Strictly speaking, there are no such things are "upper pecs" and "lower pecs." That's not to say that the chest muscles don't have their own peculiar kind of North vs. South geography.
While most chest exercises will activate most of the chest muscles, changing angles, inclines and other approaches will emphasize different parts of the chest. So instead of hurling yourself promiscuously into the saddle of every chest machine in the gym, let's get acquainted with how the muscles you're trying to recruit assist you in your quest for physical glory.
Map to the Chest Muscles
The biggest chest muscle and the one that makes up most of the mass of your chest is the pectoralis major. It originates at the collar bone, ribs and sternum and spreads in a fan-like shape toward the rib cage, connecting to the upper arm near the shoulder joint.
The pec major is involved in most shoulder movements and is a major source of your lifting and pushing power. It's a muscle that's pretty easy to activate, too. According to a study of the most popular chest exercises by the American Council on Exercise, the Barbell Bench Press elicits the greatest activation from this greatest of chest muscles. It's followed by the pec dec machine and bent forward cable crossovers.
Then there's the much smaller pectoralis minor, a triangular muscle that lies underneath its big brother toward the outer side of the upper chest. Pec minor connects the top of the shoulder blade to the third, fourth and fifth ribs and functions to pull the shoulder down and forward. When well-toned, the pec minor helps define the contours of the lower region of the pec major. It may be small, but when weak or too tight, it is the mouse that roars, causing nerve impingement, shoulder problems and even pain when breathing. It plays a major role in posture. The minors like the assisted chest dip and the cable standing fly.
What's Your Angle?
"A lot of what you'll see as you improve your upper body will be determined by the angles at which you work your muscles," says Los Angeles-based trainer David Knox, author of Body School: A New Guide for Improved Movement in Everyday Life. Bench pressing on a flat surface favors the lower region of the chest while doing the exercise at an incline emphasizes the upper region, Knox says.
Not to be overlooked in your quest for flat pecs are push-ups. They don't incite the kind of muscle activation that weight lifting does, but because they involve the whole body, they bestow the kind of strength that's useful in every day life. The lower portion of the chest gets most stimulated in a standard or incline push-up, while a decline push-up targets the upper region of the pectoralis major muscle.
Most pec training comes with a bonus in that your shoulders and triceps are usually called into action. But it's also important to pay attention to a couple of other synergistic muscles. One is the anterior serratus, the muscle just under your pecs that spans from your upper ribs to your shoulder blades. The other is the latissimus dorsi, which is directly connected to your spine. Do the barbell incline shoulder raise for the former and lat pulls for the latter. Because however well-balanced your chest muscle are, they won't be of much help aesthetically or functionally if your upper back muscles are letting them cave in.