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Luca Ruggiero, Susan Dewhurst, and Theodoros M. Bampouras

Leg stiffness is an important performance determinant in several sporting activities. This study evaluated the criterion-related validity and reliability of 2 field-based leg stiffness devices, Optojump Next® (Optojump) and Myotest Pro® (Myotest) in different testing approaches. Thirty-four males performed, on 2 separate sessions, 3 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 3 trials. For all 3 methods, Optojump correlated highly to the FP (range r = .98–.99) with small bias (range 0.91–0.92, 95%LoA 0.86–0.98). Myotest demonstrated high correlation to FP (range r = .81–.86) with larger bias (range 1.92–1.93, 95%LoA 1.63–2.23). Optojump yielded a low CV (range 5.9% to 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% to 13.0%), moderate ICC (range 0.64–0.79), and SEM ranging from 6.3 to 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.

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Luca Ruggiero, Samantha E. Pritchard, John Warmenhoven, Tavis Bruce, Kerry MacDonald, Marc Klimstra, and Chris J. McNeil

Purpose: In volleyball, jump execution is critical for the match outcome. Game-play-related neuromuscular impairments may manifest as decreased jump height (JH) or increased jump total duration, both of which are pivotal for performance. To investigate changes in JH and kinetics with game play, the authors conducted a prospective exploratory analysis using minimal-effect testing (MET) and equivalence testing with the 2 one-sided tests procedure, univariate, and bivariate functional principal component analysis, respectively. Methods: Twelve male varsity athletes completed 3-set matches on 2 consecutive days. Countermovement jumps were performed on a force platform immediately prematch and postmatch on days 1 and 2 and once on days 3 and 4. Results: Across sessions, JH was equivalent (P < .022, equivalence test), while total duration reported inconclusive changes (P > .227). After match 2, MET indicated that relative force at zero velocity (P = .036) decreased, while braking duration (P = .040) and time to peak force (P = .048) increased compared with baseline. With the first and second functional principal components, these alterations, together with decreased relative braking rate of force development (P = .092), were already evident after match 1. On day 4, MET indicated that relative peak force (P = .049), relative force at zero velocity (P = .023), and relative braking rate of force development (P = .021) decreased, whereas braking duration (P = .025) increased from baseline. Conclusions: Impairments in jump kinetics were evident from variables related to the countermovement-jump braking phase, while JH was equivalent. In addition to these experimental findings, the present research provides information for the choice of sample size and smallest effect size of interest when using MET and 1- and 2-dimensional analyses for countermovement-jump height and kinetics.