Is it better to do this excercise with your back up against a pad on the or just sitting on a bench? The latter I find much more difficult because you have to balance it, but will it have any effect on isolation? Thanks
I’m not an expert on muscle recruitment, but it seems to me that with the back up against a pad, the front delts are worked relatively more. Personally, often I like doing as many reps as possible while sitting vertically on the seat, back not touching the pad, and only then lean back and get a couple more reps that way. If I do not do this, then for freeweights I do the presses standing, or sitting vertically (no pad), but will do Smith presses leaning well back against a pad and with a wide grip. (Everyone has a different method I’m sure!)
This depends on your goal. If you want the majority of the stress placed on the targeted muscles(delts, triceps, upper traps, external shoulder rotators) then you want to have support against the back. If you are into integrated training where you divide the workload along the kinetic chain involved in the exercise (all those muscles mentioned above plus the erector spinae, mutifidus, transverse abdominus, obliques, illiac psoas, gluteals) then no back support is required. Standing is even better.
To expand on Truet’s post, I think this may be a question of inner unit activation capability. No back support while seated will cause hyperlordosis, which is fine if you have the neural capacity to fire your transversus abdominus, multifidus, diaphragm and pelvic floor musclulature simultaneously to protect the spine. However if you are less proficient at this then either use a back support or perform the movement standing wich will unload the compressive forces on the spinal column significantly. The effect at the deltoids probably won’t be significantly different in any of the options.