In fitness, a repetition is defined as the performance of a single movement or exercise, through a full range of motion. A predetermined number of repetitions performed in a focused and rhythmic manner is termed the workout set, and the amount of weight or resistance used for the exercise is termed the load. Taken together, these three variables influence the intensity of a resistance training workout.
Within these factors, there is an inverse relationship between the repetition and the load. A heavier load will require a lower number of repetitions, whereas a lighter load will allow a higher number of repetitions. Depending on the goals of the workout, repetition and load can be adjusted along a continuum with high load and low repetitions at one end, and the reverse at the other. The load is often expressed as a percentage of one repetition maximum, or 1RM — the maximum weight an athlete can lift for a single repetition of a particular exercise.
The weight of the load and the number of repetitions will largely determine the type of muscle fibers recruited during the workout. Two distinct types of muscle fibers are present in most skeletal muscles, in varying proportions. Fast twitch muscle fibers are associated with brief, intense bursts of activity, and are most capable of increases in size and strength. Their counterpart, slow twitch fibers, are used for low intensity, long duration activities and tend to remain fairly constant in size.
A lower number of repetitions with a load at or near the 1RM will tend to increase muscular power. Muscular power workouts typically employ repetitions in the range of one to six, predominantly activating the fast twitch muscle fibers. Power workouts are often performed by athletes training for sport-specific objectives, such as those involved in sprinting and power lifting.
To increase the size of the muscle fibers, a response known as muscular hypertrophy, repetitions are usually performed in the range of six to 15 per workout set. This range will typically activate a mixture of muscle fiber types and encourage the development of both strength and stamina. The lower end of the range will tend to stimulate more of the fast twitch fibers, promoting muscular power and size. Higher repetitions will also induce hypertrophy, but will recruit a higher percentage of slow twitch muscle fibers, inducing muscular endurance and cardiovascular efficiency.
Repetitions of more than 15 per set will normally promote muscular endurance and cardiovascular fitness almost exclusively. Workouts including a high number of repetitions stimulate the slow twitch muscle fibers, resulting in very little change in the size and shape of the muscle belly. This type of exercise does encourage enhanced energy metabolism within the muscle, however, as well as increased ability of the cardiovascular system to deliver oxygen and nutrients to the working muscles.
The number of repetitions performed per exercise set during a workout depends largely on personal goals, genetics, and underlying fitness level. For a balanced workout and overall fitness, a combination of power, strength, and endurance workouts over the course of a week, month, or even a year will reap the benefits of all three intensities, and result in a well-developed underlying strength base.