Stop It With the BOSU Curl
Ordinarily, I wouldn't have thought that BOSU ball curls were actually a thing that people do in gyms. Complaining about them would be like complaining about cats wearing yoga pants, because you don't generally see cats wearing yoga pants, and getting all bothered about it would be a waste of psychic energy.
But I've been trying out different gyms lately and sure enough, I see people doing BOSU ball curls fairly frequently. I want to pee on their shoes.
A Bastardized Exercise
In case you haven't seen anyone doing this bastardized exercise, they stand on an upside-down BOSU ball with a pair of dumbbells and do dumbbell curls.
In order to keep their balance, they alternate arms and pump them really fast. The effect is kind of similar to one of those spinning-top toys that toddlers play with – if the top slows down, it falls over. Same thing with the BOSU ball curlers – if they slow down their arms, they fall over and do a header onto the glute/ham raise machine.
I have no g-damn idea what muscles or skills they're actually working. Maybe their ankles get stronger, but I'm not sure. They sure as hell aren't working their biceps to any appreciable degree.
Either they have to use ridiculously low poundage so they can still maintain balance, or they use heavy weight and curl it really fast for that spinning-top effect, which allows them to use pure momentum and very little muscle strength to curl the weights.
But What About the Core, Bro?
I suppose most of them would say they're working their core, which also makes me want to pee on their shoes. You know who gets a lot of the blame for that core crap? Powerlifter Fred Hatfield. "Dr. Squat." He's the one who said, "You can't fire a cannon from a canoe."
What the hell does that even mean? I guess lifters thought it meant that you need to build up your lower trunk muscles in order to lift big, but maybe Dr. Squat just fell off a BOSU Ball, conked his head, and thought he was a canoe paddler in the French and Indian wars. In any event, he started a core craze that exists to this day, to the exclusion of all common sense.
Okay, It Kinda "Works"
Granted, BOSU ball curls "work" the arms and they "work" the core, but it's the workout equivalent of washing your clothes in raw sewage. Sure, you're using water, kind of, and even dirty water cleans things, sort of, but you'd be far better off using a different method.
For curls, work your arms with reasonably heavy weight on good ol' terra firma, without momentum. For your core, do roll-outs or any one of hundreds of suitable movements that just might give you some powerful abs in the process.