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

Abstract

Exercise performed above the lactate threshold (ΘLa) produces a slowly-developing phase of oxygen uptake ( {Mathematical expression}) kinetics which elevates {Mathematical expression} above that predicted from the sub-ΘLa {Mathematical expression}-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 {Mathematical expression} or whether it was a characteristic of supra-ΘLa exercise in a group of subjects with over a threefold range of ΘLa (990-3000 ml O2·min-1) and peak {Mathematical expression} (1600-5260 ml O2·min-1). Twelve healthy subjects performed: 1) exhausting incremental cycle ergometer exercise for estimation of ΘLa ( {Mathematical expression}) and peak {Mathematical expression}, and 11) a series of constant-load tests above and below {Mathematical expression} for determination of the {Mathematical expression} profile and efficiency of work. During all tests expired ventilation, {Mathematical expression} and carbon dioxide production were monitored breath-by-breath. The efficiency of work determined during incremental exercise (28.1±0.7%, {Mathematical expression}, n=12) did not differ from that determined during sub- {Mathematical expression} constant-load exercise (27.4±0.5%, p>0.05). For constant-load exercise, {Mathematical expression} rose above that predicted, from the sub- {Mathematical expression} {Mathematical expression}-work rate relationship, for all supra- {Mathematical expression} 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- {Mathematical expression} exercise to 22.6±0.4% (p<0.05) at the lowest supra- {Mathematical expression} work rate (i.e. {Mathematical expression}+20 W, on average). The efficiency of work generally decreased further at the higher supra- {Mathematical expression} work rates. We conclude that the {Mathematical expression} response to constant-load exercise includes an additional slow phase of the kinetics for all exercise intensities above {Mathematical expression} 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- {Mathematical expression} domain. © 1989 Springer-Verlag.