Both the lack of physical activity and excessive physical activity (over training) will result in decreased levels of testosterone. Exercise effects testosterone directly by stimulating the pituitary gland and the testes and it probably also raises levels by slowing down the normal breakdown of testosterone.
The duration, intensity and frequency of exercise will determine the circulating levels of testosterone. Testosterone levels increase most with short intense bursts, while it decreases with prolonged activity especially that of frequent endurance training. During endurance training, testosterone is needed to maintain muscle but frequent extended training doesn’t allow for repair and recovery of testosterone and tissue damage occurs.
Studies show that testosterone levels will elevate with exercise for about 45 to 60 minutes. After this time period, cortisol levels begin to increase and testosterone levels will decline. This decrease has been detected for up to 6 days.
Because you require testosterone for repair and growth, do not train for more than 45 to 60 minutes at a single session. If you feel like you want to exercise or train more, split sessions are recommended. Also do not lift weights and perform aerobic training at the same time. It is also a good idea to vary your workouts and cycle them throughout the year.
Construction workers provide a real life example of these concepts. It has been observed for many years that men who lift moderate loads all day are frequently listless and tired as well as not as strong and muscular as their co-workers who perform less frequent but more intense activity.