Interpretation of ankle joint moments during the stance phase of walking: A comparison of two orthogonal axes systems

Three-dimensional ankle joint moments were calculated in two separate coord inate systems, from 18 healthy men during the stance phase of walking, and were then compared. The objective was to determine the extent of difference s in the calculated moments between these two commonly used systems and the ir impact on interpretation. Video motion data were obtained using skin sur face markers, and ground reaction force data were recorded from a force pla tform. Moments acting on the foot were calculated about three orthogonal ax es, in a global coordinate system (CCS) and also in a segmental coordinate system (SCS). No differences were found for the sagittal moments. However, compared to the SCS, the GCS significantly (p < .001) overestimated the pre dominant invertor moment at midstance and until after heel rise. It also si gnificantly (p < .05) underestimated the late stance evertor moment. This f rontal plane discrepancy was attributed to sensitivity of the GCS to the de gree of abduction of the foot. For the transverse plane, the abductor momen t peaked earlier (p < .01) acid was relatively smaller (p < .01) in the GCS . Variability in the transverse plane was greater for the SCS, and attribut ed to its sensitivity to the degree of rearfoot inversion. We conclude that the two coordinate systems result in different calculations of nonsagittal moments at the ankle joint during walking. We propose that the body-based SCS provides a more meaningful interpretation of function than the GCS and would be the preferred method in clinical research, for example where there is marked abduction of the foot.