We go to the gym to lift weights, but "lifting" is hardly the only portion of the repetition that counts. While telling someone you also "lower weights" doesn't have the same sort of hardcore cachet, that portion of the lift—aka the negative or eccentric half—is also incredibly important. In fact, that's where the real muscle-growth magic happens.
Let's take a look at why and how both bodybuilders and strength trainers are using negative training to realize positive gains that have been documented by exercise scientists.

The Rise and Fall of Your Workouts

The focus of your workouts is normally lifting a weight, an action in which a muscle shortens. Whether you're bench pressing or doing a biceps curl, the target muscles contract concentrically, meaning they shorten. Lifting a weight is also called positive-rep training.
On the flipside, when you lower the weight, the muscle lengthens, as the external resistance is greater than the force being supplied. This is also called the negative portion of the rep.
The focus of your workouts is normally lifting a weight, an action in which a muscle shortens. Whether you're bench pressing or doing a biceps curl, the target muscles contract concentrically, meaning they shorten.
You've probably noticed that it's easier to lower a heavy weight than it is to lift it. Normally, you control the speed of the negative rep, meaning you lower it slowly, but that's usually just a long second, maybe two. In contrast, eccentric training requires you to really focus on extending the length of the negative rep to three seconds or more.
Muscles are capable of achieving a higher absolute force when contracting eccentrically—yes, it's still called a contraction even though it's lengthening— compared with concentrically. In fact, eccentric muscle action can produce 20-60 percent more force than concentric actions.1

The Science of Size

Numerous studies show greater gains in muscle mass with eccentric training compared to concentric only.2-3 One reason is the greater muscle damage that occurs with eccentric training—perhaps because of the longer time under tension. Muscle damage is believed to lead to a rapid rise in protein synthesis as well as a larger rise in insulin-like growth factor—all of which adds up to more muscle.
Eccentric training can also recruit more type II (fast-twitch) muscle fibers, which have the greatest potential for hypertrophy. This was demonstrated in a study published in the Journal of Applied Physiology that found 12 weeks of eccentric training was more effective at increasing type II fiber size compared to concentric exercise.4 Eccentric training has also been shown to elevate lactate levels, resulting in an increased anabolic hormone response.5 Again, this means more muscle.
Numerous studies show greater gains in muscle mass with eccentric training compared to concentric only.
Need more proof? Another study looked at the effect of eccentric training on concentric reps using more than 100 percent of subjects' one-rep max (1RM). They found 5-15 pound increases in 1RM strength. The increase in strength has been shown to be significantly greater when compared with concentric resistance training using lighter loads.6,7
Eccentric contractions also produce less fatigue and require less energy than concentric training, meaning you can employ a greater amount of volume in your training. This leads to—you guessed it— greater gains in size and strength.8,9
Eccentric training produces more muscle damage and soreness, as well as short-term decreases in power output. (However, with repeated bouts of eccentric training, that soreness quickly trails off.) As a result, some experts advise using the method for only short periods of time or intermittently. Heavy eccentrics should be used sparingly in your workout program to avoid overtaxing the neuromuscular system and reduce the risk of overtraining.

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