A hallmark to the modern tour pro's golf swing is its knee action. Players such as Jordan Spieth and Rickie Fowler use it to generate a lot of swing speed. Fowler, for example, keeps his right knee bent all the way to the top of his backswing in order to leverage the ground and create more "coil." He then keeps his left knee bent until the very last milliseconds of the downswing when he straightens that leg to generate maximum clubhead speed through the ball (see below). Does the technique work? Well, he did hit a drive 410 yards against Jason Dufner in the WGC-Dell Match Play on Wednesday.
Jordan Spieth uses his lower body a little differently, but also relies on the bending and straightening of those joints as a power generator (see below). His driving average is 294 yards right now and he can crank it when he wants to, says his trainer Damon Goddard.
And to that end, Spieth and many other fitness-conscious tour pros do exercises to strengthen the lower body to protect the knees and allow them to provide power and stability to their golf swings.
The knee joint itself needs to be strong, but so do the joints that bookend it—the hip and ankle. So says trainers Gray Cook and Mike Boyle in their groundbreaking work "The Joint-By-Joint Approach". If you want to improve your own knee action, start with these two simple exercises demonstrated by Goddard.