Elevated baseline VO2 per se does not slow O2 uptake kinetics during work-to-work exercise transitions

Area of Science:

• Exercise Physiology
• Human Physiology
• Cardiorespiratory Fitness

Background:

• Pulmonary oxygen uptake (VO2) kinetics exhibit characteristic slowing during "work-to-work" exercise transitions.
• The underlying physiological drivers of this phenomenon, specifically elevated baseline metabolic rate versus work rate, remain debated.

Purpose of the Study:

• To determine if the slowing of VO2 kinetics during work-to-work exercise is caused by elevated baseline VO2 or the increased work rate itself.
• To test the hypothesis that a transition to a higher work rate from unloaded cycling, with VO2 elevations mimicking work-to-work, would lengthen phase II time constant (τ(p)).

Main Methods:

• Seven healthy males (27 ± 10 years) underwent two conditions: moderate-to-high intensity (M→H) and unloaded-to-high intensity (U→H, E→H) transitions.
• Baseline VO2 was matched between M→H and the second high-intensity bout (E→H).
• Phase II time constant (τ(p)) of pulmonary oxygen uptake was calculated for each transition.

Main Results:

• The τ(p) for the M→H transition (48 ± 16 s) was significantly longer than for U→H (28 ± 8 s) and E→H (27 ± 6 s).
• No significant difference was observed between U→H and E→H τ(p) values.
• Baseline VO2 and heart rate (HR) elevations did not correlate with the observed differences in τ(p).

Conclusions:

• The slowing of VO2 kinetics during work-to-work exercise is primarily attributed to the elevated baseline work rate, not the elevated baseline VO2 or HR.
• These findings suggest that the recruitment of different muscle fiber types, dictated by work intensity, influences VO2 response dynamics.