Six fresh-frozen adult cadaveric specimens were mounted in an Instron materials testing machine with use of a cemented intramedullary rod. Angular relationships between the first metatarsal and the talus were recorded with a sonic digitizer. A flatfoot deformity was created by dividing the talonavicular joint capsule (superiorly, medially, and inferiorly), the spring ligament, the anteromedial aspect of the subtalar joint capsule, and the plantar fascia. Angular displacement in the sagittal and transverse planes was recorded at no load and at 100, 350, and 700-newton plantar loads. Each specimen was subjected to four different reconstructions with tenodesis, and the angular relationship between the first metatarsal and the talus was measured at the four levels of load. A reconstruction with use of the peroneus longus tendon was performed by preserving its insertion into the first metatarsal, rerouting the tendon and passing it from medial to lateral through a calcaneal bone tunnel, and anchoring it to the lateral aspect of the calcaneus. A reconstruction with the tibialis tendon was performed by passing the medial third of the tendon from dorsal to plantar through the navicular and from medial to lateral through the calcaneal bone tunnel and securing it to the lateral aspect of the calcaneus. The reconstruction with the tibialis anterior tendon was repeated with the tendon graft routed along the medial aspect of the navicular, directly through the calcaneal bone tunnel. The fourth reconstruction was done with use of an Achilles tendon allograft.(ABSTRACT TRUNCATED AT 250 WORDS)