Comparison of oxygen uptake kinetics during concentric and eccentric cycleexercise

O-2 uptake ((V) over dot O-2) kinetics and electromyographic (EMG) activity from the vastus medialis, rectus femoris, biceps femoris, and medial gastr ocnemius muscles were studied during constant- load concentric and eccentri c cycling. Six healthy men performed transitions from baseline to high-inte nsity eccentric (HE) exercise and to high-intensity (HC), moderate-intensit y (MC), and low-intensity (LC) concentric exercise. For HE and HC exercise, absolute work rate was equivalent. For HE and LC exercise, (V) over dot O- 2 was equivalent. (V) over dot O-2 data were fit by a two- or three-compone nt exponential model. Surface EMG was recorded during the last 12 s of each minute of exercise to obtain integrated EMG and mean power frequency. Only in the HC exercise did (V) over dot O-2 increase progressively with eviden ce of a slow component (phase 3), and only in HC exercise was there evidenc e of a coincident increase with time in integrated EMG of the vastus medial is and rectus femoris muscles (P < 0.05) with no change in mean power frequ ency. The phase 2 time constant was slower in HC [24.0 +/- 1.7 (SE) s] than in HE (14.7 +/- 2.8 s) and LC (16.7 +/- 2.2 s) exercise, while it was not different from MC exercise (20.6 +/- 2.1 s). These results show that the ra te of increase in (V) over dot O-2 at the onset of exercise was not differe nt between HE and LC exercise, where the metabolic demand was similar, but both had significantly faster kinetics for (V) over dot O-2 than HC exercis e. The (V) over dot O-2 slow component might be related to increased muscle activation, which is a function of metabolic demand and not absolute work rate.