Muscle testing of a patient with radial nerve palsy can incorrectly suggest median and ulnar weakness because of a decreased ability to stabilize the thumb and wrist. Ten adult volunteers had radial nerve blocks and their strengths were quantitatively evaluated before and after blocking for grip, key pinch, isolated thumb adduction (adduction pinch), thumb palmar abduction, finger flexion, and flexor pollicis longus (FPL) function. Data were analyzed by paired t test (p less than 0.05). All composite motions that required stabilization of the wrist or thumb showed marked weakness after the radial nerve block (grip decreased 77%, key pinch decreased 33%, and thumb palmar abduction strength decreased 53%). The strength of adduction pinch, finger flexion, and FPL showed no significant decreases after the radial nerve block. Since adduction pinch and isolated FPL function can be easily tested clinically, they should be examined to prevent confusion with median and ulnar problems.