Influence of repeated isometric contractions on muscle deoxygenation and pulmonary oxygen uptake kinetics in humans

The purpose of the present study was to compare simultaneously vastus lateralis (VL) deoxygenation and pulmonary O-2 uptake ((V) over dot O-2) kinetics during fatiguing knee extension exercise with minimal cardiac load. Eight healthy subjects realized an intermittent bilateral knee-extension exercise (3-s contraction/3-s relaxation) at 40% of maximum voluntary contraction for 10 min. VL deoxygenation was recorded by near infrared spectroscopy at 2 Hz (NIRO-300, Hamamatsu Photonics, Japan) and (V) over dot O-2 was determined breath-by-breath (K4b(2), Cosmed, Italy). After a time delay of 16 +/- 5 s, deoxygenation kinetics at the onset of exercise followed an exponential time course at a significant faster rate than (V) over dot O-2 (time constant of 5.4 +/- 4.0 s vs. 31.6 +/- 10.4 s, P<0.01) reflecting a mismatch between local O-2 consumption and perfusion. Thereafter, a rise in (V) over dot O-2 of 223 +/- 123 ml min(-1) (consistent with the mathematical model, 259 +/- 126 ml min(-1)) was observed between minutes 2 and 10. During the same exercise time, changes in tissue oxygenation index decreased significantly and were individually correlated with the corresponding increased (V) over dot O-2 (P<0.05), suggesting that the majority of the slow rise of (V) over dot O-2 arose from the exercising limbs. Averaged heart rate increased from 67 +/- 11 to 116 +/- 20 beats min(-1) during exercise. Knee extension exercise may be relevant to estimate the cardiopulmonary and deoxygenation of working skeletal muscle responses for assessment of exercise limiting factors in clinical settings or in the exercise physiology.