Originally Posted by Fazc
Good points by OR and IM; I'm going to throw this into the mix, not necessarily because I disagree (I actually don't disagree with what's been said) but because it is another side to the argument:
Thinking more long-term than just the end of the set and being able to get your reps, if a trainee regularly changes their form to 'cheat' their way to the last few reps what exactly are they training their bodies to do?
I'll assume this is Strength/Power based since we're in that forum, so what they are training the body to do is when the going gets tough to start to bounce their benches off their chest and/or raise hips off the bench. By continually (or even just regularly) switching form to get the job done you are reinforcing the wrong technique. So eventually your body will know that when the reps get tough, or even when there is sufficient weight on the bar it will resort to that style of lifting.
The term used is 'movement patterns' in biomechanics. That is a field of study partly concerned with finding efficient movement patterns. If cheating is done on the hard sets what is effectively happening is that the wrong movement patterns are being trained and the more it happens the more those movement patterns are ingrained into the body because the muscles responsible for producing that movement pattern get stronger and stronger while the other, more strict, movement pattern gets weaker and weaker (relatively).
That's just one opposing viewpoint, specifically for strength and power. The reason people cheat is *sometimes* because a stronger bodypart takes over and completes the movement, the goal of training a lift should be to make the strict movement pattern also the dominant and strong one.
I agree. We all know that guy with the 1/4 ROM leg press, and no one wants to end up like him. (...right?)
That said, there's learning the right way to deadlift and not being afraid of lifting heavy, and then there's being mortally terrified of maxing for fear of some back rounding.