The deltoid muscles are the muscles located in your shoulders. They are named "deltoid" because their shape resembles the Greek letter "delta" (∆), which is a triangle. Full definition
Saeterbakken et al. (2011) also found no difference in anterior deltoid muscle activity between the free - weight, smith machine and dumbbell bench press. (strengthandconditioningresearch.com)
In support, Kohler et al. (2010) showed no difference in middle deltoid muscle activity when comparing the seated dumbbell and barbell shoulder press. (strengthandconditioningresearch.com)