Not only do you get activation of the shoulder retractors AND the posterior rotator cuff, but you're also working on proprioception (knowing where the arm is in space).
This is great for prehab AND rehab.
If you've been suffering from shoulder pain, give this a shot (as long as it doesn't increase your pain) and let us know below if you were able to get both arms in between the kettlebells with both arms, without hitting the bells, or if your painful side had more trouble.
1) Line up 3 kettlebells with some space in between.
2) Lie face down next to them without looking at them.
3) Lift your arm using retraction of the shoulder blade over one kettlebell and return it to the ground in between the kettle bells.
Work your way from one end to the other and back to equal one rep. Perform 3-5 sets of 3-5 reps. You can hold a light dumbbell as needed.