The Quadratus Lumborum (QL) is a deep lower back muscle that sits over the kidneys. It can be hard to stretch since it is underneath larger muscles. Here are some stretches for your QL.
For the first stretch, you are going to sit on the ground with your legs crossed or out in front of you at an angle. Take your arm of the opposite side you want to stretch, and place your hand underneath and push out and away. At the same time, take the arm of the side you want to stretch, and take it up and over your head to the other side. Hold this stretch for 30 seconds, and do it three times.
For the next stretch, you are going to lie down on your back. Prop your knees up, and then rotate your legs to the opposite you want to stretch. Then take the leg closest to the ground, and hook it over the other leg on the side you want to stretch to help pull your leg over and down. Try to keep your hips on the floor and not rotate your back when you stretch. Hold it for 30 seconds and repeat it three times.
For the last stretch, you want to roll over onto your side. This time the side you want to stretch will be on the ground. Put your legs on top of each other in a perpendicular position to the floor. Push up with your arms so your upper body is off the ground. Then roll your hips forward until you feel a stretch. Hold the stretch for 30 seconds and then repeat three times.