Unless you are an endurance athlete, endurance training will only compromise your strength and performance. Researchers recorded the content of free testosterone, cortisol and muscle fibre type in three groups of men: power athletes, endurance athletes and untrained subjects. As expected, results showed that the power athletes had a much greater percentage (70 percent) of powerful type II fibres than the endurance athletes (37.5 percent), while the untrained men scored closer to the power athletes with 61 percent fast-twitch fibres.
In addition, the power athletes showed a better ratio of testosterone to cortisol correlated with greater type II fibre composition. A more favourable ratio of testosterone to cortisol indicates a more anabolic environment that is ideal for muscle growth and tissue repair. The less favourable ratio experienced by the aerobic athletes indicates a more catabolic state that leads to muscle loss. Type II fibres do more than allow you to pick up heavy weights and move explosively. Type II fibres require a greater metabolic cost, whereas type I fibres are more energy efficient, leading to a slightly slower metabolism.
