Reliability of Passive Hip Extension Mechanical Parameters in Healthy Subjects

Objective: The objective was to estimate the reliability of three stiffness parameters (moment, angle and stiffness coefficient) of the hip extension movement, for three knee positions. Design: Twenty healthy subjects were assessed on two days separated by a one-week interval. A modified Biodex dynamometer was used to move the right lower limb passively in a side-lying position and to record the hip angle and moment during the movement. Three trials with the knee positioned at 0°, 45° and 90° of flexion were performed each day. Passive stiffness was quantified by three parameters derived from the moment-angle curve of each subject. The generalizability theory was used to determine the reliability coefficients and standard errors of measurement (SEM) computed for a design comprising five trials in one day. Results: Overall, the reliability coefficients were moderate to good with values ranging from 0.58 to 0.83 at the three knee positions. The highest SEM values were 4.15 Nm, 8.4° and 0.15 Nm/° for the moment, angle and stiffness coefficient parameters, respectively. The main source of error variance was related to the subject-day interaction factor and a systematic day effect was also noted. Conclusions: This study constitutes a first step towards objective evaluation of hip extension joint stiffness. Future studies will be needed to better control the alignment of the hip to reduce the systematic day error on the stiffness parameters.