To analyse the critical level of water deficit which causes a decrease in aerobic and anaerobic exercise performance, a step test score (STS) and 10 s maximal anaerobic power (MAP) output during cycling exercise were measured in two experiments (Ex-1, n=7, and Ex-2, n=9), before and after baseball practice, using subjects who played regularly. The measurements in both Ex-1 and Ex-2 were repeated under four conditions of fluid ingestion (FI) (FI of 80%, 60%, 40%, and 20% of the total sweat loss) on hot summer days. The subjects were allowed free access to a sports beverage, maintained at 10-15 degrees C, within any given FI condition during the exercise. The [mean (SEM)] duration of the exercise and the environmental conditions (wet bulb globe temperature) were similar between Ex-1 [3.52 (0.14) h and 29.2 (0.6) degrees C, respectively] and Ex-2 [3.82 (0.12) h and 29.2 (0.4) degrees C, respectively]. In both Ex-1 and Ex-2, the loss of body mass (Delta m(b)) increased significantly as FI decreased. In Ex-1, the STS significantly decreased ( P<0.05) at values of delta m(b) in excess of 2.4 (0.2)% (40%FI). In Ex-2, the MAP remained unchanged at values of delta m(b) up to 2.5 (0.3)% (40%FI), while the MAP significantly decreased ( P<0.05) at values of delta m(b) of 3.9 (0.2)% (20%FI). These results suggest that there is a critical level of water deficit at which a decrease in aerobic and anaerobic performance occurs, and that aerobic performance may be more adversely influenced by dehydration than anaerobic power output during exercise-induced dehydration.
To clarify whether the resting background effects (genomic) or exercise-related action (non-genomic) of aldosterone (ALD) is primarily affected to an individual variation in the sweat Na+ concentration ([Na+]sweat), we analyzed the cross-sectional relationship between [Na+]sweat and the plasma ALD concentration during rest and exercise in a hot environment. Eleven college-aged male subjects with a mean maximal oxygen uptake of 48 (range 42-59) ml kg-1 min-1 performed three sessions of 20-min cycle exercise at two levels of intensity (40 or 60% VO2max) in a room maintained at 31 degrees C. The chest sweat rate (SRch) and its containing Na+ were higher and individual differences in SRch and [Na+]sweat were greater at 60% exercise than at 40% exercise. In each individual, the [Na+]sweat increased significantly (P<0.05) with the increase in the SRch. In all subjects, the mean [Na+]sweat during exercise correlated negatively with the resting plasma ALD level at either percentage, but it did not correlate with the exercising ALD. These results suggest that individual variations in the increase of the [Na+]sweat in response to a rise in the SRch may thus be more closely related to the resting ALD than to the exercising ALD. As a result, the genomic action of ALD may be affected more by the sweat Na+ variation than by the rapidly non-genomic action during exercise in humans.
To assess the relationship between atrial natriuretic peptide (ANP) and the reduction in plasma volume (PV) during exercise, we measured changes in PV and ANP in seven male volunteers during treadmill exercise in air (AE) and with water immersion (WE) together with time control studies of rest in air and in water. Blood samples were collected from a catheter in the antecubital vein at exercise intensities of 32, 49, 65, and 78% of peak oxygen consumption (VO2). Plasma ANP in AE increased significantly from the resting value [15 +/- 1 (SE) pg/ml] only at 78% of peak VO2 (29 +/- 5 pg/ml), whereas ANP in WE increased significantly at exercise levels of > 49% of peak VO2 and reached 68 +/- 9 pg/ml at 78% of peak VO2. Although PV in AE and WE decreased significantly with VO2 of > 49% of peak VO2 (P < 0.01), the decrease from the resting value in WE was significantly greater than that in AE of > 65% of peak VO2 (P < 0.01) and the decreases at 78% of peak VO2 were -9.7 +/- 0.8 and -6.1 +/- 1.7%, respectively. The difference in the decrease in PV between AE and WE at corresponding VO2 correlated strongly with that in the increase in ANP (r = -0.97; P < 0.01). These results are consistent with the hypothesis that ANP may be involved in the fluid shift from the intra- to extravascular space during exercise.
To clarify the relationship between aerobic power (VO2max), blood volume (BV), and thermoregulatory responses to exercise-heat stress, we analyzed the cross-sectional relationship between the resting BV, plasma volume (PV), erythrocyte volume (EV), VO2max, forearm blood flow (FBF), and sweating responses during exercise in a hot environment (31 degrees C, 50% relative humidity). Twelve college-aged male subjects with a mean maximal oxygen uptake of 48 (range 42-59) mL.kg-1.min-1, a mean PV of 54 (range 42-72) mL.kg-1, a mean EV of 31 (range 23-43) mL.kg-1, and a mean BV of 85 (range 67-115) mL.kg-1 (measured by the Evans Blue dye dilution method) performed three sessions of 20-min cycle exercise at two levels of intensity (40% and 60% VO2max). The BV, PV and EV correlated positively with peak FBF (r = 0.596-0.711, P < 0.05), the increase of FBF in response to a unit rise in esophageal temperature (Tes; peak delta FBF/peak delta Tes) (r = 0.592-0.656, P < 0.05) and with total sweat loss (TSL) (r = 0.599-0.634, P < 0.05) during the exercise. The VO2max correlated with TSL during exercise at 40% VO2max (r = 0.578, P < 0.05), but not with peak FBF and peak delta FBF/peak delta Tes. The VO2max per lean body mass also showed a significant positive correlation with BV (r = 0.769, P < 0.01), PV (r = 0.706, P < 0.05), and with EV (r = 0.841, P < 0.001). The peak delta FBF/peak delta Tes was correlated positively with peak FBF (r = 0.597-0.830, P < 0.05-0.01) and negatively with peak Tes (r = 0.641-0.769, P < 0.05-0.01) during the exercise at the two levels. However, the chest sweat rate (CSR), TSL, and the increase of CSR in response to a unit rise in Tes (peak delta CSR/peak delta Tes) showed no correlation with peak Tes during the exercise at the two levels. These findings suggest that 1) heat dissipation responses during exercise were related more to blood volume than aerobic power and 2) skin blood flow was related more to body temperature than sweating responses during exercise under mild heat stress.
We studied the difference of thermoregulatory responses between trained male athletes (TR, n = 9) and untrained men (UT, n = 7) during 60 min of cold exposure (15 degrees C) without shivering, and examined the effects of physical fitness and body fat on these responses. Mean skin temperature (Tsk), esophageal temperature (Tes), and skin conductance (Kb) were similar between TR and UT, and heat production (M) for TR increased significantly during exposure at 15 degrees C. The M at 15 degrees C correlated positively with maximal oxygen uptake and negatively with body fat (%BF), but not with Tes. The Kb correlated negatively with Tes and positively with Tsk. The %BF also correlated negatively with Kb and Tsk during exposure at 15 degrees C, and the slope of %BF vs. Tsk relationship was significantly steeper in TR than in UT. These results suggest that (1) body temperature is maintained by the reduction of skin conductance, and (2) heat insulation independent of body fat is enhanced in trained athletes during cold exposure without shivering.
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