A well performed shoulder press helps develop good shoulder stabilisation and encourages good shoulder movement.
To help this, it is best performed with the palms facing each other, as in the clip, instead of palms facing forwards as so often seen.
I would also recommend starting sitting on a weight bench with the back raised up and resting you back against it. The desire to arch the back as the weights are pressed upwards is tremendous, and by maintaining contact with the bench, we can feel how we want to arch the back to assist. Instead of arching the back, we learn how to flatten the abs to correctly support the back in this exercise.
The head needs to be on top of the body and not poked forwards. It is quite possible that a folded towel will be needed to rest the head on since it is not desirable to do this exercise with 15 double chins in the effort to get the head back to the bench.
Clearly the exercise is very simple – push the dumbbells up and down until form breaks down = the back is arching off the bench and the elbows are widening on the up push.
As you press upwards, the tops of the shoulders themselves do not lift. The exercise works the front and mid-deltoids and the upper fibres of the pecs.
In order to stop the shoulders from lifting, the serratus anterior and the lower part of the trapezius need to fire.
So try doing this exercise with one dumbbell. Place the fingers of the other hand on the ribs below the working armpit. As the dumbbell is pressed upwards, the ribs should feel as if they literally plump up. This is your serratus anterior working. If your shoulders are lifting as you press, as if doing a shrug, then the ribs only plump up very feebly.
You can see the trapezius muscle in red to the right of this text. The fibres of the muscle run in different directions – upwards towards the head, across towards the spine and downwards towards the lower ribs. It is these downwards running fibres that you want to fire to hold the shoulders in position. When all is working nicely, the elbows will describe a nice curve as the dumbbells move up and down. To really fire things up, imagine the elbows are magnetised. And the arms should be straight at the top, not bent.
If you arch the back as you push upwards, this encourages the lats to stabilise the shoulders instead of the lower traps. Ultimately this will lead to injury and immediately it lowers performance, even if you can push a heavier weight like that. To maximally recruit muscles, the joints need to be stable. If joints are unstable, the brain weakens the big motor muscles to protect the body.
Now some will find this exercise impossible to do in this position – with the palms facing each other and the elbows pointing forwards. This is because there is too much curve in the upper back. The problem needs to be addressed to avoid getting more and more humped as the years roll by.
Breathing: exhale as you lift and inhale as you lower.