Anaerobic energy production and O2 deficit‐debt relationship during exhaustive exercise in humans.

Abstract

1. Eight subjects performed one-legged, dynamic, knee-extensor exercise, first at 10 W followed by 10 min rest, then at an intense, exhaustive exercise load (65 W) lasting 3.2 min. AFter 60 min recovery, exercise was performed for 8-10 min each at 20, 30, 40 and 50 W. Measurements of pulmonary oxygen uptake, heart rate, blood pressure, leg blood flow, and femoral arterial-venous differences of oxygen content and lactate were performed as well as determination of ATP, creatine phosphate (CP) inosine monophosphate (IMP) and lactate concentrations on biopsy material from the quadriceps muscles before and immediately after the intense exercise, and at 3, 10 and 60 min into recovery. 2. Individual linear relations (r = 0.95-1.00) between the power outputs for submaximal exercise and oxygen uptakes (leg and pulmonary) were used to estimate the energy demand during intense exercise. Pulmonary and leg oxygen deficits determined as the difference between energy demand and oxygen uptake were 0.46 and 0.48 l (kg active muscle)-1, respectively. Limb and pulmonary oxygen debts (oxygen uptake during 60 min of recovery-pre-exercise oxygen uptake) were 0.55 and 1.65 l (kg active muscle)-1, respectively. 3. During the intense exercise, muscle [ATP] decreased by 30% and [CP] by 60% from resting concentrations of 6.2 and 22.4 mmol (kg wet wt)-1, respectively, and [IMP] increased to 1.1 mmol (kg wet wt)-1. Muscle [lactate] increased from 2 to 28.1 mmol (kg wet wt)-1, and the concomitant net lactate release was 14.8 mmol (kg wet wt)-1 or about 1/3 of the total net lactate production. During recovery 70% of the accumulated lactate was released to the blood, and the nucleotides and CP returned to about 40 and 85% of pre-exercise values at 3 and 10 min of recovery, respectively. 4. Total reduction in ATP and CP (and elevation of IMP) during the intense exercise amounted to 16.4 mmol ATP (kg wet wt)-1, which together with the lactate production accounted for 83.1 mmol ATP (kg wet wt)-1. In addition 6-8 mmol ATP (kg wet wt)-1 are made available related to accumulation of glycolytic intermediates including pyruvate (and alanine). Estimated leg oxygen deficit corresponded to an ATP production of 94.7 mmol ATP kg-1; this value included 3.1 mmol kg-1 related to unloading of HbO2 and MbO2. Resynthesis of nucleotides and CP could account for less than 10% of the leg oxygen debt, and lactate elimination including resynthesis of glycogen for another 25%. 5. The anaerobic energy contribution during the first half-minute of intense exercise accounted for 80% of the total energy turnover and this decreased to 30% during the last phase of the exercise. The mean anaerobic energy contribution was 45% for the 3.2 min of exhaustive exercise. 6. The maximal anaerobic capacity of human muscle amounted to the equivalent of close to 0.5 l O2 kg-1. An extrapolation to whole-body anaerobic capacity cannot be made, as the magnitude of neither [ATP] and [CP] reduction nor lactate release from the muscle is likely to be comparable in all muscles when the human performs whole-body exercise. 7. When exercising with a small muscle group the measurements of (i) oxygen deficit and (ii) energy yield, based on metabolic alterations of the active muscle, give similar values for the anaerobic energy release. The dominant fraction of the elevation in recovery oxygen uptake (i.e. oxygen debt) is not accounted for, as normalization of nucleotides, CP, muscle and blood lactate only amounted to about 1/3 of the debt measurement. Elevation in hormones such as adrenaline and noradrenaline as well as temperature do not appear to play a role in the high recovery oxygen uptake in the present study.