(54) NON-DOMINANT HAND GRIP STRENGTH MEDIATES PEAK FORCE OUTPUT IN ISOMETRIC MID-THIGH PULL: THE INFLUENCE OF LIFTING STRAPS

Purpose: Assess intercorrelations of strapped vs unstrapped grips on IMTP net peak force (NPF) and determine the relative contribution of hand grip strength. Methods: 29 physically active subjects (9 males, mass: 77.1±10.9 kg, height: 171.1±3.3 cm, age: 30.3±6.9 years, median training age: 5.5 yrs) performed two IMTP testing sessions 48-72 hours apart. Subjects performed a self-selected warm up followed by two familiarization reps per condition at 50% and 75% maximum volitional effort. On the first day subjects performed 3 maximal IMTPs on dual force plates (ForceDecks Lite®, Vald, Newstead, AUS, 1000Hz sampling rate) of one strapped (figure 8 or traditional) and one unstrapped condition (pronated or mixed grip), randomized. On the second day, subjects performed the reciprocal strapped and unstrapped tests. Grip strength was measured using a handgrip dynamometer on both days. Limb dominance was defined as the hand with a stronger grip. Spearman-rank correlations were used to assess relationships among all outcome variables. To account for the contribution of grip strength, determinants of unstrapped pronated NPF were identified using a stepwise linear regression model, including NPF for all grip conditions and grip strength for non-dominant (nDOM), dominant (DOM), mean of both hands, and mean of the best trial from each hand as predictors. Mediation analysis was employed to evaluate whether grip strength influenced unstrapped IMTP. Mediation significance and confidence intervals were assessed via bootstrapping (5000 simulations). Significance was set a priori at α< 0.05. Results: Strong to very strong NPF correlations were observed between all grip conditions (r=0.89 – 0.92). NPF was strongly correlated with grip strength (r=0.80 – 0.87) across all conditions. Stepwise linear regression identified NPF with traditional straps and nDOM grip strength as significant predictors of pronated, unstrapped NPF (r²=0.888). Mediation analysis revealed that nDOM grip strength significantly mediated the relationship between strapped and unstrapped NPF for traditional (ACME=0.302, 95% CI [0.13, 0.52], p=0.0016) and figure 8 straps (ACME=0.398, 95% CI [0.187, 0.59], p=0.0016). The proportion of the total effect mediated by grip strength was 46.4% (95%CI [20.0%, 73.0%]) and 54.0% (95% CI [25.8%, 81%]) for traditional and figure 8 straps respectively. The direct effect of strapped NPF on unstrapped NPF remained significant for both (ADE=0.339-0.348, p< 0.01). Conclusions: The relationship between strapped and unstrapped NPF is mediated by non-dominant grip strength, which explains 81.2% of the variance in pronated, unstrapped IMTP. Non-dominant grip strength is a better predictor than dominant grip strength or the maximum or average grip strength of both hands. Approximately half (46-54%) of the total effect of strapped NPF on unstrapped NPF is explained by grip strength of the non-dominant hand. Practical Applications: Non-dominant grip strength is a significant mediating factor in unstrapped IMTP performance. Strapped and unstrapped IMTP NPF are strongly correlated, suggesting that unstrapped IMTP could identify group rankings, but lifting straps are necessary to fully quantify total pulling strength from an IMTP. Acknowledgements: The authors report no funding for this project and no financial conflicts of interest.