Isometric strength: reliability and relationship with sprint acceleration performance

Introduction: In recent years, the popularity of multi-joint isometric strength testing has increased dramatically. The reliability of variables produced from the force-time curve is conflicting in the literature, and the reporting of reliability is not standardised across the research. The relationship of the isometric mid-thigh pull (IMTP) and isometric squat (ISqT) to sprint performance has been examined in field sports, including soccer and rugby, but there is a lack of research on track and field sprint athletes and hurling players. Further, there is no research examining how isometric strength relates to sprinting kinematics. Aim: To determine and compare the reliability of the IMTP and ISqT and investigate the relationship between isometric strength and sprint performance in track and field sprinters and field sport athletes. Methods: Study one involved a narrative review of the current literature to determine the level of reliability of the IMTP and ISqT and the reporting of normative data. Study two measured the reliability and usefulness of the IMTP and ISqT performed at the same knee and hip angles and determined the magnitude of differences between tests. Study three examined the relationship between the IMTP and ISqT and sprint acceleration performance among track and field sprinters. Study four explored the relationship of the IMTP and sprint kinematics among male sprinters. Study five examined the relationship between the ISqT, countermovement jump (CMJ) and reactive strength index (RSI) and sprint performance in hurling players and determined if principal component analyses (PCA) could be used to reduce the dataset to independent factors. Results: Study one demonstrated that peak force is the most reliable measure and can be used to determine maximum strength capabilities. Study two identified that the same measures from the force-time curve were reliable in both the IMTP and ISqT and both tests are capable of detecting changes in performance of peak force. When split by sex, female participants produced significantly greater peak force during the ISqT, with no significant differences among male participants. Among males, Study three reported large and very large correlations between IMTP and ISqT measures and 0 – 5 m time (r = -0.517 to -0.714; p < 0.05). Study four established higher force generation in the IMTP was associated with faster velocities at rear block clearance and step 2 (r ≥ 0.544, p < 0.05) and a longer rear block clearance relative step length (r ≥ 0.547, p < 0.05) in male sprinters. Study five established correlations between force at 100 ms and 0 – 5 m and 5 – 10 m (r ≥ -0.407, p < 0.05), CMJ and all splits up to 30 m (r ≥ -0.391, p < 0.05) and between RSI and splits over 5 m (r ≥ -0.557, p < 0.05) in hurling players. The PCA identified three principal components explaining 86.1% of the variation in the dataset [SSC capabilities and sprint times (33.3%), time specific force values (29.9%) and absolute and relative strength (29.9%)]. Conclusion: The IMTP and ISqT are reliable tests for peak force and time dependent measures ≥ 150 ms when measured at the same knee and hip angles. Male sprinters that produce greater force are faster over the first 5 m of a sprint from the blocks and higher force generation during the IMTP is associated with faster velocities at rear block clearance and step 2 and longer rear block clearance relative step length. Among hurlers, ISqT force at 100 ms distinguished between performance levels from 0 – 5 m, players that produced higher force were faster. Isometric tests are reliable tests that correlate with early acceleration and this is important for coaches and practitioners monitoring specific types of strength that relate to initial acceleration ability.