Since the front squat requires a more upright torso angle to support the bar across the front of the shoulders, many lifters tend to bring their weight too far forward.
As the body drifts forward, two issues arise. First, the torso loses its stability and the shoulders protract, internally rotate, and flex at the thoracic spine. This lack of stability drips force output, making you weaker. It also places your shoulders and spine in risky positions.
The second concern? As the body falls forward, the tibial angle of the lower leg is increased, causing the knees to drive too far forward, taking the brunt of a force through these joints. This can have a negative impact on patella-femoral health and can flare up the soft tissues surrounding the knees, causing chronic inflammation.
The Fix: Elevate Your Heels
Using small plates like fives or tens under the heel, lifters will be able to sit back into their hips during the squat, decrease the forward lean and instability, and decrease the amount of shearing on the patella.
If you think the problem here is a lack of ankle mobility, think again. Sure, limited dorsiflexion can alter squat mechanics, but the more intriguing phenomenon during the heels-elevated squat is the alteration of the kinetic chain and the distribution of weight over the center of mass that provides more stable "non-threatened" positions at the knees, hips, pelvis and spine.
Don't be surprised if using a slightly elevated heel position unlocks squat depth, decreases that ugly butt wink, and leads to hypertrophy of the quads and glutes. Oh yeah, and it'll also feel a lot better.