This one is called Prone T’s, and it will help to strengthen the muscles of your mid-back — typically one of the weakest muscle groups for many people. This exercise is excellent for your scapula stabilizer muscles and your rotator cuff muscles.
Lay flat, face down, on some kind of stable surface that has open sides and a narrow width if possible. Find an arrangement for your head that allows your neck to remain in a comfortable position, without straining it through the exercise. A small pillow under your face works great, so long as you can breathe!
For this exercise, use the muscles at the middle and base of your shoulder blades primarily, and avoid using your upper trap (upper shoulder) muscles, neck muscles, and low back muscles — these should stay as relaxed and unused as possible.
Put your hands directly out from your shoulders to your sides, forming a “T” (see image above). Make fists with your hands, then stick out your thumbs and point them toward the ceiling. Breathe out, slowly pinch your shoulder blades together, and drive your thumbs upward slowly toward the ceiling until you can’t pinch anymore.
Hold this pinching for 3 seconds, then slowly relax back down to the starting position. Repeat 10-15 times, as you are able.
You can slowly add weight to your hands if this is very easy for you, but if you start to feel you’re losing good form by using anything more than just a little of your neck muscles, upper trapezius muscles, or low back muscles — any of the red arrows in the picture — you’re done with this one for the day!