I was stretching my legs in taekwondo and they started to shake when i stood up. b4 i started taekwondo i took a selfie and my whole body was shaking?

Multiple reasons: Individuals can get muscle cramping or muscle "shaking " because of electrolyte imbalances and low magnesium. Especially if coupled with excessive muscular exertion and or dehydration. Our muscle groups is a large of in our bodies reservoir of water and vulnerable when dehydrated. Muscles can also "shake" if you are over exerting large muscle groups that are weak and lack endurance.