Weight lifting can improve performance in endurance sports such as running, cycling or rowing. A study from Spain divided highly trained, competitive rowers into four groups 1) four arm exercises leading to repetition failure, 2) four exercises not leading to failure, 3) two exercises not to failure, and 4) no resistance training.
All four groups did the same endurance training (Medicine and Science in Sports and Exercise, June 2010). Those who did the four exercises not to failure improved the most in *rowing performance, *lifting the heaviest weight that they could lift once, and *the highest muscle power output.
Athletes who train primarily for strength must train to muscle failure. That means that they lift weights repeatedly until they can barely lift that weight another time. This causes muscle damage that is necessary for maximum muscle growth and strength. The next-day muscles soreness tells them that their muscles are damaged and when thy heal, their muscles will be stronger. Then they take easier workouts until the soreness goes away and repeat their muscle-damaging workouts.
However, athletes in endurance sports must take long hard workouts lasting many hours. If lifting weights causes so much muscle damage that they cannot do their endurance workouts, they cannot compete in their sports. This study shows that athletes in endurance sports can benefit from lifting weights, but they should not go to failure so often that it reduces their workouts that are specific for their sports.