Final answer:
In exercise physiology, heart rate (HR) is the variable that plateaus at levels below 40% of VO2 max.
Step-by-step explanation:
In the context of exercise physiology, a variable that plateaus at levels below 40% of VO2 max is heart rate (HR).
During exercise, HR initially increases linearly with intensity until it reaches a plateau, meaning it no longer increases despite further increases in exercise intensity. This plateau typically occurs at levels below 40% of maximal oxygen uptake (VO2 max).
For example, in healthy young individuals, HR can increase up to 150 bpm during exercise, but it may not increase above this threshold even with higher exercise intensities below 40% of VO2 max.