Skeletal muscle volume following dehydration induced by exercise in heat

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.

Effect of Hypohydration on Peripheral and Corticospinal Excitability and Voluntary Activation

“…Although no change in Na + -K + ATPase activity was reported by Green et al [31], this has been observed in other studies [32]. No data are currently available on the effects of hypohydration and exercise in the heat on ion exchange, but Hackney et al [33] recently demonstrated that knee extensor muscle volume was significantly reduced by ∼4% when a 3% hypohydration was induced by cycling in the heat with no fluid replacement. This was suggested to be due to sequestration of intracellular water to the extracellular space.…”

Skeletal muscle water and electrolytes following prolonged dehydrating exercise

“…Using MRI technique, Hackney et al. () reported a 1.2% reduction in leg volume for each 1% decrease in body mass. Costill et al.…”

“…However, the ratio between whole‐body and muscle dehydration seems to depend on the intensity of exercise. Both investigators (Costill et al., ; Hackney et al., ) used very hot environments (37–39 °C) and low workload (97 ± 39 W; Hackney et al., ) to achieve up to 5% dehydration. In contrast, Neufer et al.…”

“…At low workloads, glycogenolysis rate is low (Horowitz et al., ) and less water may be liberated and available to be shifted away from the muscles. Prior studies (Costill et al., ; Neufer et al., ; Hackney et al., ) established that muscle water participates in the losses of fluid that take place during prolonged exercise in the heat. Our study helps to define the factors that influence the rate of muscle water loss during prolonged dehydrating exercise (e.g., exercise intensity).…”

“…With similar degree of dehydration, Hackney et al. () report only 5% reduction in knee extensor water content [measured by magnetic resonance imaging (MRI)]. The exercise intensity was higher in the Neufer et al.…”