BACKGROUND: Plantar soft tissue stiffness and thickness are important biomechanical variables to understand stress concentrations that may contribute to tissue injury. OBJECTIVE: The purpose of this study was to determine the effects of passive metatarsal phalangeal joint (MPJ) extension on plantar soft tissue stiffness and thickness. METHODS: Seventeen healthy participants (7 male, 10 female, mean age 25.3 y, SD 4.4 y, mean BMI 24.7 kg/m(2), SD 3.2 kg/m(2)) were tested. Plantar soft tissue stiffness and thickness were measured at the metatarsal heads, midfoot and heel using a custom-built indentor device and an ultrasound machine. RESULTS: Indicators of soft tissue stiffness (K(1) values) at the metatarsal heads and midfoot showed increases in stiffness of 81-88% (SD 20-33%) in the MPJ extension position compared with the MPJ neutral position. Soft tissue thickness measures at the metatarsal heads with the MPJ in neutral ranged from a mean of 8.9-13.5 mm and decreased, on average, by 8.8% (SD 2.9%) with MPJ extension. CONCLUSIONS: MPJ extension has a profound effect on increasing forefoot plantar soft tissue stiffness and a consistent but minimal effect on reducing soft tissue thickness. These changes may help transform the foot into a rigid lever at push-off consistent with the theory of the windlass mechanism.