Moving your muscles through their full range of motion will engage far more muscle fibres than doing partial reps or cheat reps (where momentum moves the weight). The more fibres you fatigue, the faster your muscles grow. Lifting a weight that’s too heavy for you is a mistake regardless of the part of the body you’re working, but it can be truly disastrous when performing shoulder exercises. The shoulders are delicate and complicated joints that are not especially easy to target. ~ Berceste