A Technical Report on Reliability Measurement in Asymmetry Studies

Bailey, CA, Sato, K, and McInnis, TC. A Technical report on reliability measurement in asymmetry studies. J Strength Cond Res 35(7): 1779-1783, 2021-Much of the current literature on asymmetry in sport performance may be biased by evaluating reliability before quantifying the asymmetry metric. This technical report aimed to evaluate the reliability of asymmetry measurements in countermovement jumps (CMJs) by providing measures before production of asymmetry metrics, after production of scalar- and vector-based asymmetry metrics, and to analyze the resulting differences that can lead to misinformed decision making. Thirteen collegiate baseball players (19.9 ± 1.3 years, 82.2 ± 10.9 kg) participated in CMJ testing on 2 force plates to evaluate symmetry index (SI) scores as scalar (asymmetry magnitude only) and vector (asymmetry magnitude and direction) quantities. Relative and absolute reliability were evaluated for peak force (PF), scalar PF SI, and vector PF SI. Results showed that reliability measures produced on the constituent force-time variable provide much more favorable results compared with evaluating reliability of the asymmetry measure itself (coefficients of variation of 10.4-15.7% vs. 63.2-1,497.1%). Findings also showed that reliability is altered depending on whether asymmetry is quantified as a scalar or vector. Asymmetry should likely be quantified as a vector for reliability purposes because that allows variability in both magnitude and direction. These findings demonstrate that inadequate evaluation of reliability in asymmetry leads to biased results. Practitioners should use caution when considering the results of asymmetry assessments as they may not be as reliable as they are often portrayed.