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

Exercise performed above the lactate threshold (OLa) produces a slowly-developing phase of oxygen uptake (VO2) kinetics which elevates VO2 above that predicted from the sub-OLa VO2-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 VO2 or whether it was a characteristic of supra-OLa exercise in a group of subjects with over a threefold range of OLa (990-3000 ml O2.min-1) and peak VO2 (1600-5260 ml O2.min-1). Twelve healthy subjects performed: 1) exhausting incremental cycle ergometer exercise for estimation of OLa (OLa) and peak VO2, and 11) a series of constant-load tests above and below OLa for determination of the VO2 profile and efficiency of work. During all tests expired ventilation, VO2 and carbon dioxide production were monitored breath-by-breath. The efficiency of work determined during incremental exercise (28.1 +/- 0.7%, means +/- SE, n = 12) did not differ from that determined during sub-OLa constant-load exercise (27.4 +/- 0.5%, p greater than 0.05). For constant-load exercise, VO2 rose above that predicted, from the sub-OLa VO2-work rate relationship, for all supra-OLa 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-OLa exercise to 22.6 +/- 0.4% (p less than 0.05) at the lowest supra-OLa work rate (i.e. OLa + 20 W, on average).(ABSTRACT TRUNCATED AT 250 WORDS)