Although the chest muscle appears to be one continuous muscle sheath, it is actually comprised of two muscle heads, the upper and lower pecs. The lower chest muscle is much larger and more visible than it's upper counterpart. Most chest exercises work both muscle heads to some degree, but you can adjust the angle of the weight bench to target the lower pecs more.
The clavicular head, or upper pec, originates at the middle portion of your clavicle bone, extends across your chest, and attaches to the top part of your humerus, or upper arm bone. The sternal head, or lower pec, attaches to the front of the sternum and the ribs, extends across your torso, and attaches to your upper arm bone. This head covers most of your chest, extending down from the second to the sixth rib.
Chest exercises on a decline bench -- your head is lower than your legs -- target the lower pecs more than chest exercises on an incline bench. If a bench is set at approximately a 15-degree decline, you can perform decline barbell presses, decline dumbbell presses and decline flyes to work your lower pecs. For safety reasons, have a partner available to hand you the dumbbells after you are in position on the bench.
If you don't have access to a decline bench, a flat bench is preferable to an incline for targeting the lower pecs. A study published in the "Journal of Strength and Conditioning Research" tested the activity of the sternal head, clavicular head and the anterior deltoid during the chest press exercise with the bench set at horizontal, a 28-degree incline, a 44-degree incline and a 56-degree incline. The lower pecs were more active during the flat bench press than they were during any of the incline bench presses.
To thoroughly target the chest muscles, include a variety of bench and fly exercises at various angles. You can also use machines and cables to work your lower pecs. Incorporate body weight exercises, such as pushups and chest dips, to round out your chest program. If your training goal is to improve strength or power, the American College of Sports Medicine recommends choosing a weight that fatigues the muscle in eight to 12 repetitions. For muscle growth, complete six to 12 repetitions and for muscular endurance, do 15 to 20 reps.