VO2 Max Decreases
The heart is hard at work but pumping out less blood than usual; the muscles aren’t getting as much oxygen. These responses play a role in decreasing the all-important VO2 max, which is the maximum amount of oxygen that the body can take in and use to fuel working muscles. Again, for endurance athletes, the higher VO2 max, the better; it’s a marker of exercise efficiency.
Because VO2 max decreases, at any exercise intensity you’ll be working at a higher percent of VO2 max. In other words, relative stress increases at a given workload because you’re less efficient.