The in vivo elastic properties of the plantar fascia during the contact phase of walking were determined experimentally by integrating a pressure-sensitive optical gait platform with a radiographic fluoroscopy system for recording skeletal motion. In order to calculate the fascia's tension-deformation relation, lateral images of the foot's skeleton that allowed evaluation of the fascia's transient length from the arch-contact to toe-off stages of walking were obtained simultaneously with the vertical foot-ground contact forces. The plantar fascia was shown to undergo continuous elongation from arch-contact to toe-off, reaching a deformation of 9 to 12% between these positions. Rapid elongation of the fascia, at a strain rate of about 0.9±0.1 Sec-1, was observed before and immediately after midstance, while a significantly slower elongation occurred at a strain rate of approximately 0.2±0.1 Sec-1 around push-off and toe-off. The average stiffness of the fascia at the slow-to-moderate walking velocities was 170±45 N/mm, which is similar to reported stiffness values for cadaver fascia specimens. The present technique may be useful for validation of computational models of the soft tissues of the foot as well as for testing the effectiveness of orthoses and shoe types for relieving excessive strain of the fascia in the treatment of plantar fasciitis.