You are able to produce more force while doing eccentric lifts compared to concentric. Studies have shown an greater increase in strength with eccentric contractions. For optimal results, one should incorporate both concentric and eccentric exercise into a workout. Eccentric phase of a lift is the most important part of the lift(aside from technique).
You must produce as much force as your muscle is capable of producing for this to work. You cannot load a weight you can pump 5 times and move as slow as possible and expect results. Yes you may burn, but you are not overloading the tissue.
