Anatomy of A Rep

How to master and manipulate the most fundamental element in weight training.

By Timothy C. Fritz

The Basics

At the most rudimentary level, a rep comprises three phases: the concentric contraction (lifting the weight), the transition and the eccentric contraction (lowering the weight). For most exercises, the concentric phase comes first, although on exercises such as the squat and bench press, you actually descend through the eccentric half to assume the real start position.

A common misconception is that a muscle contracts during the first half of the movement and then relaxes as you return the weight to the start position. In fact, a muscle contracts during both phases. The difference is that the muscle shortens during the concentric half and lengthens during the eccentric half.

A contraction is initiated by impulses sent via nerves from the brain and spinal column to muscle cells. A single nerve, or neuron, is connected to several muscle cells, or fibers. Collectively, the neuron and the fibers it innervates constitute a motor unit. When a nerve fires, all muscle fibers constituting the unit contract. Each muscle comprises many such motor units, of course, but only during maximal contraction do they all fire simultaneously. During a less-than-maximal movement, only a certain number of motor units respond at one time, depending upon the force and motor skills required.

PHASE ONE: The Concentric Contraction

Closing the Gap

During the concentric contraction, the working muscle shortens, pulling the bones on either side of the joint being used closer together. (Picture your forearm and upper arm coming together when you curl a dumbbell.) At the start of this concentric contraction, only a small number of motor units are activated, generating minimal force. As more force is required, additional motor units are called upon.

If the weight being lifted is relatively light, many motor units will remain inactive; only a fraction of the total muscle contracts. However, if the weight is heavy, the muscle is fatigued or both, the muscle must recruit as many motor units (and fibers) as possible to accommodate the demands being placed upon it. The amount of force that a muscle is able to generate increases with the number of motor units that are utilized.

Use a rep speed at which the movement is completely controlled -- with no swinging -- to get maximum recruitment. If you let momentum do some of the work for you, you won't use as many muscle fibers to lift the weight. Breathing is another important aspect of any rep. As a rule of thumb, exhale during the concentric contraction, the period of greatest exertion.

PHASE TWO: The Transition

Stuck in the Middle with You?

At the end of the concentric contraction, a muscle is in its shortest position. Some exercise physiologists and many bodybuilders recommend that you pause here for a second or two to contract the working muscle as intensely as possible, a technique called peak contraction. "During my precontest phase, I rely heavily on peak contraction, particularly for biceps and triceps," says Garrett Downing, winner of the heavyweight division at the 1999 NPC USA Championships.

"For example, holding something like a triceps extension at the bottom, as opposed to just lightly 'tapping' there, gives me more of a pump and makes my muscles look harder and more striated. It's definitely an asset."

Two-time Mr. Olympia Larry Scott agrees. "I believe peak contraction has a lot of physiological benefits in terms of activating hard-to-hit muscle fibers, establishing new neural pathways and so forth," he says. "More fundamentally, it helps you get more in touch with your body. I use it for every bodypart, not just arms."

Others question the need to stop at any point during the rep. Steven Fleck, PhD, CSCS, former head of the physical conditioning program for the U.S. Olympic Committee, believes that using the appropriate resistance is more important than generating a peak contraction. "If the weight is light, you can never reach maximal contraction," he says. "But if you manage the resistance right, you'll get near-maximal contraction at some point during the range of motion."

Your best bet is probably to include peak contraction as a tool in your training program without relying on it to produce maximum stimulation and contraction. Instead, depend on heavy weights to promote the highest levels of contraction, activate the highest number of fibers and hence fatigue your muscles.

PHASE THREE: The Eccentric Contraction

Letting It Down Slow

Whether or not you pause at the end of the concentric half of the rep, eventually you have to return the weight to the start position. This half of the rep is called the eccentric phase, which many bodybuilders mistakenly treat as an afterthought. As you lower a dumbbell during a curl, for example, the biceps lengthens, even though it's still contracted to some degree. (Were it not for this contraction, the weight would simply fall back to the start instead of returning in a controlled manner.)

During the eccentric phase, nerve impulses continue to signal motor units to fire, even though fewer motor units are incorporated than during the concentric contraction. As a result, more stress is placed upon each of the activated muscle fibers.

This has important implications for muscle soreness and tissue breakdown, key issues in muscle-building. "One of the theories of size development is that you must have some minute muscle damage followed by an inflammatory response," explains Fleck. "The inflammatory response is one of the triggers for protein synthesis, which results in more muscle.

With normal weight training, during an eccentric contraction you lower the same weight with fewer muscle fibers, and that means that each fiber involved has to sustain greater force. Therefore, a higher percentage is damaged." In theory, increased muscle-fiber damage could lead to increased growth. Greater tissue damage would also explain the increased incidence of delayed-onset muscle soreness often associated with eccentric training.

Research confirms that the eccentric component of a lift may be just as important as the concentric phase for promoting muscle growth. One study showed that, when compared to normal (concentric and eccentric phase) weight training, concentric-only training required twice as many repetitions to produce similar results.1 Breathing is as important during the eccentric phase as it is during the concentric phase. Inhale during the eccentric phase or between reps.

Full Range of Motion

Combining the concentric and eccentric phases of the rep produces an exercise's range of motion. To ensure maximal contraction and promote joint flexibility, you want to fully utilize this. Limiting factors can include joint properties and body composition, both of which will provide a natural "stopper," letting you know when you've taken a movement far enough. Movements that fall short at either end of the range of motion will limit the number of muscle fibers involved, and may actually lead to decreased flexibility. Exercising through a full range of motion is safe as long as the rep is slow and under control.

One Final Rep

Although you needn't -- and shouldn't -- contemplate muscle physiology each time you lift, you can benefit greatly from a basic understanding of what happens when you complete a repetition. Realize that the eccentric contraction is at least as important as the concentric contraction when it comes to building muscle. Remember to breathe naturally and execute movements through a full range of motion in a slow, controlled manner.

Maintain tension (contraction) in the working muscle during the entire movement, and don't feel like you have to stop at the midpoint of the rep to accentuate the contraction, although don't hesitate to do it if it feels good. If each set you perform consists of intelligent reps based on good form, your training sessions will quickly peak, as will your muscles.