What type of exercise mainly stimulates an increase in muscle mass?

Resistance training is the type of exercise that primarily stimulates an increase in muscle mass. This form of training involves using weights or resistance to create tension in the muscles, leading to micro-tears in the muscle fibers. As the body repairs these fibers, it builds them back stronger and larger, ultimately resulting in hypertrophy, or an increase in muscle size. Resistance training can include various forms such as lifting free weights, using weight machines, or performing body-weight exercises, and is typically characterized by a lower number of repetitions with heavier weights for muscle growth.

In contrast, endurance training, low-intensity aerobic training, and flexibility training focus on other fitness components. Endurance training generally emphasizes cardiovascular efficiency and stamina rather than muscle size. Low-intensity aerobic training primarily works on improving aerobic capacity and fat oxidation, which does not significantly contribute to muscle hypertrophy. Flexibility training, while important for overall fitness and injury prevention, focuses on improving the range of motion in joints and does not involve the muscular tension required for stimulating muscle growth.