We can all recognise that feeling of muscle fatigue after taking a brisk walk, cycling, running or playing soccer.

Our ability to engage in physical activity for long periods of time is thanks to efficient energy production in the mitochondria — the small “powerhouses” in our muscles.

The more mitochondria we have, the longer we can exert ourselves.

Source: New research on the muscles of elite athletes: When quality is better than quantity – ScienceDaily