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1 Department of Health, Exercise and Sport Sciences, Texas Tech University, Lubbock, TX, USA
2 Department of Kinesiology, Kansas State University, Manhattan, KS, USA
* To whom correspondence should be addressed. E-mail: barry.scheuermann{at}utoledo.edu.
Compared to moderate and heavy intensity exercise, the adjustment of VO2 to exercise intensities that elicit peak VO2 (VO2,peak) has received relatively little attention. This study examined the VO2 response of 21 young, healthy subjects (25 ± 6 yr; ± SD) during cycle ergometer exercise to step transitions in work rate (WR) corresponding to 90, 100 and 110% of the peak WR achieved during a preliminary ramp protocol (15-30 W/min). Gas exchange was measured breath-by-breath and interpolated to 1 s values. VO2 kinetics were determined using a 2 or 3 component exponential model in order to isolate the time constant (
2) as representative of VO2 kinetics and the amplitude (Amp) of the primary fast component independent of the appearance of any VO2 slow component. No difference in VO2 kinetics was observed between WRs (90 = 24.7 ± 9.0; 100 = 22.8 ± 6.7; 110 = 21.5 ± 9.2 s, p>0.05); nor in a subgroup of 8 subjects was 2 different from the value for moderate intensity (<lactate threshold) exercise (2 = 25 ± 12 s, p>0.05). As expected, the Amp increased with increasing WRs (Amp90 = 2089 ± 548; Amp100 = 2165 ± 517; Amp110 = 2225 ± 559 ml/min; Amp90 vs. Amp110, p<0.05). However, the gain of the VO2 response (
VO2/WR) decreased with increasing WRs (G90 = 8.5 ± 0.6; G100 = 7.9± 0.6; G110 = 7.3 ± 0.6 ml/min/W; p<0.05). The Amp of the primary component approximated 85%, 88% and 89% of VO2,peak during 90%, 100%, and 110% WR transitions, respectively. The results of the present study demonstrate that, compared to moderate and heavy intensity exercise, the gain of the VO2 response (as VO2/WR) is reduced for exercise transitions in the severe intensity domain, but the approach to this gain is well-described by a common time constant which is invariant across work intensities. The lower VO2/WR may be due to an insufficient adjustment of the cardiovascular and/or pulmonary systems which determine O2 delivery to the exercising muscles, or due to recruitment of motor units with lower oxidative capacity, following the onset of exercise in the severe intensity domain.
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