the way he hits hammer curls in the last excercise is so lame and stupid. Many people do this to show off big weights. When I see this I know for sure they have low IQ
There is jsut no full contraction and not the stimuli it needs to grow fully.
I know exactly what you mean but cheating movements do place "a lof of stress" on muscle at certain points in the execution. For example there can be tremendous stress just from the negatives if done somewhat correctly. The real negative is the high injury risk. If there was zero injury risk I would do proper sets but also insanely high loads, even sets of pure negatives for low reps but I just can't, everything would tear

To illustrate what I mean, here's an idiotic bench press, but imagine the stress placed on the pec and triceps, as he reverses the motion. Lots of shot putters seem to do these idiotic, from an injury risk POV, benches.
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