Variability of the dynamic stiffness of foot joints. Effect of gait velocity variation.

BACKGROUND: Comparison of dynamic stiffness of foot joints was previously proposed to investigate pathological situations with changes in the properties of muscle and passive structures. Samples must be controlled to reduce the variability within groups being compared, which may arise from different sources, such as gait speed or foot posture index (FPI).

METHODS: In this work, variability in the measurement of the dynamic stiffness of ankle, midtarsal and metatarsophalangeal joints was studied in a controlled sample of healthy adult male subjects with normal FPI, and the effect of gait speed was analyzed. In Experiment I, dynamic stiffnesses were obtained in three sessions, five trials per session, for each subject, taking the mean value across trials as representative of each session. In Experiment II, five trials were considered at slow, comfortable, and fast velocities RESULTS: Results showed similar inter- and intra-session errors and intra-subject errors within sessions, indicating the goodness of using five trials per session for averaging. Intra- and inter-subject variability data provided can be used to select the sample size in future comparative analyses. Significant differences with gait speed were observed in most dynamic stiffnesses considered, with a general rise when gait speed increased, especially at the midtarsal joint, this being attributed to an active modulation produced by the central nervous system.

CONCLUSIONS: Differences from gait speed were higher than intra- and inter-session repeatability errors for the propulsion phases at the ankle and midtarsal joints, comparative analyses at these phases needing a more exhaustive control of gait speed to reduce the required sample size. s.