While doing bicep curl with a friend at the gym, I notice his hip would always swing front and back everytime he did a concentric on the bicep curl. We tried to correct the problem by reducing the load.
What could be the most probable reason for this issue to take place?
Concentration curls require you to sit on a bench with your elbow on your knee or thigh, so it would be physically impossible to swing your hips to the front. The entire point is to stabilize everything as much as possible so the bicep is the only thing used.
I imagine what your friend is doing is a standing bicep curl where hip swinging is very common. People do it because they're lifting too heavy, and they're using their hips to push the weight a bit to build momentum to help push the weight.
If you want to stop doing that, then you can do a curl variation that's more stable like the concentration curl above, a preacher curl, a spider curl, or you can do a standing bicep curl against a wall.
It would be near impossible to use any body momentum doing any of those forms.
While "most probable" is tending towards the discussion type of answer, there are two main reason for a hip swing in a biceps curl, and both have the same root cause.
Reason 1 is to create momentum in the weights, so that rather than starting from a dead stop there is motion forward and a bit up. This makes it easier to complete each rep.
Reason 2 is to counterbalance, there is enough weight out away from the body that the hips move to compensate.
Both of these are generally caused by using too much weight for ability. If you are having to "cheat" a weight to complete a rep (In pretty much any given exercise) you are not getting as much benefit as you would from a lighter weight and stricter form. There are a few examples where cheating the exercise is correct, but those are few and specific.
