Muscle oxygen uptake and energy turnover during dynamic exercise at different contraction frequencies in humans

1. It has been established that pulmonary oxygen uptake is greater during Cycle exercise in humans at high compared to low contraction frequencies. However, it is unclear whether this is due to more work being performed at the high frequencies and whether the energy turnover of the working muscles is higher. The present study tested the hypothesis that human skeletal muscle oxygen uptake and energy turnover are elevated during exercise at high compared to low contraction frequency when the total power output is the same.

2. Seven subjects performed single-leg dynamic knee-extensor exercise for 10 min at contraction frequencies of 60 and 100 r.p.m. where the total power output (comprising the sum of external and internal power output) was matched between frequencies (54 +/- 5 vs. 56 +/- 5 W; mean +/- S.E.M.). Muscle oxygen uptake was determined from measurements of thigh blood now and femoral arterial - venous differences for oxygen content (a-v O-2 diff). Anaerobic energy turnover was estimated from measurements of lactate release and muscle lactate accumulation as well as muscle ATP and phosphocreatine (PCr) utilisation based on analysis of muscle biopsies obtained before and after each exercise bout.

3. Whilst, a-v O-2 diff was the same between contraction frequencies during exercise, thigh blood flow was higher (P < 0.05) at 100 compared to 60 r.p.m. Thus, muscle (V)over dot(O2) was higher (P < 0.05) during exercise at 100 r.p.m. Muscle increased (P < 0.05) by 0.06 +/- 0.03 (12%) and 0.09 +/- 0.03 l min(-1) (14%) from the third minute to the end of exercise at 60 and 100 r.p.m., respectively, but there was no difference between the two frequencies.

4. Muscle PCr decreased by 8.1 +/- 1.7 and 9.1 +/- 2.0 mmol (kg wet wt)(-1), and muscle lactate increased to 6.8 +/- 2.1 and 9.8 +/- 2.5 mmol (kg wet wt)(-1) during exercise at 60 and 100 r.p.m., respectively. The total release of lactate during exercise was 48.7 +/- 8.8 and 64.3 +/- 10.6 niniol at 60 and 100 r.p.m. (not significant, NS). The total anaerobic ATP production was 47 +/- 8 and 61 +/- 12 mmol kg(-1), respectively (NS).

5. Muscle temperature increased (P < 0.05) from 35.8 +/- 0.3 to 38.2 +/- 0.2 degreesC at 60 r.p.m. and from 35.9 +/- 0.3 to 38.4 +/- 0.3 degreesC at 100 r.p.m. Between 1 and 7 min muscle temperature was higher (P < 0.05) at 100 compared to 60 r.p.m.

6. The estimated mean rate of energy turnover during exercise was higher (P < 0.05) at 100 compared to 60 r.p.m. (238 +/- 16 vs. 194 +/- 11 J s(-1). Thus, mechanical efficiency was lower (P < 0.05) at 100 r.p.m. (24 +/- 2%) compared to 60 r.p.m. (28 +/- 3%). Correspondingly, efficiency expressed as work per mol ATP was lower (P < 0.05) at 100 than at 60 r.p.m. (22.5 +/- 2.1 vs. 26.5 +/- 2.5 J (mmol ATP)(-1)).

7. The present study showed that muscle oxygen uptake and energy turnover are elevated during dynamic contraction at a frequency of 100 compared with 60 r.p.m. It was also observed that muscle oxygen uptake increased as exercise progressed in a manner that, was not solely related to the increase in muscle temperature and lactate accumulation.