Rolling shrugs

I see lots of highschool kids doing this at the gym that i workout at.

I've heard it doesn't work well and i dont do them this way myself.

Anyway, here is the question, why doesn't it work well, and what else is bad about this technique? Is it dangerous? shoulder impigment? what?

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There is no real advantage to rolling the shoulders in a shrug. A shrug should be done with a direct up & down motion while keeping the body erect. I'm not sure if there is any actual evidence of injury due to rolling the shoulder other than anecdotal (I've not come across any although it may be out there somewhere), but it does not really add anything to the exercise. I'd prefer a strict shrug, or a high pull if you want to go for a more power move, if you wanted any 'drawing back' (scapula retraction) you'd be better doing a kelso shrug - where you go into a bent over barbell or dumbbell bottom position & then retract the scapula(e), personally I find the single arm dumbbell version gives greater range of motion.