While an anatomy lesson isn't included with your gym membership, maybe it should be. Failure to understand and work opposing muscles causes imbalances that then lead to poor results, bad posture or, at worst, joint injury. The agonist muscle isn't necessarily the one that causes the most pain in an exercise such as the overhead press; it's the muscle that is primarily responsible for the movement. The antagonist muscle is the one that opposes the prime mover and is responsible for the opposite movement.
Your anterior, or front, deltoids are the prime movers, or agonists, for shoulder flexion. Whether you use a barbell or two dumbbells, that's what you're doing with the overhead press. Another way to think of it is moving your shoulder blades, or scapulae, up and down, which you would notice if you could get a back view of yourself working out in a tank top -- that is, if you're doing them right.
Your latissimus dorsi, commonly called the lats, are the antagonists to your anterior deltoids. The technical term for what they do is "shoulder adduction" -- an impressive way of saying that they pull your shoulder blades down and in -- as in lat pulldown. So, not surprisingly, a standard exercise for the lats is the cable lat pulldown. Another exercise is wide-grip pullups with a pronated grip: palms facing away from you.
If your main concern is how you look in a sleeveless dress or tank, chances are you're spending more time on your shoulders than you are on your lats. Maybe you don't even know your lats exist or see lat exercises, especially pullups, as more of a guy thing. What if you end up dangling from a bar unable to pull yourself up? The problem is that overdeveloped delts and underdeveloped lats can lead to shoulder pain and injury. So if you're regularly doing overhead presses or front lifts, you have to work your lats to the point that they're equally as strong as your delts.
While they may not be as familiar to the average gym rat as overhead presses and pullups or pulldowns, several other exercises also work the lats and are easier to do correctly. One is the lat pullover done lying on your back on a bench with your feet propped on the end. With your arms extended, you'll hold one dumbbell with a sweetheart grip above your chest, then pass it over your face -- keeping your arms extended -- and behind you until your arms are next to your ears. As you can probably guess, this may not be the best exercise if you already have shoulder or back problems. Another option is the bent-over row. For this you'll have one hand and the opposite knee on the bench. Holding a dumbbell with your arm extended, bend your elbow to bring the weight in toward your ribs, keeping your arm tucked into your body.
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