Skeletal muscle volume following dehydration induced by exercise in heat

Kyle J Hackney,Summer B Cook,Lori L Ploutz-Snyder,Timothy J Fairchild

doi:10.1186/2046-7648-1-3

Abstract

BackgroundIntracellular skeletal muscle water is redistributed into the extracellular compartment during periods of dehydration, suggesting an associated decline in muscle volume. The purpose of this study was to evaluate skeletal muscle volume in active (knee extensors (KE)) and less active (biceps/triceps brachii, deltoid) musculature following dehydration induced by exercise in heat.MethodsTwelve participants (seven men, five women) cycled in the heat under two conditions: (1) dehydration (DHYD) resulting in 3% and 5% losses of estimated total body water (ETBW), which was assessed by changes in body mass, and (2) fluid replacement (FR) where 3% and 5% losses of ETBW were counteracted by intermittent (20 to 30 min) fluid ingestion via a carbohydrate-electrolyte beverage. During both conditions, serum osmolality and skeletal muscle volume (assessed by magnetic resonance imaging) were measured at baseline and at the 3% and 5% ETBW loss measurement points.ResultsIn DHYD, serum osmolality increased at 3% (p = 0.005) and 5% (p < 0.001) ETBW losses, while FR decreased serum osmolality at the 5% loss of ETBW time point (p = 0.009). In DHYD, KE muscle volume declined from 1,464 ± 446 ml to 1,406 ± 425 ml (3.9%, p < 0.001) at 3% ETBW loss and to 1,378 ± 421 ml (5.9%, p < 0.001) at 5% ETBW loss. The largest decline in KE volume in DYHD occurred in the mid-belly (31 ml, p = 0.001) and proximal (24 ml, p = 0.001) regions of the grouped vasti muscles. There were no changes in volume for the biceps/triceps (p = 0.35) or deltoid (p = 0.92) during DHYD. FR prevented the loss of KE muscle volume at 3% (1,430 ± 435 ml, p = 0.074) and 5% (1,431 ± 439 ml, p = 0.156) ETBW loss time points compared to baseline (1,445 ± 436 ml).ConclusionsFollowing exercise in the heat, the actively contracting muscles lost volume, while replacing lost fluids intermittently during exercise in heat prevented this decline. These results support the use of muscle volume as a marker of water loss.

Highlights

Intracellular skeletal muscle water is redistributed into the extracellular compartment during periods of dehydration, suggesting an associated decline in muscle volume
Body mass was significantly lower in the DHYD compared with that in the fluid replacement (FR) at 3% (p = 0.007) and 5% (p < 0.001) estimated total body water (ETBW) loss measurement time points
Serum osmolality was significantly greater in the DHYD condition compared with that in FR at both the 3% (p < 0.001) and 5% (p < 0.001) ETBW losses

Summary

Introduction

Intracellular skeletal muscle water is redistributed into the extracellular compartment during periods of dehydration, suggesting an associated decline in muscle volume. During prolonged exercise in the heat, there is an increase in plasma and serum osmolality [10,11], which resulted from a greater net loss of water as compared with sodium (Na+). This elevation in osmolality is the primary hypothesized mechanism for the mobilization of water across the muscle cell membrane [12]. Consumption of water or a carbohydrate-electrolyte beverage during prolonged exercise counteracts this increase in serum osmolality [10,11], negating one of the primary mechanisms whereby water is sequestered across the muscle cell membrane. The implications of fluid replacement via water or carbohydrate-electrolyte beverages on muscle water during exercise in heat are currently not known

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