Glute Bridge...with Leg Extension.
There is a saying throughout the Titleist Performance camp..."Glutes are King" as in king of the swing. Your glutes are more than just a punchline for jokes. Fat is one thing, Muscle is a whole other beast.Your glutes are the most powerful muscle you use from the ground up. They are the power source for your hips. You must have stability for your hips to generate the power needed during transfer to your upper body.
Do this exercise for great glutes...
Lying flat on your back, get your feet together with knees at about 90 degrees. With your hands on your chest, lift your butt off the ground to make a straight line with your torso and your upper legs. Hold for 10 seconds. To make this harder, lift one leg off the ground and keep upper legs parallel to each other. Also dont let your hips "sag" to one side. Keep your weight in your heels for maximum glute activity.

