A supinated grip at the forearm allows the biceps to contribute more strongly as an elbow flexor.

Both recordholders.org and Guinness World Records maintain that pull-ups use a pronated grip, while chin-ups use a supinated grip.

A supinated grip externally rotates the humerus, which puts the shoulders in a much more favorable position for the lift, decreasing injury potential without compromising range of motion.

Performing the chin-up correctly can be tricky with a supinated grip, because of the natural tendency to do most of the work with the elbow flexors (largely biceps) rather than the shoulder extensors (largely lats).