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When lifting a dumbbell or barbell, you need to accelerate it at the beginning of the movement, and slow it down at the end of the movement. This means that the start of the movement is harder, the end of the movement is easier.
Momentum can be used to "cheat". For example, the start of a dumbbell row is quite easy, while the end if hard due to the leverages at the top. Experienced lifters will often use momentum to lift heavier weights. Some would call this "ego lifting". I call it overloading the eccentric.
Cable machines show an interesting effect. The energy in the moving weight is mass * velocity squared. A cable stack with a 2:1 pulley uses twice the weight, but has half the kinetic energy of a cable stack with 1:1 pulley, as the weight moves at half speed.
The bad news about momentum is that bad technique can lead to injury.
Resistance bands are very light, so momentum becomes negligible. With bands, you can't rely on momentum to get past sticking points.
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