Although plantar flexor muscle strength is known to affect gait characteristics, the effect of plantar flexor (PF) muscle stiffness on gait is unknown. The purpose of this study was to determine the effect of PF muscle stiffness, in addition to PF strength, on the gait characteristics of subjects with diabetes and peripheral neuropathy (DM and PN) and age-matched controls. Thirty-four subjects were tested (17 DM and PN, 17 controls), 10 males and seven females in each group. PF muscle strength (concentric peak torque) explained a significant amount of variance in all gait variables (20- 37%). Passive stiffness added a unique contribution to plantar flexor peak moment and walking speed (10-11%). Plantar flexor stiffness, in addition to strength, may be an important factor contributing to gait characteristics. Treatment techniques designed to preserve or increase stiffness may be indicated for patients with limited PF strength. (C) 2000 Elsevier Science B.V.