Stand with your back against a wall and step out about a foot and a half and squat down until thighs are parallel to the floor. Interlock your fingers and press your palms up trying to straighten your arms and press them back into the wall. Pull the stomach to the spine and spine to the wall trying to get tall and straight up the wall. Breathe deep smooth breathes and hold for time.
Perform the wall sit in this manner and you will work your legs, calves, tighten your midsection, improve your posture and strengthen your upper back! I learned this style of wall sit from Matt Furey and definitely recommend his materials! Long holds don't just train endurance, they will make you stronger and build muscle size as well. Enjoy the burn!