Any time you perform squats, pick up something from the floor, walk up the stairs, or even jump and land your body is making a compromise depending on how you initiate this movement. The result may leave you with you anterior knee pain (think Patellofemoral Pain Syndrome), glute inhibition, and poor force production. And that leaves you landing from a jump like a 6 year old, which is super lame.
So here's the problem: The tissue or joint loaded first in a movement is the tissue or joint loaded the most. Too often we see athletes and patients (even "strong" ones) initiating squats with the knees coming forward - loading their quads instead of their high posterior chain (glute/hamstring complex). This quad dominance sets off a cascade in which the quads continue their powerful force and literally crush the patella against the knee as the athletes descends into the squat, jump land, etc. There is good evidence out there to suggest that the greater the knee flexion angle, the greater the compression forces on the patellofemoral joint - in a nearly linear fashion - and we're not really into crushing our own joints. Once the tissue/joint is loaded, it is virtually impossible to unload it during the middle of the movement - especially with an external load (think a heavy back squat).
You'll see how the athlete load orders their squat in the first few inches of the descent. Knees coming forward first in the descent is an immediate fault. By loading the knees first, the athlete is essentially rendering their glutes and hamstrings incompetent and preventing their substantial ability to not only protect the knee from gnarly compressive and valgus forces, but also greatly limiting power production and stability as you are unable to generate sufficient hip extension force (think jumping). The glutes control the hip and knee in all 3 planes of motion, rather than the quads which control the knee in the sagittal plane only, limiting their ability to stabilize and protect the knee. Quad dominance is seen often in athletes (particularly females - no wonder they are 4-6 times more likely to tear their ACL) and is a faulty movement
Try this yourself: Perform an air squat in front of the mirror. Your initial movement should come from the hip/trunk complex, not the knees coming forward. If you're knees are the first thing to come forward... epic fail. Think about those landing from a jump or descending into a squat while coming forward onto their toes - it's an unstable, weak position. Going into the squat in a poor position makes the finish even uglier: dropping vertical jump height, weakening triple extension, and killing power. Why? Because once a movement is loaded, poor muscle activation and poor position cannot be overcome! Trying to come out of the loaded knee forward position by shooting the hips back will only overextend the athlete and exponentially increase shearing forces.
1) Prioritize motor control and emphasize loading the glutes and hamstrings first. Tilt forward at the torso to tension the posterior chain and stabilize pelvic position. Following the load-ordering concept, at the bottom of the squat the posterior chain tissues are now under high amounts of tension having been loaded first. This allows for some highly modulated force production (i.e. a lot of oomph) and reducing forces on the patella.
A common error is to have the athlete stick their butt back first and keep the chest up during the squat descent. This faulty movement instruction can compromise spinal position resulting in overextension of the lumbar spine. A wide-open chest and overextended spine is commonly coached but woefully incorrect - causing the athlete to complain of low back pain (big surprise) during/after squats.
The first 6-8 inches of the squat is crucial to appropriately load the movement
2) Have the athlete shove their knees out laterally while keeping feet forward to pre-activate the glutes and create torque to take up tension in the system (glutes, hip capsule, hip adductors all control the pelvis) rather than finding tension with the knees tracking forward - this unloads the knee . Think about pulling yourself into the bottom position rather than dropping into it.
3) The shins should stay vertical as long as possible during the squat, tracking forward only at maximum depth (and that's butt to heel - limiting squats to 90 deg in perpetuity is NOT normal nor natural for humans). Tilting the torso forward at the hips while maintaining a stable, neutral spine is key and will assist in loading the posterior chain first in squatting.
4) Box squats are a great way to teach and improve control of the squat as you can vary the height of the box while teaching midline spinal stability. Box squats also help to cue driving back with the hamstrings as targets are useful in motor planning and control. Just don't reach with the butt causing overextension - instead coach tilting forward at the hips and shoving the knees out to stabilize the pelvis.
Elite and non-elite athletes alike need to be able to squat correctly and avoid this "quad dominance" seen so often in athletes - otherwise you're putting dangerous forces through your knee complex and gutting athletic potential. Appropriate load ordering is critically important in both human movement and mobility exercises and can/should be applied to all movements.