Impact of Correcting for Pelvic Tilt on the Assessment of Concentric Hip Extension Torque: Validity and Reliability of Isokinetic Dynamometry.

A low hip extensor strength has been associated with various musculoskeletal injuries and an impaired motor performance. Hip strength is often assessed by using an isokinetic dynamometer. However, the validity and reliability of such testing have not been examined comprehensively. In this study, 14 participants performed maximal concentric contractions on a dynamometer in supine position at 60° s-1, 120° s-1, 180° s-1, and 350° s-1 on two different days to assess inter-day reliability. Hip angle and angular velocity were determined using 3-D motion analysis (3-D MOCAP) and were compared to the angle and angular velocity of the dynamometer. Additionally, we corrected dynamometer kinematics using linear regressions incorporating pelvic tilt angle and angular velocity. Our results showed that the dynamometer angle was lower than the hip angle based on 3-D MOCAP (p < 0.001). This was mostly explained by pelvic tilt (R2 = 0.81). From 60° to 180° s-1 the torque was lower when the range of motion was defined based on 3-D MOCAP (d = 1.0-1.2, p < 0.001) compared to the dynamometer, which difference was reduced significantly after correcting the hip angle for the pelvic tilt by applying regression models (d = 0.12-0.35, p > 0.05). From 45° to 20° hip angles the reliability of torque was good-to-excellent except at 350° s-1. In future studies, considering pelvic tilt may improve the accuracy and reliability of examining hip extension torque.