater than that of the lumbricals. Comparing the Medical Research Council (MRC) scale (0-5) with the dynamometry measurements shows that MRC grade 3 correlates with about 0.8 kg, while grade 5 correlates with about 6.5 kg of MCP joint flexion strength.