The trapezius is a large, triangular-shaped muscle located at the back of your neck and upper back. It helps to move and stabilize your shoulders and neck. Full definition
Comparing the effect of grip technique, Beggs et al. (2011) investigated the use of either the double overhand or over-under grip technique on upper trapezius muscle activity and found no difference between conditions at either 60 or 80 % of 1RM. (strengthandconditioningresearch.com)
They found that each successive increase of 10 % relative load produced significantly greater upper trapezius muscle activity. (strengthandconditioningresearch.com)
However, the pronated pull - up generally requires more effort than the supinated chin - up and therefore the difference in lower trapezius muscle activity may not exist when equal relative loads are utilised. (strengthandconditioningresearch.com)