What characterizes overtraining?

Overtraining is characterized by a long-term decline in performance. This occurs when an individual exceeds their training capacity without allowing for adequate recovery time, leading to physical and emotional fatigue. The body's systems become stressed, and recovery mechanisms are overwhelmed, resulting in decreased athletic performance, increased risk of injury, and a general sense of burnout.

In contrast, the other options describe states that are not associated with overtraining. An immediate increase in physical conditioning suggests a positive response to training, which does not align with the effects of overtraining. A temporary plateau indicates a stable performance level rather than a decline, and an improvement in physical functioning suggests effective training without the negative impacts of overtraining. Thus, the defining feature of overtraining is the sustained decrease in performance, making the first option the correct answer.