Fig. 1.

Salient features of the human foot, and the windlass mechanism in action. (A) A medial view of the human foot bones highlighting the pronounced longitudinal arch (LA, dashed line) and a schematic illustration of the Cal-Met angle that we used as a measure of dynamic arch compression (the angle formed between the calcaneus and metatarsal segments of the foot model, as defined in ref. 43). (B) Superior view of the human foot bones with a depiction of how the human hallux (bold outline) is greatly adducted from the opposable hallux found in fossil remains of our hominin ancestors (e.g., dashed outline). (C) A plantar view of the human foot showing the largest superficial PIMs that span the LA and MTP joints: Abductor hallucis (AH) and FDB. The PIMs also include abductor digiti minimi, quadratus plantae, flexor hallucis brevis, the lumbricals, and adductor hallucis (17), which have not been included here for clarity. (D) Depicts the windlass mechanism in action from mid to late stance in human walking. From left to right, the foot rotates about the MTP joints, tensioning the plantar aponeurosis (PA) and raising the LA (decreasing the Cal-Met angle) before the toes are plantar flexed as the PA recoils just before toe-off.