Oxygen uptake kinetics for moderate exercise are speeded in older humans by prior heavy exercise

This study examined the effect of heavy-intensity warm-up exercise on O-2 uptake ((V) over dot O-2) kinetics at the onset of moderate-intensity (80% ventilation threshold), constant-work rate exercise in eight older (65 +/- 2 yr) and seven younger adults (26 +/- 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) over dot O-2 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 Mod(1), the time constant (tau) for the predominant rise in (V) over dot O-2 (tau(V) over dot O-2) was slower (P < 0.05) in the older adults (50 +/- 10 s) than in young adults (19 +/- 5 s). The older adults demonstrated a speeding (P < 0.05) of (V) over dot O-2 kinetics when moderate-intensity exercise (Mod(2)) was preceded by high-intensity warm-up exercise ((V) over dot O-2, 27 +/- 3 s), whereas young adults showed no speeding of (V) over dot O-2 kinetics (tau(V) over dot O-2, 17 +/- 3 s). In the older and younger adults, baseline HR preceding Mod(2) was elevated compared with Mod(1), but the tau for HR kinetics was slowed (P < 0.05) in Mod(2) only for the older adults. Prior heavy-intensity exercise in old, but not young, adults speeded (V) over dot O-2 kinetics during Mod(2). 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.