As the name of the golf swing error implies, locking up the back knee involves gradually straightening the right leg during the backswing. At the top of the swing, the error is clearly visible through a right leg that is completely straight, with no knee flex to be found. This is in contrast to a better swing that sees the flex in the right knee maintained more or less throughout the swing. Indeed, at address both knees are set in an athletic position using a reasonable amount of flex. That flex is to be kept throughout most of the swing until impact is made with the ball.
Problems Associated with Locking up your Right Knee
Changes the Angle in your Hips – Leads to Inside-Out Club Path
The main issue with losing the flex in your right knee during the backswing and at the top of the swing is that it changes the angle and direction in your hips. Indeed, whereas the hips are supposed to coil and uncoil along a central axis without moving much sideways, the act of straightening the right leg will pull your hips towards your right leg. In doing so, it will also open up your hips to the right side. From that point on, it will be difficult to bring the club towards impact in anything but an inside-out swing and club path. And while that club path is not completely undesirable in itself, it does render hitting straight shots very difficult.
How to Stop Locking up your Right Knee
Lean in Your Right Knee
There is a trick that helps set the right knee in its place and that is to lean in your right knee at address. As a useful swing trigger, the act of leaning your right knee towards the inside (towards the left knee) brings the knee flex – and the location of that knee – into your subconscious. As a swing trigger, it cements that position since it is the last movement made while setting up for a shot before the takeaway can proceed. It helps in preventing the knee from moving laterally and it also helps in setting and keeping the angle of the knee, which is what is being sought here.