chuk. Oxygen uptake kinetics for moderate exercise are speeded in older humans by prior heavy exercise. J Appl Physiol 92: 609-616, 2002; 10.1152.japplphysiol.00186. 2001.-This study examined the effect of heavy-intensity warm-up exercise on O2 uptake (V O2) kinetics at the onset of moderate-intensity (80% ventilation threshold), constantwork rate exercise in eight older (65 Ϯ 2 yr) and seven younger adults (26 Ϯ 1 yr).Step increases in work rate from loadless cycling to moderate exercise (Mod 1), heavy exercise, and moderate exercise (Mod 2) were performed. Each exercise bout was 6 min in duration and separated by 6 min of loadless cycling. V O2 kinetics were modeled from the onset of exercise by use of a two-component exponential model. Heart rate (HR) kinetics were modeled from the onset of exercise using a single exponential model. During Mod1, the time constant () for the predominant rise in V O2 (V O2) was slower (P Ͻ 0.05) in the older adults (50 Ϯ 10 s) than in young adults (19 Ϯ 5 s). The older adults demonstrated a speeding (P Ͻ 0.05) of V O2 kinetics when moderate-intensity exercise (Mod 2) was preceded by high-intensity warm-up exercise (V O2, 27 Ϯ 3 s), whereas young adults showed no speeding of V O2 kinetics (V O2, 17 Ϯ 3 s). In the older and younger adults, baseline HR preceding Mod 2 was elevated compared with Mod 1, but the for HR kinetics was slowed (P Ͻ 0.05) in Mod2 only for the older adults. Prior heavy-intensity exercise in old, but not young, adults speeded V O2 kinetics during Mod2. Despite slowed HR kinetics in Mod 2 in the older adults, an elevated baseline HR before the onset of Mod 2 may have led to sufficient muscle perfusion and O 2 delivery. These results suggest that, when muscle blood flow and O 2 delivery are adequate, muscle O 2 consumption in both old and young adults is limited by intracellular processes within the exercising muscle.aging; heart rate; oxygen transport; oxygen utilization AGING IS ASSOCIATED WITH a slowing of pulmonary O 2 uptake (V O 2 ) kinetics during the on-transition to a step increase in work rate (WR) of moderate intensity (2,7,25), implying that the rate of O 2 utilized at the muscle is also slowed with increasing age. Although it has been demonstrated that, for older adults, V O 2 kinetics responses due to exercise in muscles that are chronically active remain similar to those of young adults (8) or speed toward values seen in young adults as a consequence of chronic exercise training (1), it is unclear whether these adaptations are a consequence of improved blood flow and/or O 2 delivery or of a faster activation of the biochemical reactions in muscle, factors that have been implicated as limiting muscle O 2 consumption in young adults (20,34).Recently, it was shown in young adults that V O 2 kinetics after the start of heavy-intensity exercise became faster as a consequence of a prior "warm-up" bout of heavy-intensity exercise, whereas, in contrast, V O 2 kinetics during moderate-intensity exercise were not affected by a warm-up bout of exercise (...

Cross-sectional studies have compared the oxygen uptake (VO2) kinetics during the on-transient of moderate intensity exercise in older and younger adults. The slower values in the older adults may have been due to an age-related reduction in the capacity for O2 transport or alternatively a reduced intramuscular oxidative capacity. We studied: (1) the effects of ageing on VO2 kinetics in older adults on two occasions 9 years apart, and (2) the effect of hyperoxia on VO2 kinetics at the second test time. After a 9 year period, follow-up testing was undertaken on seven older adults (78 +/- 5 years, mean +/- S.D.). They each performed six repeats of 6 min bouts of constant-load cycle exercise from loadless cycling to 80% of their ventilatory threshold. They breathed one of two gas mixtures (euoxia: inspired O2 fraction, FI,O2, 0.21; hyperoxia: FI,O2, 0.70) on different trials determined on a random basis. Breath-by-breath VO2 data were time aligned and ensemble averaged. VO2 kinetics, modelled with a single exponential from phase 2 onset (+20 s) to steady state and described by the exponential time constant (tau) were compared with data collected from the same adults 9 years earlier. One-way repeated measures analysis of variance revealed that tau was slowed significantly with age (from 30 +/- 8 to 46 +/- 10 s), but was unaffected by hyperoxia (43 +/- 15 s). We concluded that: (1) in older adults studied longitudinally over a 9 year period, the on-transient VO2 kinetics are slowed, in agreement with, but to a greater extent, than from cross-sectional data; and (2) the phase 2 time constant (tau) for these older adults was not accelerated by hyperoxic breathing. Thus the expected hyperoxia-induced increase in the capacity for O2 transport was not associated with faster on-transient VO2 kinetics suggesting either that O2 transport may not limit VO2 kinetics during the 8th decade, or that O2 transport was not improved with hyperoxia.

Summary. The

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