This is an interesting article about the different types of movements. This applies to those of you that lift weights...
As early as I can remember doing weight bearing exercise, I nearly always emphasized the “pushing” or contractive part of the movement. If I was doing a push-up, my effort was on pushing myself off the floor. Then I quickly lowered myself and began the next one. If I was doing a sit-up, my effort was on sitting up; then I’d drop back to the floor. If I was doing curls with dumbbells, I was focused on raising the weight, bringing by hand toward my shoulder. After each curl, I’d let my arm drop without much restraint, and then start the next rep. This type of exercise focuses on what’s called the concentric aspect of muscle work – the part where muscle fibers are shortening.
There’s another kind of work that our muscles do in exercise, which is to restrain the re-lengthening of the muscle after a contraction. This is called eccentric (“ek-sentrik,”) exercise, and it has unique benefits and even some advantages over concentric exercise. In the three examples in the previous paragraph – pushups, sit-ups, and curls – the part of the exercise I wasn’t paying attention to (lowering my body in the push-ups and sit-ups, and lowering my forearm in the curls) was the eccentric phase.
In studies comparing strength and muscle mass gains between concentric-only and eccentric-only exercise, the two forms produced about equal results.  The main difference was that participants found eccentric exercise easier to do. One study, published in January 2011, found that women who did 30 minutes of eccentric exercise per week experienced an improvement in their Resting Energy Expenditure – a measure of how many calories the body burns in a 24 hour period while resting – and their fat oxidation – the breakdown of fat into energy. 
Because it’s so much easier to do eccentric exercise versus concentric, many studies utilized considerably more weight for the eccentric phase than the participants could possibly lift concentrically. For example, if a 100 pound barbell is the most weight you could press off your chest, it’s likely that you could (eccentrically) lower 120 pounds or more to your chest.
However, because of the heavier loading and high number of reps certain studies have employed, some researchers have concluded that eccentric exercise has a greater potential for muscle damage than concentric exercise. In a small percentage of cases, the muscle damage resulting from very intense, high-force eccentric exercise has been severe enough to flood the blood stream with muscle tissue particles that are toxic to the kidneys (a dangerous condition called rhabdomyolysis). These studies also demonstrated the potential for prolonged muscle weakness and severe swelling.  However, this is quite unusual, and not to be expected with regular exercise programs.
Intense eccentric exercise tends to produce significant muscle soreness, and possibly weakness and reduced range of motion, beginning about 24 hours later. However, in a paper by Priscilla Clarkson, PhD, of the Department of Exercise Science at my alma mater, the University of Massachusetts at Amherst, she states, “The damage that is induced by these exercises is completely repairable in a short amount of time.” Clarkson states that even after high-force eccentric exercise, within 7 to 10 days, muscle function is fully recovered. Furthermore, “ the damage also results in an adaptation in the muscle making it more resistant to damage from subsequent strenuous exercise.” In subsequent bouts of high-force eccentric exercise, study participants experienced better performance, quicker recovery, less muscle damage, and less soreness afterwards. 
Luckily, even if you stay on the safe side, and engage only in moderate eccentric exercise that produces little muscle damage, this “training effect” still occurs. No such adaptation occurs with concentric training. 
So, what do you do with this information? The simplest and most immediate way to apply this research to your fitness regimen is to start placing equal emphasis on the concentric and eccentric phases of the exercises you do. Don’t switch to only eccentric training, because eccentric and concentric exercises benefit us in slightly different ways. A balance is best. Spend just as much time on the eccentric phase as you do on the concentric. As a starting point, I recommend 3 seconds on each phase for each rep. For example, if you’re doing a pushup, that means it should take you 3 seconds to go up and 3 seconds to go down.
Later I’ll write more about how to increase the weight for the eccentric phase and some of the interesting uses of eccentric contraction to treat injuries.
Dr. Peter Borten