This is a great exercise for anyone who runs, skis, or participates in any sport that involves running, jumping, or sustaining a squatting position. It is a good way to train and develop strength in the hips in order to protect the knees and ankles. If you do not have good hip control, you run the risk of developing injuries further down the chain like anterior knee pain and achilles tendinosis (see yesterdays post http://ascentpt.wordpress.com/2013/10/22/achilles-pain/)
For the single leg squat, you should have good enough balance that you can stand on one leg with light support of one hand on a handrail if needed, but no more than this. If you are unable to stand on one leg without holding onto a surface tightly, you should practice balancing on one leg first.
For the single leg squat, balance on one leg with the other leg extended in front of you. Imagine that there is a box around the foot of your standing leg. Slowly bend your knee, keeping it inside this box; in other words, do not let the knee translate in front of the toes, or inside the foot. Bend as far as you can maintaining this neutral position of the knee. Raise back up to the original position of standing on one leg, and repeat.
The single leg squat activates the gluteus medius, which helps to prevent the knee from collapsing inwards and absorbing excessive stress. It also activates the gluteus maximus, which is a powerful muscle that helps control impact through the knee and joints of the body. And finally, it strengthens the quadriceps, which is the main muscle that crosses the knee and allows for controlled knee bending necessary in jumping, skiing, running, etc. This exercise uses a combination of concentric and eccentric contraction, both of which are necessary in sport performance.
This is a great exercise to start getting ready for ski season! Stay tuned for more exercises to help you get ready for ski season over the next few weeks.