The supine posture has been shown to limit exercise-induced plasma volume expansion. Differences in hydrostatic pressure gradients between the standing and seated position indicate that treadmill exercise might promote a greater plasma volume expansion than cycle ergometer exercise. To test this hypothesis ten subjects performed intermittent high intensity exercise (4 min at 85% VO2max, 5 min at 40% VO2max repeated 8 times) on separate days on the treadmill and cycle ergometer. Changes in plasma volume expansion were calculated from changes in hematocrit and hemoglobin. Stroke volume (SV), trans-thoracic impedance (Z0), HR, and arterial blood pressure (non-invasive arm cuff, SBP & DBP) were assessed in the seated position before and postexercise. Zo increased (p<0.05) as subjects started exercise (both treadmill and cycling), indicating a reduction in central blood volume (CBV), which returned to baseline towards the end of exercise. Postexercise Zo returned to control levels within 30 min regardless of the previous exercise mode. A significant post-exercise hypotension was observed following cycle ergometer exercise (p<0.05) but not following treadmill exercise. Plasma volume increased 6.1±1.0% and 7.0 ± 1.1% (p<0.05) following treadmill and cycle ergometer exercise, respectively. The increase in PV was similar for both exercise modes. Initial differences in central blood volume disappeared over the course of the exercise protocol and during recovery, possibly indicating that there is a postural threshold and moving beyond it yields no further effect. The lack of differences between modes of exercise on plasma albumin content and Z0 indicate that the upright postures were not different from each other. As such, PV expansion following high intensity intermittent exercise appears to be independent of upright exercise mode.

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