Muscle strength was assessed in 11 patients with radial or posterior interosseus palsy. Apparent weakness was found in the dorsal and palmar interossei and the abductor digiti minimi. These muscles insert on the extensor expansions, and their activation is associated with concomitant contraction of finger flexors and extensors. This apparent weakness may be due to their unopposed traction on the extensor expansion by the paralyzed extensor digitorum.