Side planks are magic, they can highlight so many different stability issues with people and how well someone can link their core and hips together. They can be used to retrain hip shifts, lack of stabilisation with strength exercises and should make regular appearances in your warm ups. If someone has a major difference on one side it will usually present itself elsewhere even when standing up. When you boil it down to what it is, if you struggle to do side planks, you literally cannot hold you

Double Plank Challenge

Spinning Plank Challenge