This is an incredible exercise for working the lateral deltoids. If you have back problems this is ideal because your spine is straight and relaxed.
Anchor your band to the door at your knee height. Grab a handle in each hand and lie on your back, away from the door. Bend your knees and place your feet flat on the floor. Keep your spine straight and your head aligned.
Exhale as you contract your deltoids and move your arms horizontally until your elbows are in line with your shoulders.
Inhale and slowly return to the starting position.