Let me begin by saying, if you workout, you need to read this article! The only ones of you who are exempt from this article are those who know the terms concentric, eccentric, and motor unit recruitment like the backs of your hands.

According to world-renowned training expert and all-around cool guy, Dr. Clay Hyght, the definition of a repetition is “moving the resistance in an exercise from the starting position to the finish position and back to the starting position”. The textbook Exercise Physiology: Theory and Application to Fitness and Performance by Powers and Howley defines a repetition as “the number of times an exercise is repeated within a single exercise ’set.’” You get the idea. Although you may not have known the formal definition, you probably already knew what a repetition was, and that it is usually abbreviated ‘rep.’

A repetition, or rep, has two distinct phases: the concentric phase and the eccentric phase. During the concentric phase (aka the ‘positive’) the muscle shortens, thus bringing its origin and insertion (its ends) closer together. During the eccentric phase (aka the ‘negative’) the muscle lengthens and its origin and insertion get further apart.

While giving a seminar to a group of personal trainers, I came up with an easy way to keep that straight. During the concentric portion of a rep the muscle is contracting, and during the eccentric portion the muscle is elongating. Silly I know, but it helped them to keep it straight.

Here’s yet another way to look at it: during the positive you are applying force, and during the negative you are resisting force.

Let’s look at a couple of real-world examples to make sure you’ve got it straight. When you performing a squat, lowering yourself from the top, legs-straight position to the bottom, sitting-in-a-chair position is the negative and pushing yourself from the bottom to the top starting position is the positive.

Don’t think, though, that positive always means up and negative always means down. During a lat pulldown, pulling the bar down to your chest is the positive and letting the bar go from your chest back up to the starting position is the negative. That’s why I like to look at it as applying and resisting force; positive/concentric and negative/eccentric respectively.

If you had to pick one phase of the repetition that was more important, which would you pick? Most people would understandably choose the positive or concentric part of the rep where you apply force. Besides, on the negative aren’t you just letting go? No way, Jose.

The negative portion is actually much more stressful to the muscle and therefore more beneficial in terms of eliciting damage that will result in growth, if not overdone. For that reason, you should perform the eccentric portion of a repetition rather slowly – typically three to four seconds. I mean a full three to four seconds as in counting - one Mississippi, two Mississippi, three Mississippi, four Mississippi. (You can use one one-thousand, etc… if you prefer.)

This will ensure that you get maximum bang for your buck out of each rep, as well as go a long way in keeping you injury free. Going through the negative very quickly (dropping the weight) is one of the most dangerous things you can do in the gym.

What about the concentric portion? Is it the red-headed step child of the rep phases? Not at all. The concentric portion is very important as well. Ironically, you want to perform the concentric or positive portion rather quickly - sometimes as fast as possible (while staying in control).

Doing so, especially with a heavy weight, actually maximizes the number of muscle fibers that you use during a giving contraction. More muscle fibers used = more muscle fibers stimulated = more muscular progress; you get the idea. If you’d like to simplify things and put a number on how long the concentric phase should take - around one second would be your answer.

Rep Phase Force Application Time Muscle Damage
Concentric (positive) Applying Force ~ 1 second Very Little
Eccentric (negative) Resisting Force 3 – 4 seconds Lots

To continue our discussion of a repetition, we should talk about the interval between the concentric and eccentric portions of the rep. For example: the top and bottom parts of the squat. My rule for whether or not to pause after the positive part of the rep (in the contracted position) is as follows: if there is tension on the working muscle during the contracted position, then pause slightly on each rep.

Some examples of exercises where you would have tension on the working muscle would be glute bridges for glutes and lying leg curls for hamstrings. Milk the most out of each rep and pause slightly in the peak contracted position.

On the other hand, exercises like lunges and squats do not put much tension on the working muscle in the top or contracted position; therefore I would try to avoid pausing after the positive part of the rep on these type exercises. We tend to do so as a means of “resting,” which could be counter productive in the long run.

As for the portion of the rep between the negative and the positive, I would generally pause for up to (but no more than) a second in that position. However, do make sure that you change directions (from negative to positive) rather slowly to avoid injury. Decelerate the weight before the end of the negative, and then gradually accelerate into the positive phase. Never bounce!

Look around the gym next time you’re there. An alarming number of our fellow trainees will training sloppily, simply moving the weight from point A to point B with no concern for proper form and maximum safety and effectiveness.

Don’t worry, know matter how well built they are, they don’t know something you don’t know. In fact, you now know something they don’t know. The only reason they train so sloppily is to feed their ego by being able to use more weight.

Training is for stimulating your muscles, not your ego!

Train hard! Train smart! - Dr. Clay