Often mistaken for ribs, the serratus (as in, serrated) anterior muscle is the ridge like muscles which run from the upper 8 or 9 ribs at the front-side of the ribcage – covered by the arms when by the side – and attach to the wing-like bone at the back of the shoulder blades (scapula).
The main function of the serratus anterior is to keep the scapula held against the thoracic wall, and also to pull the scapular around the ribcage when a punch is thrown, hence the informal term for the serratus anterior – the ‘boxers muscle’. It also plays a role in rotating the scapula upward for overhead press movements.
The movements I’ve detailed should give you clues as to the types of exercises which may be possible to hit the serratus anterior.
I recommend the following few exercises…
Incline shoulder raises
Take up a regular incline bench press position. De-rack the barbell at around shoulder width and bring over upper chest. Now, rather than having your shoulder blades (scapula) pinched back as you would a regular bench press, what you do is keep your arms straight – that’s right, don’t lower the bar with the elbows – and lift the weight solely by allowing your shoulder blades to travel forwards. Go as far forwards as you can, and then as far back as you can, for a good range of motion. That is putting direct work on the serratus anterior muscle. Remember, elbows straight throughout.
These exercises may also be done with dumbbells.
Overhead shoulder raises
Standing or seated, you perform these with dumbbells or a barbell much as you would in the aforementioned incline shoulder raises, except directly overhead in the fashion of a military or shoulder press.
The trick is to keep the arms locked and give a little bit of extra movement upwards from the sheer movement of the scapula via the serratus anterior. Pull the weight up, and pull it down. Get as good of a range of motion as possible just from the movement of the scapula.
As you may imagine, regular overhead presses and such tend to hit the serratus anterior muscle, too, but this is a better way to isolate serratus anterior without going through the entire range of motion and involving other muscles.