Well, you've probably done one of them. I know that I have and it wasn't until I was reading about it later (after not seeing or feeling results) that I realized that what I was doing was a total waste of time. So yeah, that was embarrassing.
Anyway, I am going to try to save all of you some time and potential injury by pointing out a couple of really useless exercises that I see people doing frequently at the gym. I don't know what compels them to do this but I would imagine it is because they saw someone else doing them.
Please help to stop this circle of time-wasting on the part of people at the gym, including me.
Side bend with dumbbells
Sadly, I pulled this image from a trainer's website where she is telling people how to do this exercise correctly and then proceeds to tell them how to do it incorrectly. The fact of the matter is that you are basically creating a see-saw of sorts when you have weights in both hands and therefore you are not creating any additional resistance than if you just had nothing in your hands. Think about it: It's like one of those old-timey scales but you put the exact same weight on both sides - nothing changes, right?
Also it is worth noting that even if you do this exercise "correctly" it still creates a very unnatural movement that could be damaging to the spine and it is best if you don't do it at all.
Basically anything using a BOSU ball
The fact that this piece of equipment exists at all and is present in most of the gyms I have ever been to (gathering dust in the corner for the most part) is a testament to how people will manufacture just about anything to try to take advantage of the billions that are spent on fitness products each year.
The idea behind BOSU balls is that it increases your stability and "core strength" and I don't know if that is true or not but I do know one thing: You will not be able to lift as much weight if you are on an unsteady surface! When I see people in the gym that are doing bicep curls while struggling to remain upright and not fall over I can't help but roll my eyes a little. What exactly do you think you are accomplishing here? Then there are the people that do squats with them
Screw it! Use 2 of them to double the core strength building!
You are going to be lifting significantly less than you normally would be capable of lifting while putting yourself on unsteady ground and while this might seem like it is helping core strength we already have a ton of exercises that are meant for precisely that. In the meantime, if you do exercises on a BOSU ball you are getting less benefit from the exercise that you are doing and you are also opening yourself up to injury. In my mind and the mind of most trainers, this is a very stupid combination.
Everything I've just said about BOSU balls also applies to Pilates balls or whatever those things are called. In my gym if anyone stands on top of these things trainers will intervene and tell them to stop.
This isn't just my opinion though a study was done back in 2010 that shows that higher weights on stable surfaces have a much greater impact on muscle development in an overall sense than introducing an unstable surface like a BOSU ball.
I think that if there is one thing I have learned from my years of being involved in fitness is that if a new product starts appearing and no one has ever used anything like that before, there is a very good chance that said item is full of crap.
That'll do me for now. I hope you aren't doing these exercises and if you are, it might be time to stop. Use that time to focus on something that actually does work instead.