The pectoralis major is the most prominent chest muscle, but it has different regions that must be developed evenly to look balanced and full in your upper body. You may know how to target the upper chest area with incline presses and the lower chest area with decline flyes, but the outer portion of your chest isn't developing as you'd like.
Some simple adjustments to the chest exercises you already use helps develop the outer pec region.
The dumbbell fly has you lying on your back on a bench with a dumbbell in each hand. Extend your arms over your chest and turn the palms to face one another. The classic version has you then open your arms in an arc and squeeze them back together as if you were executing a hug.
The trick to making this move work your outer pecs is to open as widely as you can to maximize the stretch at the bottom of the exercise. Be careful not to go below the height of the bench, which can be injurious to the shoulders. When you return to start, stop before your palms hit — go only about two-thirds of the way up to keep the activation of the outer pecs.
Perform chest dips on parallel bars. Mount the apparatus with a hand on each bar and your feet linked to keep your whole body elevated. Lean slightly forward as you bend your elbows to feel a stretch in your chest. Straighten the joints to return to the start.
To make the dip most effective for the outer pecs, seek the lowest drop you can safely achieve. Instead of coming all the way up as in the classic variation, rise just three-quarters of the way up so you keep the outer region of the pec major activated for the entirety of the set.
Wide-Grip Bench Press
The bench press is the go-to exercise for your pecs. Your grip affects how you'll target the muscles, however. A wide grip — meaning beyond your shoulders — on both the flat bench and incline bench activates your outer pecs. After you've lowered the bar to your chest, stop short of straight elbows to keep the outer pecs engaged. Extend the elbows about three-quarters of the way up before bending them to lower the bar back to your chest.
Note that a wide grip can irritate the shoulders and wrists in some people.
The regular push-up activates your pecs, yes — but a subtle change will also better stimulate your serratus anterior, a muscle that wraps around your ribs, under your armpits, and resembles shark gills when cut and defined. Building the serratus anterior, along with the outside of the pecs, give you that full chest you're after.
The push-up plus is a standard push-up during which, when you rise up to straight elbows at the top, you press extra hard into the earth. This broadens the backs of your shoulders — an action for which the serratus anterior is partly responsible. A 2014 study published in the Journal of Strength and Conditioning Research confirmed that the push-up plus is effective in maximizing serratus anterior activation.