Effects of prior heavy exercise on V(over dot)O2 kinetics during heavy exercise are related to changes in muscle activity

We hypothesized that the elevated primary O-2 uptake ((V) over dot O-2) amplitude during the second of two bouts of heavy cycle exercise would be accompanied by an increase in the integrated electromyogram (iEMG) measured from three leg muscles (gluteus maximus, vastus lateralis, and vastus medialis). Eight healthy men performed two 6-min bouts of heavy leg cycling (at 70% of the difference between the lactate threshold and peak (V) over dot O-2) separated by 12 min of recovery. The iEMG was measured throughout each exercise bout. The amplitude of the primary (V) over dot O-2 response was increased after prior heavy leg exercise (from mean +/- SE 2.11 +/- 0.12 to 2.44 +/- 0.10 l/ min, P < 0.05) with no change in the time constant of the primary response (from 21.7 +/- 2.3 to 25.2 +/- 3.3 s), and the amplitude of the (V) over dot O-2 slow component was reduced (from 0.79 +/- 0.08 to 0.40 +/- 0.08 l/ min, P < 0.05). The elevated primary (V) over dot O-2 amplitude after leg cycling was accompanied by a 19% increase in the averaged iEMG of the three muscles in the first 2 min of exercise (491 +/- 108 vs. 604 +/- 151% increase above baseline values, P < 0.05), whereas mean power frequency was unchanged (80.1 +/- 0.9 vs. 80.6 +/- 1.0 Hz). The results of the present study indicate that the increased primary (V) over dot O-2 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.