The video below demonstrates the Single Leg Squat Test, which is part 4 of a 5 step Functional Movement Screen I taught to a group of personal trainers. This set of movement tests is designed to identify predispositions to injury so they can be addressed before a problem occurs.
VIDEO TRANSCRIPTION (Please excuse grammatical errors, as this is simply a transcription of conversational speaking)
Going right into the next test, with the single leg squat. So the single leg stance that we just did was looking a little bit more at frontal or coronal plane stability, kind of side to side. When you add the squat in, you’re adding in the transverse plane and the rotational instability components. I like to do the one legged squat with the other leg out rather than back because it mimics going downstairs. It’s just a little more functional. You have them lower down and first of all: “does it hurt?” If it’s just mildly uncomfortable usually that’s going to probably work itself out as you get stronger, but if they have debilitating pain or can’t go very low, that’d be something to have them check out before you get deep into a leg program or something like that. The main thing with this is, “does the knee stay over the toes?” If they drop it in, it’s a really big problem because what it does, in most cases, its showing that the hip external rotators are not doing their job to stabilize the leg. I would guess that component or that injury predisposition is present in … I haven’t looked at studies on this… but I would guess more than half of your major knee injuries (except for maybe a football player that gets swiped in the legs). If you had that person do this before the injury, they’d be dropping in like that. In those cases, there’s tons a great exercises to strengthen the hip rotators.