Mark Belio, Kyle M. Edgar, Sasha Riley, Mohamdod S. Alzer, Andrei Sergeyev, Sarah Blount, Alain Aguilar, Joshua Beaver, Erik D. Hanson, FACSM. University of North Carolina at Chapel Hill, Chapel Hill, NC.

BACKGROUND: Magnesium (Mg) supplementation can improve recovery of maximal muscle force in the days following fatiguing exercise. However, the ability of Mg to enhance recovery during or immediately following strenuous activity has received far less attention. With new forms of Mg supplementation now available, this study examined if transdermal Mg chloride (tMgCl2) improves muscle endurance during a fatigue protocol (FP). Additionally, the role of tMgCl2 on short-term recovery of maximal isometric handgrip force (MIHF) was determined. METHODS: 40 healthy, recreationally active subjects (age 22±3 y, 50% female) with resistance training experience completed a double-blind, placebo-controlled randomized crossover trial with familiarization. Treatment and limb dominance sequencing were randomized and counter-balanced. MIHF was evaluated with handgrip dynamometry. The FP consisted of 3 sets of isometric holds to failure at ~50% MIHF with one minute (min) of rest. Recovery of MIHF was assessed immediately after the FP and every 45 seconds (s) for 4.5 min. MIHF was normalized to baseline and expressed as a percentage. Isometric hold time during the FP and recovery of MIHF were analyzed using a linear mixed model with time and condition as fixed factors and subject as a random factor. Area under the curve (AUC) for MIHF recovery was analyzed using a paired-samples t-test. RESULTS: Time to failure for set one of the FP was 68.9s, with decreases of 49.3% (p<0.001) and 62.4% (p<0.001) at the second and third set, respectively. There was no difference in time to failure between conditions [MD = 2.1s; 95%CI (-1.4, 5.7); p=0.240] or interaction effect present (p=0.762). Immediately following the FP, MIHF decreased to 58.6% of baseline, steadily recovered to 64.7% at 45s, 71.3% at 135s, and 77.6% at 270s, but force remained suppressed relative to starting values (all p<0.001). There was no difference in recovery between conditions [MD=0.5%; 95%CI (-0.8, 1.9); p=0.457] or interaction effect (p=0.743) present. There was also no condition effect on MIHF recovery AUC [MD=60 kg·s; 95%CI (-837, 957); p=0.896]. CONCLUSIONS:These results suggest that tMgCl2 does not improve muscle endurance, nor did it hasten MIHF recovery after a FP. Under the present conditions, tMgCl2 does not enhance muscle function, with additional outcomes and direct comparisons between Mg supplementation type still being required.

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