Changes in kinematic chain dynamics between calcaneal pronation/supination and shank rotation during load bearing associated with ankle position during plantar and dorsiflexion

[Purpose] This study aimed to elucidate the dynamics of the kinematic chain of shank rotation accompanying calcaneal pronation/supination during weight-bearing changes associated with ankle positions during plantar/dorsiflexion and whether this biomechanical phenomenon is affected by age. [Participants and Methods] The study included 54 participants (108 legs, 25 young and 29 old participants). A 3-dimensional motion analysis system measured the calcaneal pronation/supination and the shank rotation angles during pronation/supination of the foot when upright. The kinematic chain ratio was defined as the linear regression coefficient (shank rotation angle/calcaneal pronation/supination angle). The kinematic chain ratio was measured during plantar flexion, in an intermediate position, and dorsiflexion. [Results] Significant differences in the kinematic chain ratio were related to ankle position but not age. The mean kinematic chain ratio in all participants was 0.9 ± 0.3 for plantar flexion, 1.0 ± 0.2 for an intermediate position, and 1.3 ± 0.4 for dorsiflexion, showing significant differences between the 3 ankle positions. [Conclusion] During dorsiflexion, the long axis of the shank and that of the subtalar joint are nearly parallel; thus, shank rotation increases (larger kinematic chain ratio). During plantar flexion, a larger angle is created; thus, shank rotation decreases (smaller kinematic chain ratio). When analyzing the kinematic chain between calcaneal pronation/supination and shank rotation, it is essential to consider the ankle position during plantar/dorsiflexion.