Dynamic joint stiffness of the ankle in healthy and rheumatoid arthritis post-menopausal women.

The purpose of this study was to compare rheumatoid arthritis post-menopausal women (RAPW) with pathological involvement of the lower limb joints and age-matched healthy post-menopausal women (AHPW) in regard to the dynamic joint stiffness of the ankle (DJS) during the stance phase of gait. Data were collected from 18 RAPW and 18 AHPW. Gait was assessed by a three-dimensional motion analysis system synchronised with a force plate. Subjects walked barefoot at natural and self-selected speed, performing 14 valid trials (comprising 7 left and 7 right foot-steps on a force plate). The stance phase was split into three sub-phases that corresponded to the three angular displacements of the ankle that occurred during this phase, namely, controlled plantar flexion (CPF), controlled dorsiflexion (CDF), and powered plantar flexion (PPF). A linear model represented each sub-phase and computed DJS. Model fitting was assessed by the coefficient of determination (R). The coefficient of variation (CV) was used to assess intra-individual variability. In all sub-phases, R values for both groups were higher than 0.85. There were no differences in the R values among groups. RAPW showed a higher DJS during the CPF (p < 0.05). CDF and PPF yielded no differences among groups. During CPF, RAPW yielded a higher CV for DJS (p < 0.01). RAPW also yielded lower ankle angular displacements during CPF and PPF (p < 0.05). Findings suggested that the stance phase of RAPW and AHPW can be studied by a linear ankle 'moment of force -- angle' relationship. During CPF, RAPW exhibited excessive ankle stiffness and presented a higher intra-individual DJS variability.