Fact of the matter is that most pros use a ton of momentum on everything, much worse than this. Like Ronnie on his T-bar and barbell rows. He used momentum to get the weight moving then slammed his body down to meet the weight lol.
That's why I think, or suspect, that it's the beginning of the movement where you really stretch and maybe have those micro-tears on the muscle. Remember those guys who promoted "Xreps" on Iron Man, where you focus on partials in the stretch position? Or Jason Huh who just does those short partial reps in the stretched position. I swear, when doing tricep extensions he seems to just be using 15 degrees of motion.
When you look at, say Ronnie, doing those T-bar rolls, his back/lats are under tremendous force at the beginning, stretched part, of the movement. The tremendous weight he is using plus the force of him blasting it up must really put a ton of force on the muscle. This force diminishes quickly as momentum takes over. So he's really only stressing the muscle in that beginning part of the movement.
I remember reading how it was that stretched position that cause the most muscle damage. Arthur Jones had a similar notion which he called the prestretch. Say, when doing a bench-type movement. Just before you fully lower it you almost kind of pause, for rather control it, and then sort of bump it up. It's kind of hard to describe. It's the same principle when you throw a baseball or swing a baseball bat. Your waist, shoulder, hand don't move in unison like when a door swings on its hinges. You throw your hips out first creating a tension, a prestretch, on your obliques, chest, arms, and hands creating more force. This "loading up" is clearly seen by that small step forward when swinging a baseball bat. Watch a discus thrower. How the hips precede the motion which causes the pecs to stretch out and like a rubber band snaps the arms and hands forward. This prestretch supposedly activates more muscle fibers.
Scroll to about 3:00.
