When I'm doing standing bar curls for a high rep-count (and especially during the 21-rep challenge), my shoulders are absolutely burning by the time I get close to finished.
I'm focusing on keeping my shoulders back, chest out and up, and keeping my elbows static at all times.
I'm not sure of the name of the shoulder muscle here, but it's the one located at the front, and flexes if you push your elbow diagonally forward/up. I know this should indicate what I'm doing wrong during the curl, but it's like I can't help but tighten it when curling.
For reference, this does not happen during concentrated curls or hammer curls.
Any tips on correcting the issue, or alternative measures?