Kinematics of the Talocrural Joint
The talocrural joint is a single-degree-of-freedom joint, and its movement patterns are highly characteristic. All motions occurring at this joint take place around a specific axis of rotation that passes through the talar body and the distal ends of both malleoli.
From an anatomical perspective, the lateral malleolus is positioned lower and more posterior compared to the medial malleolus (which can be directly verified through palpation). Due to this structural arrangement, the axis of rotation deviates slightly from a perfect medial-lateral axis.
From an anatomical perspective, the lateral malleolus is positioned lower and more posterior compared to the medial malleolus (which can be directly verified through palpation). Due to this structural arrangement, the axis of rotation deviates slightly from a perfect medial-lateral axis.
As illustrated, the axis of rotation (marked in red) passes from the lateral to the medial side through the talus and the malleoli. This axis follows a path that is slightly tilted upward and forward. Specifically, the axis deviates approximately 10 degrees in the frontal plane (Figure A) and about 6 degrees in the horizontal plane (Figure B) from a pure medial-lateral axis. Because of this distinctive inclination of the axis, dorsiflexion is accompanied by slight abduction and eversion, whereas plantarflexion is accompanied by slight adduction and inversion.
Due to these complex motion characteristics, the talocrural joint biomechanically generates pronation and supination movements. However, since the axis deviates only slightly from a pure medial-lateral orientation, the primary components of pronation and supination at the talocrural joint are naturally dorsiflexion and plantarflexion.
The horizontal and frontal plane components of pronation and supination are relatively minor in magnitude. As such, these components are generally not considered in most clinical evaluations and therapeutic interventions.
The neutral position of the talocrural joint, which serves as the reference for defining 0 degrees, is when the foot is at a right angle (90 degrees) to the leg. Although research methodologies and measurement techniques vary, the talocrural joint typically allows for 15–25 degrees of dorsiflexion and 40–55 degrees of plantarflexion relative to this neutral position.
Additionally, secondary movements from the adjacent subtalar joint are known to contribute approximately 20% to the total range of motion. In practical daily activities, dorsiflexion and plantarflexion of the talocrural joint can be observed in two primary scenarios: first, when the foot is freely moving in the air, and second, when the foot is fixed on the ground during gait (e.g., during the stance phase when the leg rotates forward).