A lot of potential muscle growth comes from the resistance of the weight on the down force of the gravitational pull of the weight. Whether it is free weights or a machine, you have the negative resistance. When you lower a bar to your chest during a press or you lower the weight back to the starting position when doing a curl. Doing this in a slower manner is called Negative Reps and it helps build up the muscle fiber which otherwise does not get as much work as the fiber pushing or pulling th