Validity and reliability of two field-based leg stiffness devices: implications for practical use

Leg stiffness is an important performance determinant in several sporting activities. This study evaluated the criterion-related validity and reliability of two field-based leg stiffness devices, Optojump Next® and Myotest Pro® in different testing approaches. Thirty-four males performed, on two separate sessions, three trials of 7 maximal hops, synchronously recorded from a force platform (FP), Optojump and Myotest. Validity (Pearson’s correlation coefficient, r; relative mean bias; 95% limits of agreement, 95%LoA) and reliability (coefficient of variation, CV; intraclass correlation coefficient, ICC; standard error of measurement, SEM) were calculated for first attempt, maximal attempt, and average across three trials. For all three methods, Optojump correlated highly to the FP (range r = 0.98-0.99) with small bias (range 0.91-0.92, 95%LoA 0.86-0.98). Myotest demonstrated high correlation to FP (range r = 0.81-0.86) with larger bias (range 1.92-1.93, 95%LoA 1.63-2.23). Optojump yielded a low CV (range 5.9%-6.8%), high ICC (range 0.82-0.86) and SEM ranging 1.8-2.1 kN/m. Myotest had a larger CV (range 8.9%-13.0%), moderate ICC (range 0.64-0.79) and SEM ranging from 6.3-8.9 kN/m. The findings present important information for these devices and support the use of a time efficient single trial to assess leg stiffness in the field.