A Systematic Meta-Analysis of the Effectiveness of (Left) Hand Contractions on Motor Performance.

Unilateral left hand contractions (LHCs) have been used as a neurophysiologically motivated intervention for optimizing and stabilizing a wide range of behaviors, including motor performance. What is currently unknown however is the efficacy and neurophysiological basis of LHC interventions. The aim of this systematic meta-analysis was to review and synthesize the evidence on the effectiveness of LHCs, as a pre-performance routine, on motor performance. We, therefore, discuss the theoretical background, highlights key methodological considerations, and suggest areas of future research. An electronic search of the PubMed, PsycInfo, Google Scholar, Scopus, and SPORTDiscus databases was conducted to identify peer-reviewed literature relating to LHCs and motor performance, skilled performance, peak performance, and choking. Ten studies (12 effect sizes) met inclusion criteria and were retained for quality assessment and synthesis. The findings indicate a moderate effect of LHC interventions on performance (g = 0.59, 95% CI = 0.14, 1.03); further, moderator analyses revealed that the effect was significant in experimental design studies, among sub-elite and elite athletes, and when participants were exposed to high-pressure situations. A trim-and-fill analysis was conducted to estimate and correct any potential publication bias, and the results suggest the effect of the LHC intervention may not be as robust as initially appeared. We conclude that this moderate effect should be interpreted with caution. Specifically, concerns regarding study quality and lack of neurophysiological specificity appear to limit the impact and significance of the LHC literature as it currently stands.