Fitness as a determinant of oxygen uptake response to constant-load exercise

Exercise performed above the lactate threshold (Θ _La) produces a slowly-developing phase of oxygen uptake ( \(\dot V_{O_2 } \) ) kinetics which elevates \(\dot V_{O_2 } \) above that predicted from the sub-Θ _La \(\dot V_{O_2 } \) -work rate relationship. This phenomenon has only been demonstrated, to date, in subjects who were relatively homogeneous with respect to fitness. This investigation therefore examined whether this behaviour occurred at a given absolute \(\dot V_{O_2 } \) or whether it was a characteristic of supra-Θ _La exercise in a group of subjects with over a threefold range ofΘ _La (990–3000 ml O₂·min⁻¹) and peak \(\dot V_{O_2 } \) (1600–5260 ml O₂·min⁻¹). Twelve healthy subjects performed: 1) exhausting incremental cycle ergometer exercise for estimation ofΘ _La ( \(\hat \Theta _{La} \) ) and peak \(\dot V_{O_2 } \) , and 11) a series of constant-load tests above and below \(\hat \Theta _{La} \) for determination of the \(\dot V_{O_2 } \) profile and efficiency of work. During all tests expired ventilation, \(\dot V_{O_2 } \) and carbon dioxide production were monitored breath-by-breath. The efficiency of work determined during incremental exercise (28.1±0.7%, \(\bar x \pm SE\) ,n=12) did not differ from that determined during sub- \(\hat \Theta _{La} \) constant-load exercise (27.4±0.5%,p>0.05). For constant-load exercise, \(\dot V_{O_2 } \) rose above that predicted, from the sub- \(\hat \Theta _{La} \) \(\dot V_{O_2 } \) -work rate relationship, for all supra- \(\hat \Theta _{La} \) work rates. This was evident above 990 ml O₂·min⁻¹ in the least fit subject but only above 3000 ml O₂·min⁻¹ in the fittest subject. As a consequence the efficiency of work was reduced from 27.4±0.5% for sub- \(\hat \Theta _{La} \) exercise to 22.6±0.4% (p<0.05) at the lowest supra- \(\hat \Theta _{La} \) work rate (i.e. \(\hat \Theta _{La} \) +20 W, on average). The efficiency of work generally decreased further at the higher supra- \(\hat \Theta _{La} \) work rates. We conclude that the \(\dot V_{O_2 } \) response to constant-load exercise includes an additional slow phase of the kinetics for all exercise intensities above \(\hat \Theta _{La} \) irrespective of the fitness of the subject. Consequently, measurements of the aerobic efficiency of work during constant-load exercise must rigorously constrain the exercise intensity to the sub- \(\hat \Theta _{La} \) domain.