You have been dancing for six years. And the basic skills are also very good, so it is of course very easy to practice. Because your ligaments have been completely stretched. So you can demonstrate basic skills without warming up. Your sister is in pain because she hasn't practiced basic skills. Her ligaments haven't opened yet. She is still young, so her ligaments may be okay and not too hard, but it will still hurt if she exercises. Because I haven’t practiced basic skills, the ligaments will be pulled apart all of a sudden. The ligaments have not yet gone through a buffering stage, so of course it will hurt. But you'll get better as you get used to it. Another reason may be that you are pushing too hard. It exceeds the limit of ligament stretching, so it hurts. This is very dangerous. If the force is too strong, it may cause ligament strain, which will take several months to recover from. Human ligaments are like rubber bands. They need to be taken slowly. After they adapt to this buffer period, they can add more force.