Mechanical Linkage between Achilles Tendon and Plantar Fascia Accounts for Range of Motion of Human Ankle–Foot Complex

PURPOSE: The human ankle–foot complex possesses a passive range of motion (ROM) through changes in tibiocalcaneal (θ(cal)) and foot arch (θ(arch)) angles. Based on the anatomical linkage between the Achilles tendon (AT) and plantar fascia (PF), we hypothesized that AT and PF with different mechanical properties conjointly modulate the passive ROM of the human ankle–foot complex. We examined the association of AT and PF stiffness with passive ankle–foot ROM and further addressed differences between sexes. METHODS: A series of sagittal magnetic resonance images of the foot and passive ankle plantar flexion torque were obtained for 20 men and 20 women with their ankle–foot passively rotated from 30° of plantar flexion to 20° of dorsiflexion. Based on the measured changes in AT and PF lengths, θ(cal), θ(arch), and passive torque, AT and PF stiffness were determined. RESULTS: Upon passive ankle dorsiflexion, AT and PF were lengthened; their length changes were inversely correlated. Men showed a stiffer AT, more compliant PF, less calcaneal rotation, and greater foot arch deformation compared with women. Furthermore, we found inverse correlations between AT stiffness and ROM of θ(cal), and between PF stiffness and ROM of θ(arch) in men and women. CONCLUSIONS: Passive AT and PF extensibility counter each other. AT and PF stiffness and passive ROM of ankle–foot components were countered between sexes; however, associations between stiffness and passive ROM of the ankle–foot complex were consistent between sexes. Our findings support the notion that the balanced mechanical interaction between the AT and PF can account for the passive ROM of the human ankle–foot complex in vivo, and the differences between sexes.