Positional difference of malleoli-midpoint from three-dimensional geometric centre of rotation of ankle and its effect on ankle joint kinetics.

Backgrounds: Joint kinetic calculations are sensitive to joint centre locations. Although geometric hip and knee joint centre/axis are generally developed, the ankle joint centre (AJC) is conventionally defined as the midpoint between the malleolus lateralis and medialis (AJC) in most gait analyses.

Research Question: We examined the positional difference of the AJC from the geometric centre of rotation (AJC) and its effect on the ankle joint kinetics in representative human gaits.

Methods: In the first experiment, we calculated the AJC and indicated its location on the ankle MRI in 14 (seven male and seven female) participants. In the second experiment, we compared ankle kinematics/kinetics based on AJC and AJC during walking and hopping at 2.6 Hz in 17 (nine male and eight female) participants.

Results: In both experiments, AJC was located at positions significantly medial (-9.2 ± 5.4 mm and -10.1 ± 4.4 mm) and anterior (17.0 ± 7.4 mm and 15.3 ± 5.2 mm) from the AJC. Furthermore, the AJC underestimated peak dorsiflexion (AJC/AJC: 52.6 ± 17.1%) and inversion (AJC/AJC: 62.2 ± 11.5%) torques and their durations in walking. Additionally, AJC overestimated the plantar flexion torque in both gait modes [AJC/AJC: 111.3 ± 4.8% (walking) and 112.7 ± 6.3% (hopping)].

Significance: We therefore concluded that the positional difference between the geometric and landmark-based AJC definitions significantly affected ankle kinetics, thereby indicating that the functional method should be used for defining AJC for gait analysis.