Ahh, curls, the beach muscle guy’s second favorite exercise next to the bench press.
I have seen this guy at the gym many times and his back is just permanently rounded at the top. Maybe from doing curls like this? A perfect demonstration on limiting range of motion and an even more perfect example of how NOT to stabilize your body before a lift through you core.
Stand up straight, tighten that core and then do a full curl: from arms hanging STRAIGHT down to hands near your shoulders. Rinse and repeat. And I better not see you swaying from back to front during the lift.
And why waste your time with curls? One of the smallest muscles in your body so if you are looking to burn calories this isn’t ideal. If you want to get bulky, well, OK, have at it. How about doing pullups and working your bi’s along with other muscles?