Gym, legs
Lie on your back with knees bent and lift your hips until your body makes a straight line.
Best on camera: a slow, steady move with your joints in clear view.
Phone at hip height, about 3 metres away, filming you from the side.
Your whole body, head to feet, needs to be in the picture. The coach will remind you if it can't see you.
Hips not high enough
Squeeze harder at the top until your body is straight from shoulders to knees.