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_O2) of five older humans (77 ξt 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_O2 kinetics (phase 2, τ) 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_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 μmol (g wet wt)^-1 min^-1; week 9, 11.4 ± 3.6 μmol (g wet wt)^-1 min^-1) but was unchanged in the untrained leg (week 0, 5.9 ± 0.5 μmol (g wet wt)^-1 min^-1; week 9, 7.9 ± 1.9 μmol (g wet wt)^-1 min^-1). These data suggest that the acceleration of V_O2 kinetics was due to an improved rate of O₂ utilisation by the muscle, but was not a result of increased O₂ delivery.