CARDIORESPIRATORY KINETICS AND FEMORAL-ARTERY BLOOD VELOCITY DURING DYNAMIC KNEE EXTENSION EXERCISE

The kinetics of femoral artery mean blood velocity (MBV; measured by p ulsed Doppler) and whole body oxygen uptake (VO2; measured breath by b reath) were assessed from the time constant during the on (tau(on)) an d off (tau(off)) transients to step changes in work rate between compl ete rest and dynamic knee extension (KE) exercise. Six healthy men per formed 5 min of seated KE exercise, with each leg alternately raising and lowering a weight (10% maximum voluntary contraction) over a 2-s d uty cycle. Because kinetic analysis of VO2 kinetics during KE exercise is a new approach, the VO2 responses were also evaluated during the o n and off transitions to the more familiar upright cycling exercise in which the magnitude of increase in VO2 and cardiac output was similar to that during KE exercise. During KE exercise, VO2 tau(on) [mean 72. 2 +/- 11.2 (SE) s] was slower than VO2 tau(off) (33.3 +/- 1.8 s; P < 0 .01). Cardiac output, measured with impedance cardiography, was not di fferent for tau(on) (67.1 +/- 20.0 s) compared with that for tau(off) (52.9 +/- 7.6 s). Likewise, MBV tau(on) (34.5 +/- 3.9 s) was not diffe rent from tau(off) (35.3 +/- 3.2 s). During cycling, the VO2 tau(on) ( 18.0 +/- 2.4 s) and tau(off) (30.7 +/- 1.2 s) were both faster than KE VO2 tau(on) (P < 0.01). Even though the MBV kinetics indicated a rapi d adaptation of blood flow during KE exercise, there was a slow adapta tion of VO2. A transient hyperemia immediately on cessation of KE exer cise, indicated by both MBV and calculated systemic vascular conductan ce responses, suggested that blood flow might have been inadequate and could have contributed to the delayed adaptation of VO2 at the onset of exercise, although other explanations are possible.