INTRODUCTION: Hip extensors have a major contribution to sprint running propulsion in the early stance phase when glutes and hamstrings shorten at high velocities (1). Considering the difficulties to perform isokinetic testing of this muscle group, hip extensors are typically tested in isometric conditions, which may not reflect the role of hip extensors in sprinting. The aim of this study was to 1) develop a protocol for testing hip extensors in isometric and concentric conditions, and 2) examine the association between hip extension torque measured at different velocities and sprint horizontal force in elite athletes. METHODS: A total of 82 elite athletes from sprint-based sports (track and field, rugby 7) participated in this two-part study. In Part 1, 13 sprinters/hurdlers performed maximal isometric (50 deg hip flexion) and concentric hip extensions on a dynamometer at four velocities (60, 120, 180, and 350 deg/s) with their dominant leg, in supine position. Contractions were performed with knee flexed to 30 deg (externally fixed), as well as without fixing the knee but asking the athletes to keep it at ~90 deg flexed position. Torque and dynamometer position, and 3-D displacements of reflective markers were recorded. A custom model was used to calculate hip angles, which was used to define relevant dynamometer positions. Differences and correlation between torque in extended and flexed knee positions were calculated. In Part 2, 69 athletes performed isometric and concentric hip extensions (60 and 350 deg/s) without fixing the knee while torque and dynamometer position were recorded. Torque in each condition was analysed over the range of motion defined in Part 1, and was related to the maximal horizontal force in sprinting as measured using a radar during 40m acceleration sprints. RESULTS: Based on kinematic analysis, dynamometer range of motion of interest was 50 to 0 deg, over which torque was analysed in all conditions. In Part 1, hip extension torque was 29 to 42% higher in the relatively more extended knee position than in relatively more flexed knee position at all velocities (p<0.01). The torque measured in the two knee positions significantly correlated in all conditions (r =0.62 to 0.89, p < 0.025), with the weakest correlation at 350 deg/s. In Part 2, isometric and concentric torque at 60 deg/sec significantly correlated to horizontal force in sprinting (r=0.53, p<0.01, and r = 0.48, p<0.01, respectively), however, this association was not significant at 350 deg/s (r=0.22, p=0.06). CONCLUSION: In isometric and at slow concentric contractions, evaluating hip extension torque in ~90 deg flexed knee position provides valuable information for elite athletes from sprint-based sports. However, at high velocity, hip extensors should be tested in a relatively extended knee position.
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