You are squatting in a flat posture. How can you get backache without exerting strength? With the chest pull, you just feel standardized, but it's not. Pull your chest and back, and feel the whole body when you exercise to a certain class. Without this feeling, you just pose, and your feet should not be left out. At first, you can put your feet slightly outward at 15, and then straighten them after you have a foundation. The most important thing is to stand flat. If no one teaches you, your knee or other parts of your body will be broken.