Lowering the weight during a rep is often an afterthought. You work hard on the “working” part of the rep, regarding the second half as simply a time to reset.

If you’re guilty of this, you’re only getting a portion of the benefits of the exercise—especially if you’re trying to build muscle. Here’s why.

How Muscles Work

When you hear someone mention a muscle contraction, your first inclination might be to think of a muscle shortening. After all, the definition of contraction is “the process of becoming smaller.”

However, this only tells part of the story. Muscles have three types of contractions, each of which produce forces, but the muscle doesn’t necessarily shorten. For example, here’s how the biceps function during a Curl:

1. As you curl the weight up, your biceps concentrically contract, causing them to shorten. You can see this in action as your biceps literally tighten up and get shorter.

2. At the top of the movement, you briefly hold the weight without moving. This is called an isometric contraction, where your muscles create force without actually moving.

3. The final phase of the movement is the eccentric contraction. Your biceps lengthen but they’re still creating tension. If they didn’t eccentrically contract, the weight would just flop down.

The eccentric portion of the lift is actually about 30 percent stronger than the concentric portion. So if your max Curl is 50 pounds, the most your muscles could handle on the eccentric action is 65 pounds.

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What This Means For Building Muscle

If you lower the weight haphazardly, you fail to train your muscle during the eccentric portion of the lift. Realistically, you can think of this as performing only half of a rep.

It’s been found that focusing on the eccentric portion of the lift causes greater muscle hypertrophy (i.e., muscle growth) than when you perform the concentric portion. Also, a study determined that performing both the eccentric and concentric portions is more effective than performing only the concentric action. This is likely due to an increase in time under tension, or the amount of time the muscles have to work.

There are a few strategies you can try to train the eccentric action.

Since you’re weaker in the concentric portion of the exercise, you’re limited by how much weight you can lift in that phase. Negatives allow you to train the eccentric action by eliminating the concentric portion. For example, if you’re doing the Bench Press, you can load up more weight than your max, lower it to your chest and have spotters help you raise the bar back up. Although it’s effective, this method only trains the eccentric action.

To work both, you must choose a weight you can handle throughout the entire set, but you manipulate the timing of each phase. The most common strategy is to lower the weight over two to four seconds, which creates a serious pump in your muscles.

However, there’s another way that combines the best of both worlds.

Partner Manual Resistance

Partner manual resistance is just what it sounds like—you work against a counterforce provided by a partner. On some exercises you use a weight, and your partner simply resists your movement on the way down. On other exercises, your partner provides the source of resistance for both parts of the rep.

The result? You effectively work both the concentric and eccentric muscle contractions, causing your muscles to grow.

Because of the limitations of working with a partner, you can’t do this on every exercise. However, here are 8 of our favorites.

1. Manual Push-Up

How to: Assume a push-up position. Have your partner stand straddling your torso with his hands on your upper back. Slowly lower into a Push-Up, fighting your partner’s attempt to push you to the ground. Drive up out of the Push-Up.

2. Manual Towel Row

How to: Lie with your stomach on an incline bench holding one end of a towel with both hands. Your partner sits in front of you also holding the towel. Perform a row while your partner resists your movement. Slowly extend your arms, fighting your partner’s attempt to pull them.

3. Manual Overhead Press

How to: Sit on the ground with your elbows bent at a 90-degree angle and position your hands in front of your shoulders. Have your partner place his hands on your fists. Moving against your partner’s resistance, drive your arms overhead. Slowly lower your arms to the starting position, fighting your partner’s attempt to press your arms down.

4. Manual Chest Press

How to: Lie on a bench as if you were about to perform a Dumbbell Bench Press and clench your fists. Have your partner stand behind you with his hands on your fists. Moving against your partner’s resistance, drive your arms up in front of your chest. Slowly lower your arms to the starting position, fighting your partner’s attempt to press your arms down.

5. Manual Pull-Ups (or Dips)

How to: Grasp a pull-up bar with a shoulder-width grip. Have your partner stand behind you and hold your hips. Pull your body up until your chin is over the bar. Slowly lower your body until your arms are straight, fighting your partner’s attempt to pull you down.

6. Manual Curls

How to: Hold a barbell or EZ bar with a shoulder-width grip. Have a partner stand in front with his hands on the bar between your hands. Keeping your elbows to your sides, perform a Curl. Slowly lower the bar to the starting position, fighting your partner’s attempt to push the bar down.

7. Manual Skullcrushers

How to: Lie on your back with your arms extended in front of your chest and clench your fists.  Have your partner stand behind you with his hands on your fists. Bend your elbows as if performing a Skullcrusher, fighting your partner’s attempt to press your arm down. Straighten your arms against your partner’s resistance to return to the starting position.

8. Manual Hamstring Curl

How to: Lie on your stomach. Have your partner sit next to your right knee with his hands on your ankle. Bend your knee to curl your lower leg up against your partner’s resistance until it’s perpendicular to the ground. Slowly extend your knee, fighting your partner’s attempt to push your leg down.