Lastly, they showed that the standing rather than the seated dumbbell shoulder press produced superior middle deltoid muscle activity, and a non-significant trend to greater muscle activity from the standing barbell press compared with the seated variation. (strengthandconditioningresearch.com)
Ackland et al. (2008) reported the anterior and middle deltoid display similarly large peak shoulder abduction moment arm lengths (anterior = 30.2 mm; middle = 29.1 mm), while the posterior deltoid displayed a peak shoulder adduction moment arm length of -15.9 mm. (strengthandconditioningresearch.com)
In contrast, they reported superior middle deltoid muscle activity when performing the free - weight bench press. (strengthandconditioningresearch.com)