Introduction. Exercise and head-out water immersion (HOI) have consistently reported an increase in central blood volume associated with the cephalad shift in blood volume. This causes an increase in left ventricular end diastolic volume and greater stroke volume during exercise compared to exercise in air at similar metabolic costs. In contrast, the metabolic response, specifically, blood lactate accumulation during exercise combined with HOI has yielded varying results depending on the mode of exercise. At present it appears that during exercise at similar metabolic costs, cycle ergometry exercise augments plasma lactate over treadmill running while HOI reduces the plasma lactate response to cycle ergometry exercise. The interaction between treadmill running and HOI appears less certain. Thus, we tested the hypothesis that running on a treadmill on land would result in a lesser accumulation of lactate than during HOI treadmill running.
Methods. Eleven subjects' lactate thresholds were determined while running at a 0% grade at increasing speeds on a treadmill on land or during HOI on an underwater treadmill in a randomized cross-over design. Exercise tests were separated by a minimum of 3 days. Lactate concentrations were expressed in mM• kg-1 H2O after correcting for plasma solid concentration. During exercise changes in plasma volume were calculated from changes in hematocrit and hemoglobin. Lactate threshold was estimated from a log-log plot of lactate concentration (mM• kg-1 H2O) as a function of relative oxygen consumption (ml O2•min-1•kg-1 BW).
Results. The energy cost and heart rate response to running at speeds between 5.5 and 7.5 mph was similar for land and HOI. During treadmill running on land, plasma volume decreased by 6.4 ± 4.0% at a speed of 7.5 mph. The decrease in plasma volume was significantly greater during HOI and averaged 18.7 ± 1.7% (p <0.05) at 7.5 mph. Plasma lactate was higher at any given treadmill speed ≥ 5.5 mph during HOI compared to land (p <0.05). Lactate threshold during HOI running (21.8 ± 1.6 mM• kg-1 H2O) was lower (p <0.05) than during running on the land treadmill (27.0 ± 1.6 mM• kg-1 H2O).
Discussion. HOI running resulted in a consistent shift to the left (rise in plasma lactate occurred at a lower ) in the lactate threshold and elevated plasma lactate concentration at speeds between 5.5-7.5 mph despite similar metabolic and HR responses to the exercise.



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Life Sciences; Exercise Sciences



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Lactate threshold, underwater treadmill running