The posterior delts might not be first on your list of muscles to work, but they certainly should get some role in your shoulder or back workout routine. These muscles contribute to broad-looking shoulders, so you appear strong and confident. They're also important to posture; weak back shoulders can cause you to slouch, hunch and cave in at the chest. You might even experience tightness and stiffness in your neck as a result.
Two exercises commonly used to train this area are the bent-over rear lateral raise and the reverse pec deck. Both are effective at isolating the rear deltoids, but appeal to different training focuses. If you find you just can't decide, consider alternating them week to week — doing the reverse pec deck for a couple of workouts, then switching to the bent-over rear lateral raise for the next few. This means you'll benefit from the subtleties of both exercises.
If you do choose the bent-over row, be sure to keep your head and neck neutral so as not to cause strain and compromise your form. Look forward at the floor just a few feet instead of checking out your muscles in the mirror.
In the bent-over rear lateral raise, you've got a lot going on. You're either seated or standing, hinged from your hips, grasping weight and opening and closing your arms to squeeze your shoulder blades together. Your balance and body angle must be spot on to get the most for your rear delts and upper back.
With the pec deck, you're seated in the pec deck resistance machine backward, with your face and torso turned to the back rest. With the levers at shoulder height, place your upper arms or elbows in the lever pads and push them back, to draw the shoulder blades together. You have to focus on little else than squeezing your shoulder blades against the machine's resistance to train the rear delts. Minimal energy is wasted trying to maintain proper alignment.
The stability of the pec deck could also be viewed as a drawback, depending on your training focus. For the people who are after more functional strength, rather than singular muscle development, the balance and maintenance of the hinged position of the bent-over rear lateral raise enhances the workout's value. Your triceps, wrist extensors, hamstrings, erector spinae, glutes and adductors — or inner thighs — play the important role of stabilizers. The reverse pec deck engages no notable stabilizers.
Go For Convenience
The bent-over rear lateral raise is more versatile than the reverse pec deck. Instead of being beholden to a single machine, use dumbbells, resistance bands or a cable machine to do a variation of this move to target the backs of the shoulders.
That being said, you may prefer a machine that provides you with the ease of the pec deck. You simply adjust the seat and the arm levers, move the pin to the desired weight and get to exercising. No heavy weights to lug, no pulleys to move and no fear that your form is terribly off.
The reverse pec deck offers one way to target the backs of your shoulders. Some machines will allow you to work one arm at a time, but it can be tricky to accomplish while seated in the machine. The bent-over rear delt fly provides you with options. Work one arm at a time while standing or sitting.