Prior heavy exercise increases the amplitude of the primary oxygen uptake (ΩO2) response and reduces the amplitude of the ΩO2 slow component during subsequent heavy exercise with no change in the primary ΩO2 time constant (Burnley et al. 2000; Scheuermann et al. 2001). The mechanism underpinning these effects is unclear, but it has been suggested that changes in motor unit recruitment may be responsible (Burnley et al. 2001). This study was designed to test the hypothesis that the increase in the amplitude of the primary ΩO2 response consequent to prior heavy exercise would be associated with an increase in neuromuscular activity measured using electromyography.
Eight healthy male subjects (aged 20-31 years) participated in the study, which was approved by the Manchester Metropolitan University ethics committee. Subjects completed two 6 min bouts of heavy cycling exercise at 70 % of the difference between the lactate threshold and ΩO2,max separated by 12 min of recovery. This protocol was repeated on a separate day. Pulmonary ΩO2 was measured breath-by-breath and the electromyogram (EMG) of three muscles responsible for power production during cycling (gluteus maximus, vastus lateralis and vastus medialis) was measured on one of the days using a short-range telemetry system. The ΩO2 response was averaged over the two repeat protocols and modelled using three independent exponential terms. The integrated EMG (iEMG) and mean power frequency (MPF) were calculated over 30 s intervals throughout each exercise bout. Arterialised venous blood samples were drawn for the analysis of blood [lactate] at the onset of each bout. Results were analysed using Student’s paired samples t tests.
Baseline blood [lactate] was significantly elevated at the onset of the second exercise bout (from 1.2 ± 0.1 to 4.9 ± 0.4 mM; P < 0.05). Prior heavy exercise significantly increased the primary ΩO2 amplitude (from mean ± S.E.M. 2.11 ± 0.12 l min-1 to 2.44 ± 0.10 l min-1; P < 0.05) and reduced the ΩO2 slow component amplitude (from 0.79 ± 0.08 to 0.40 ± 0.08 l min-1, P < 0.05) without a change in the primary time constant (21.7 ± 2.3 vs. 25.2 ± 3.3, P = 0.36). The increased primary ΩO2 amplitude was accompanied by a 19 % increase in the iEMG averaged across all three muscles (491 ± 108 vs. 604 ± 151 % increase above baseline values, P = 0.04), whereas MPF was unchanged (80.1 ± 0.9 vs. 80.6 ± 1.0 Hz, P = 0.38). The iEMG profiles during the two heavy exercise bouts were remarkably similar to the ΩO2 profiles: in the second heavy exercise bout, iEMG was higher in all three muscles during the first 3 min while the increase in iEMG with time was less steep in the final 3 min.
The results of the present study indicate that the increased primary ΩO2 amplitude and the reduced ΩO2 slow component amplitude observed during the second of two bouts of heavy exercise is related to a greater recruitment of motor units at the onset of exercise.