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_ankle) 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_ankle. Model fitting was assessed by the coefficient of determination (R²). The coefficient of variation (CV) was used to assess intra-individual variability. In all sub-phases, R² values for both groups were higher than 0.85. There were no differences in the R² values among groups. RAPW showed a higher DJS_ankle during the CPF (p < 0.05). CDF and PPF yielded no differences among groups. During CPF, RAPW yielded a higher CV for DJS_ankle (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_ankle variability.