Mirrors and squats
I remember reading somewhere that mirrors can actually increase the chance of a newbie doing squats with incorrect form; as a newbie, I always used to look up at the ceiling, especially while pressing up, to make sure the back stayed as straight as possible. If you don't have good squat form down and stare at yourself or your legs in a mirror while squatting, you tend to lean forward too much, putting your back in a dangerous place.
So, mirrors for newbie squats = bad.
But say you're doing heavy barbell bicep curls. I like to stand looking parallel to a mirror, so I can look to the side and see exactly where my elbows are, and make sure I'm not swinging the weight too much. Or shoulder dbell presses, you really want the weights to go up in unison, I think, and mirrors help. Or bent over bbell rows, I'll look to a mirror on the side and make sure my back looks straight.
But, mirrors for arms/shoulders/back days = good.
Really, though, on all days, mirrors are a necessity for looking down the sports bras of sweaty fitness hotties when they're doing bent over dbell rows.
On all days, mirrors = stealth peeping aid
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