The kinematics of both the first metatarsophalangeal joint and the articulation of the hallux sesamoid bones with the metatarsal head were investigated with fifteen fresh-frozen below-the-knee amputation specimens using a radiographic technique. Six feet were of normal structural anatomy, six displayed hallux valgus, and three had hallux rigidus. Normal specimens demonstrated an average total range of motion in the sagittal plane of 111 degrees, with about 76 degrees of dorsiflexion and 34 degrees of plantar flexion. The abnormal specimens revealed a decreased total arc of motion, with a limitation of plantar flexion in feet with hallux valgus and a loss of dorsiflexion in feet with hallux rigidus. Motion analysis of the normal metatarsophalangeal joints demonstrated minimum scattering of instant centers of rotation. This was in contrast to the diseased articulations, which displayed markedly displaced instant centers of rotation located eccentrically about the metatarsal head. Surface motion in the normal joints was characterized as tangential sliding from maximum plantar flexion to moderate dorsiflexion, with some compression at maximum dorsiflexion. The feet with hallux valgus and the feet with hallux rigidus displayed distinctive patterns of distraction and jamming throughout specific portions of the range of motion in the sagittal plane. Motion of the metatarsophalangeal joint in the transverse plane concomitant with motion in the sagittal plane, which has been hypothesized by other investigators, was confirmed and quantified in this study. The feet with hallux rigidus displayed a reduction in this motion.(ABSTRACT TRUNCATED AT 250 WORDS)