1. Come to all fours, hands under shoulders, knees under hips.
2. Extend one leg straight back behind you with the toes pointing to the floor. Activate that glute and make sure to not arch the back. If your back is arching, your can lower your leg a few inches until the lower back arch is gone.
3. Grab a dumbbell or band in the opposite hand to the extended leg. Slowly raise the dumbbell/band to shoulder height, squeezing that shoulder blade. Keep your arm straight as you move. Keep abs tight as you move. Keep your leg extended back with your glute engaged.
4. Slowly lower the dumbbell/band back down to the floor with control.
5. Repeat the dumbbell/band movement for indicated number of reps.
You should feel this working your shoulder blade muscle.