O2 uptake kinetics above and below the lactic acidosis threshold during sinusoidal exercise

Area of Science:

• Exercise Physiology
• Cardiovascular Physiology
• Muscle Metabolism

Background:

• Describing oxygen uptake (VO2) kinetics during exercise above the lactic acidosis threshold (LAT) is challenging due to difficulties in identifying steady-state responses.
• Understanding the O2 transport system's capacity above the LAT is crucial for exercise performance.
• Previous methods struggled to accurately assess VO2 kinetics in this intensity domain.

Purpose of the Study:

• To investigate and compare oxygen uptake (VO2) kinetics during sinusoidal work rate changes below and above the lactic acidosis threshold (LAT).
• To determine if VO2 kinetics are altered at exercise intensities exceeding the LAT.
• To identify potential limitations in O2 delivery or utilization above the LAT.

Main Methods:

• Applied sinusoidal work rate variations with a 4-minute period, both below and above the LAT.
• Collected VO2 and heart rate data during these controlled exercise protocols.
• Analyzed the amplitude and phase lag of VO2 and heart rate responses relative to work rate changes.

Main Results:

• VO2 response amplitude was significantly reduced above the LAT compared to below the LAT (5.4 vs. 7.6 ml.min-1 x W-1).
• Phase lag of the VO2 response increased significantly above the LAT.
• Heart rate amplitude decreased and phase lag increased for above-LAT work rate changes, with dramatically reduced VO2/heart rate fluctuations.

Conclusions:

• VO2 kinetics are demonstrably slower in the work rate domain above the LAT compared to below the LAT.
• The findings suggest that O2 transport mechanisms to the exercising muscle may limit VO2 kinetics at higher exercise intensities.
• This study provides insights into the physiological constraints on oxygen utilization during strenuous exercise.