Summary
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.
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Supported by grants from the John D. and Catherine T. Mac-Arthur Foundation, USPHS RR 00865-15, and NIH HL 07694-01
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Henson, L.C., Poole, D.C. & Whipp, B.J. Fitness as a determinant of oxygen uptake response to constant-load exercise. Europ. J. Appl. Physiol. 59, 21–28 (1989). https://doi.org/10.1007/BF02396575
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DOI: https://doi.org/10.1007/BF02396575