Nicolas M. Philipp1, Derek A. Crawford2, Matthew J. Garver2, & H. Scott Strohmeyer2

1University of Kansas, Lawrence, Kansas; 2University of Central Missouri, Warrensburg, Missouri

Over the last few years, researchers and sport scientists have expressed increased interest in the effects of interlimb asymmetry on sport performance. PURPOSE: The purpose was to evaluate the utility of three different methods of classifying asymmetry to highlight if a 6-week resistance training intervention can meaningfully reduce levels of asymmetry, and to determine if reductions in asymmetry are associated with improvements in change of direction (CoD) performance. METHODS: Eighteen, division-two collegiate American football skill position players (age = 21.2 ± 1.4) completed all procedures. These procedures involved the completion of the Bulgarian Split Squat (BSS) exercise (measured by a Tendo) from which asymmetries in relative average power (Rel.AP) and relative peak power (Rel.PP) were derived ([(strong limb – weak limb) / strong limb x 100]). Methods of classifying asymmetry were as follows: asymmetry greater than a) intra-individual coefficient of variation, b) population mean + smallest worthwhile change (SWC), and c) population mean + standard deviation (M+SD). Participants also completed three repetitions of the 505 and L-drill COD tests with the average being used to assess performance. Odds ratios (OR; likelihood for improvement or regression), linear mixed models (fixed factors of time and asymmetry), generalized linear models (categorical change: improve or regress), and Pearson’s correlation coefficient were used. RESULTS: Participants classified as asymmetrical, exhibiting observed asymmetry in Rel.PP scores larger than M±SD, had the greatest likelihood of reducing asymmetry (OR = 6.99, 95% CI: 1.4, 12.5) and improving L-drill performance (OR = 1.33, 95% CI: -2.1, 4.8). Further, our training intervention meaningfully reduced Rel.AP asymmetry (p = 0.027, Cohen’s d = 0.73). At the group level, these reductions in asymmetry were accompanied by improvements in L-drill performance that were larger than the sample SWC. At the individual level, change scores in asymmetry and change scores in CoD performance only showed small, non-significant correlations. CONCLUSION: Interlimb asymmetry is specific to the method of asymmetry classification. Our results suggest that practitioners only intervene in athletes whose asymmetry scores exceed the sample M+SD using the Rel.PP metric.

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