Wall Sit and Stay (for core, back, shoulders, and leg muscles). 

Wall sits: Perform 1-3 repetitions of 30 seconds per sit, every day.

1. Stand with your back pressing against a wall, arms along the sides of your torso with hands flat against the wall. Your feet are about 2-ft away from the wall, legs hip-width apart or a little wider. Toes may be pointed slightly outward.
2. Slide your back and arms down the wall until your hips and knees are at a 90-degree angle. Your shoulders, upper back, lower back and back of your head should all be flat against the wall. Distribute your weight evenly between both feet.
3. Engage your abdominals, relax your shoulders and breathe evenly while counting to thirty. Recover for 15-seconds by standing up, shaking legs out, walking around.
   Then, repeat the remaining repetitions.

For progression: Hold arms out in front of your chest. Keep shoulders and lower back pressed against the wall.