The posterior or rear delts are notoriously hard to target, leaving many lifters with underdeveloped physiques. If that's you, try this:
Landmine Bent Reverse Flye
Brace your non-working arm on your thigh and keep your torso motionless. Rather than standing right beside the sleeve of the bar, orient your body slightly toward it. This will help you better connect with the posterior deltoid and de-emphasize the rhomboids and middle trapezius.
If you find yourself lifting your trunk, allow more bend in your elbow.
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Regardless of the profession in question, your "rep" (reputation) is usually what gets you where you want to go. In the world of physique and performance enhancement, building your rep is equally important, except that we're talking about an altogether different type of "rep" here.
Go light, be meticulous with your form, and focus on contracting all the small muscles working in your upper back and shoulders.