Gabe Mirkin, M.D.
If you want to become very strong, you have to lift weights heavy enough to make your muscles burn. It doesn't make any difference whether you move the weights slowly or rapidly. Just exercise intensely enough so your muscles feel sore the next day. The soreness indicates that you have injured the fibers. When they heal, they will be stronger than before you injured them. But muscles heal only when you let them rest. If you put great pressure on a muscle that is already sore from a previous workout, it cannot heal and will tear. Then you are injured and will not be able to lift at all.
When you move very slowly with a weight, your muscles fatigue and weaken so that the weight feels much heavier than it is. Lifting a weight slowly causes the same muscle damage as lifting a heavier weight more quickly. One problem with slow lifting is that sports require fast movements and training is specific. You have to exercise against resistance moving fast to be able to use your muscle fast when hitting a baseball, throwing a half court pass in basketball and hitting someone in football. So slow lifting can make you strong, but it is not the best way to prepare you for fast-moving competitive sports such as tennis, ping pong, basketball and volleyball.