For this explanation, I am going to focus on a bicep curl. The concentric or flexion (shortening of the muscle) of the movement in the bicep curl is lifting the dumbbell up towards the shoulders keeping the elbows tight at your side. Most people think that exercising is just a concentric movement (flexing), but that is not the case at all. The eccentric or extension (lengthening of the muscle) of the movement is lowering the dumbbell away from the shoulders towards the starting point by your side. This movement is just as important as flexing the dumbbell upward, but most people don’t realize the importance and usually just drop the weight. You want to keep the muscle tension on the bicep for the entire movement, instead of letting gravity take over. Constant tension on the muscle is where you really get the greatest benefit of the exercise. Try and change your workout by curling the weight up quickly but controlled and then eccentrically lowering the weight very slowly (5-7 second count) focusing on keeping the muscle engaged the whole way. Repeat the movement for repetitions. You can do this with any exercise. It will work the muscle different than you may be used to and you will benefit greatly from it. Enjoy!