Your least frequent, most extreme exertions will have the greatest influence on your fitness. The peak moments of a workout count far more than the amount of time you spend working out. This is why a series of 40-yard sprints at full speed benefits you more than half an hour of jogging. It's also the reason why lifting a weight heavy enough to make your heart pound and your muscles burn counts more than spending hours at the gym always in your comfort zone, never truly challenging your body. When a work-out becomes an unvarying, monotonous routine, it loses its effectiveness.