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Decreased VO2 max is an expected outcome of aerobic endurance overtraining because consistent, high-intensity training without adequate recovery can lead to a state of fatigue and overreaching. This state negatively impacts the body’s ability to take in and utilize oxygen effectively, resulting in a lower VO2 max.

When an athlete is overtrained, their performance can diminish, and one of the first signs is often a decrease in the capacity for aerobic work, as measured by VO2 max. This can occur due to several physiological factors, including muscle damage, hormonal imbalances, and reduced energy availability. Therefore, monitoring VO2 max can be an important indicator for trainers to determine if an athlete is overtraining.

In this context, other options such as increased muscle glycogen and increased body fat percentage generally do not align with overtraining signs. Instead, overtraining often results in decreased glycogen stores due to insufficient recovery and may lead to a more catabolic state rather than accumulating fat. Similarly, the sympathetic nervous system might show heightened reactivity to stress rather than a decreased response, making those options less accurate regarding the effects of overtraining.