Determinants of oxygen uptake kinetics in older humans following single-limb endurance exercise training

We hypothesised that the observed acceleration in the kinetics of exercise on-transient oxygen uptake (V(over dot)(O2)) of five older humans (77 +/- 7 years (mean +/- S.D.) following 9 weeks of single-leg endurance exercise training was due to adaptations at the level of the muscle cell. Prior to, and following training, subjects performed constant-load single-limb knee extension exercise. Following training V(over dot)(O2) kinetics (phase 2, tau) were accelerated in the trained leg (week 0, 92 +/- 44 s; week 9, 48 +/- 22 s) and unchanged in the untrained leg (week 0, 104 +/- 43 s; week 9, 126 +/- 35 s). The kinetics of Mean blood velocity in the femoral artery were faster than the kinetics of V(over dot)(O2), but were unchanged in both the trained (week 0, 19 +/- 10 s; week 9, 26 +/- 11 s) and untrained leg (week 0, 20 +/- 18 s; week 9, 18 10 s). Maximal citrate synthase activity, measured from biopsies of the vastus lateralis muscle, increased (P < 0.05) in the trained leg (week 0, 6.7 +/- 2.0 mu mol (g wet wt)(-1) min(-1); week 9, 11.4 +/- 3.6 pmol (g wet wt)(-1) min(-1)) but was unchanged in the untrained leg (week 0, 5.9 +/- 0.5 mu mol (g wet wt)(-1) min(-1); week 9, 7.9 +/- 1.9 mu mol (g wet wt)(-1) min(-1)). These data suggest that the acceleration of V(over dot)(O2) kinetics was due to an improved rate of O-2 utilisation by the muscle, but was not a result of increased O-2 delivery.