This exercise will help strengthen your inner thighs and give you better posture and stability in your setup and throughout your swing.
For this exercise you'll need a basketball and an exercise mat or carpeted floor where you can lie down. Deflate the basketball slightly to make it easier to squeeze with your knees.
Position 1 - Lie on your back with feet flat and knees raised
Lie down on the mat, making sure to keep your upper body flat on the ground. Your lower back has a natural curve to it, and the "arc" of the curve should not touch the ground when you lie flat. Your body weight will be supported by the upper pelvis.
Keep your feet flat on the floor and raise your knees. Place the basketball between your knees as shown in the photo above. Squeeze the ball with your knees and hold for a one second count. Do 15 one-second reps, then 15 short, quick reps.
Position 2 - Keeping knees bent, raise legs until the knees are over the hip bones
For the second phase of the exercise, keep your knees bent and pull your legs up until your knees are lined up over your hip sockets, as shown in the photo above. Your thighs will be in a vertical position, directly over your hips.
Once again, do 15 one-second squeezes followed by 15 short, quick reps. As before, make sure that the curve of your lower back is just barely off the ground as you perform the repetitions.
Position 3 - With feet on the ground and knees raised, contract glutes to raise hips
For the final position, place your feet flat on the floor with knees raised, as before. Now contract your glutes to lift your hips up off the ground, as shown in the final photo. Again, do 15 one-second reps and 15 quick, short bursts.