The frog legs movement is a good exercise to assist with hip and knee joint mobility by strengthening the supporting muscles of the inner thigh and groin while also increasing strength in the lower abdominals and therefore core strength.

The exercise:

Starting position:

Lay on your back with your legs bent upward so that your knees are roughly aligned with your pelvis.
Have your knees apart and your heels pressed firmly together.
Your stomach should be tight with your lower back pressed against the mat. you are squeezing your stomach into your spine.
Your shoulder blades flat on the mat and hands behind your neck.

The movement:

Take a deep breath.
As you slowly exhale, slowly lengthen your legs while maintaining the pressure on your heels.
When the legs are fully extended, slowly inhale as you move back to the starting position, pressure should be maintained during this phase also.

Notes:

Ensure that your stomach remains tight and the pelvis is aligned throughout this exercise, If you have trouble doing this the angle og your legs can be adjusted so that it is possible.