AKA Adductor plank
The Copenhagen plank is a simple exercise for adductor strength.
In a side plank position, place the top leg on a bench with the bottom leg under the bench. Lift the hips up until your body is in a straight line. Keep the lower leg off the floor.
Reduce the difficulty to scale by keeping the foot of the lower leg on floor to assist, and/or by moving the bench closer toward the hips.
The Copenhagen plank strengthens the adductors for strength balance of the hip and to help protect against adductor strains.
Do 3-4 sets of 10-30 second holds.