Exercise normally stimulates the body to recover and through this process improve endurance, strength or other variables through super compensation. Well performed fitness training will increase growth hormone secretion, which stimulates recovery and rejuvenates tissue. A higher training frequency and training intensity normally results in better results. This is true up to a point. While it does not happen too often, it is possible to break down the body instead of improving performance by training too much.

This can occur when someone for example performs resistance exercise two times a day targeting the same muscle groups. This is rare however, since resistance exercise stimulates anabolic hormone production, such as growth hormone and testosterone, which stimulate recovery.

In addition the intensity of resistance exercise, when performed right, is too high to sustain for longer periods or very often because of muscle soreness. When performing cardiovascular exercise, one runs a greater risk of overtraining, because the total exercise duration is often longer and relative intensity is lower than resistance training. The longer duration increases cortisol production, a stress hormone which breaks down muscle for energy and inhibits testosterone production. The lower intensity does not stimulate growth hormone and testosterone production as much as resistance exercise.

So on the one hand, cardiovascular exercise can break down the body and inhibits recovery and on the other hand, not much growth hormone and testosterone is produced to stimulate recovery. As a result, people get weaker by training too much. In addition, the immune system is suppressed, increasing the risk of infection. This does not mean that cardiovascular exercise should be avoided altogether, it merely means that one should be wary of performing too much exercise without adequate rest. The negative effects of too much training can already occur by training 8-10 hours per week, which is normal for quite a few endurance athletes.