From the inimitable Bill Hartman:
"Unilateral leg work, including the single leg squat, is an important element in the development of an athlete's training program.
The single stance and the single leg squat are also a great diagnostic tools for athlete's in regard to shoulder function.
It has been shown that in 49% of athletes with a arthroscopically diagnosed posterior-superior labral tears there is also a hip rotation range of motion deficit or abduction weakness. (W Kibler, Joel Press, Aaron Sciascia. The Role of Core Stability in Athletic Function. Sports Medicine, Volume 36, Number 3 (2006), pp. 189-19
This is one of the reasons we included combined hip and shoulder mobility exercises in the Inside-Out DVD.
Here's some quick tests you can do on yourself:
1. Stand in front of a full length mirror. Pick up your right foot. If the right hip drops, it indicates a left hip weakness. Repeat on the other side.
2. Pick up your right foot and perform a single leg squat with the left leg. If the left knee drops down and in as you descend, you have a left hip weakness.
3. Lie on your back with your hips bent to 90 degrees and knees bent to 90 degrees. Keeping your knees together, push your feet apart to internally rotate the hips. You should have about 35 degrees of rotation on each side. Check for the amount of rotation and even more importantly, check for symmetry.
Any unilateral deficits in range of motion and strength should be addressed as they will typically affect the function of the shoulder on the opposite side (right hip to left shoulder and vice versa).
Great stuff, and who woulda' thunk it?