Weightlifting routines are built around repetitions (a single execution of a particular exercise) and sets (a group of successive repetitions). Lifting an amount of weight that you can handle comfortably for about eight repetitions in a set builds strength, or the amount of force the muscle can exert in a single contraction. When you limit the number of repetitions, the action produces hypertrophy, a condition in which the muscles grow larger. Performing with an amount of weight that allows you to complete 15 or more repetitions creates muscular endurance -- the ability of the muscle to repeatedly contract over a period of time without fatiguing. Guys looking to add size to their arms should spend the most time in the hypertrophy range [source: Malvesti].