Comparison of ankle dorsiflexion using XROMM and external angular kinematics in a quadrupedally walking macaque

Movement at complex joints, such as the ankle, can be challenging to quantify from external kinematics alone. We compare x-ray reconstruction of moving morphology (XROMM) and angles derived from high-speed video footage to study dorsiflexion in the ankle of a rhesus macaque (Macaca mulatta) during stance phase. We find inconsistent correspondence between angles measured on standard videos using five approaches and those measured directly on the talocrural joint using XROMM, indicating that different measurement methods are capturing different kinematic processes. Many externally measured angles indicate a range of motion of 30° during stance phase. The XROMM data, however, demonstrate that the talocrural joint itself only dorsiflexes about 15° through stance phase, with much greater mobility during swing phase. This suggests that other parts of the foot, likely the midfoot, contribute substantially to dorsiflexion in stance phase, while the talocrural joint is actively held in place.