The quadratus lumborum muscles are large muscles on each side of the low back that bend the upper body toward the side they are located on. When they work together, they bend the low back backward. The side plank exercise engages the core muscles including the quadratus lumborum by raising the upper body while keeping the legs straight. Holding this position for one minute on each side can strengthen these muscles. Stretching the quadratus lumborum involves standing with straight legs and arms raised overhead, bending slowly from the hip to one side while keeping the arms straight. Child's pose in yoga also stretches the quadratus lumborum.