Which of the chairs in the picture looks stable?
You can concentrate only when your feet touch the ground.
Only when the soles of your feet touch the floor stably creates a sense of psychological stability and improves your concentration. Even with a footrest, you can't concentrate on a chair that floats on the floor. Think about when you sit in a bar stool. If you sit for a long time, your thighs will not only weigh down, but you will not be able to concentrate on your thighs. The height of the seat where the soles of your feet touch the ground is the secret of your concentration.
Adjust your desk and chair to fit your height.
Step 1 Keep your feet on the ground.
Step 2 Your knees and thighs should be level.
Step 3 with the elbow slightly touching the top of the desk.