In Greek mythology, the renowned athlete Milo carries a baby calf on his shoulder each day. As the calf grows, Milo gets stronger, and eventually he can walk long distances with a full-grown cow on his back.  

This is a bit of a facetious example of progressive overload, a basic tenet of exercise training. Simply adding 1 pound per day doesn’t make us 365 pounds stronger at the end of the year. However, the heart and muscles need to be overloaded or stressed beyond their current capacity to get stronger.

Each “dose” of training causes acute fatigue, but also fitness down the line. Periods of intensified training cause substantial short-term fatigue, but with adequate recovery, we capitalize on that training with a boost in performance. But sometimes this process leads to overtraining.

What is overtraining?

When we’re not able to adapt positively to increased training, the emphasis shifts toward fatigue rather than fitness. This causes the fatigue-fitness cycle to become unbalanced and overtraining syndrome (OTS) can develop. OTS may affect around 60% of elite athletes at least once in their career, particularly in endurance sports such as long-distance running. The result is reduced performance and substantial physiological and psychological changes that can take months to recover from.

Signs of overtraining syndrome

Common signs and symptoms of OTS include fatigue, decreased performance, lac of motivation, depression and irritability. There is no accurate, globally accepted diagnostic criteria for OTS. Proposals to diagnose OTS include using surveys and measuring hormones, metabolites, autonomic nervous system metrics such as heart rate variability. However, there is conflicting evidence among studies, which makes it hard to diagnose OTS.

Other medical issues such as iron deficiency, low energy and infections may mimic and/or contribute to OTS, but the treatments are different.

Elite athletes may also have a mismatch between the energy they expend through exercise and the energy they take in through food. This condition, called relative energy deficiency in sport (RED-S), can pose serious health problems. RED-S has some crossover with OTS and often occurs when athletes attempt to lose weight while performing high training volumes.

Importance of recovery

A classic study on horses, which are extremely athletic animals, illustrates the importance of recovery. They trained every day, alternating high-intensity interval training (HIIT) and 20 minutes of easy running. The HIIT sessions got progressively harder and, after eight to nine months, the horses kept getting more fit with no signs of OTS. However, once the trainers increased the exercise intensity of the easy recovery day to include 20 minutes of hard running, the horses developed severe OTS within days.

Spacing out your hardest workouts every two to three days is a good general guideline based on the physiological time course of recovery. Other recovery recommendations include:

• Taking at least one day off from exercise per week.
• Eating carbohydrates and protein before and after exercise.
• Sleeping enough.
• For single-sport athletes (e.g., runners), including cross-training (i.e., cycling, rowing, tennis, etc.)

Exercise is vital for health, but training for health and for performance are not the same. Week in and week out, athletes need to learn how to navigate the line between doing too much and doing too little. Rigorous training is a prerequisite to success—but working with qualified coaches and medical professionals educated in OTS helps ensure a training program is structured appropriately for your goals. Sometimes that may mean doing a little less.