When muscles contract against resistance (usually a weight) with sufficient force, the muscle cells adapt to the strain by synthesizing protein—and thereby increasing in size and strength. Tendons and ligaments (connective tissue) are also strengthened. In addition, strength training can help enhance nerve activity and muscle fibers in various ways that improve physical performance.
There are two basic types of muscle fitness—muscle endurance and strength. Most workouts build both to some degree, though you can emphasize one or the other, as follows:
Light resistance, many repetitions. Lifting light-to-moderate weights (50 to 75 percent of the maximum amount you can lift) many times primarily builds muscle endurance—that is, the ability to contract a muscle repeatedly in quick succession, as in lifting a suitcase 20 times in a minute or two. To a lesser extent, this also builds muscle strength. Such training can improve performance in endurance activities such as brisk walking or cycling.
Heavy resistance, few repetitions. Lifting a heavy weight (more than 75 percent of your maximal lift) a few times, in contrast, primarily increases muscle strength (and size). Strength is the force a muscle produces in one all-out effort—as when you swing a mallet to ring a carnival bell. This type of training can be useful—for instance, when preparing for an activity that requires explosive strength, such as a jump.