They reported superior muscle activity of the upper trapezius when performing deadlifts with 80 % of 1RM compared with 60 % of 1RM. (strengthandconditioningresearch.com)
Performing deadlifts with greater stance width (using a sumo technique instead of a conventional technique) does not appear to alter EMG amplitude in the gluteus maximus (Escamilla et al. 2002; Sorensen et al. 2011). (strengthandconditioningresearch.com)
A fully underhand grip should never be used when performing the deadlift as this increases the load on the bicep muscle as well as increasing risk of rolling. (mystrengthtraining.com)