A good kind of training momentum is when you train consistently, day after day, month after month, year after year. A bad kind of training momentum is when you do your exercises too fast.
Doing your exercises too fast means that in certain portions of the movement, the muscles are only steering and guiding, rather than actually lifting weight. For example, if I do a squat and come up really quickly, there are entire angles of the movement where my leg muscles aren’t lifting any weight at all.
To kill your exercise momentum, slow your rep speed down. Take two seconds to lift the weight and three seconds to lower it. Focus on the muscles you’re working out rather than blindly going through the motions.
You’ll notice a huge difference in terms of how strong you feel and how much muscle control you have. Fast reps are often sloppy reps.
Of course, there are situations where you are doing fast reps for a particular reason, within the context of sport-specific training. That’s different.