I have a belief that momentum is the great injurer and the object of resistance training is to work the muscle. With free weights and most weight machines it's all too easy to "cheat" and gain that next step up in weight by "exploding" in the beginning of a rep. It's a behavior that takes a lot of disipline to control. Something that I lack. The Y got in a 10 station rubber band based circuit. My joints are much happier. The rubberband resistance has very little mass so changing direction doesn't jerk. There is much less benefit to exploding. You still can if you want to but it will deaccelerate much quicker.