I call this the Quad Slide.It’s pretty simple, they are sliding off the hamstrings and onto the quads.
Your body wants to take the path of least resistance like electricity, if your posterior chain is weak, or you aren’t able to activate it correctly, you will slide onto the quads and squat less.
You’ll also greatly increase shear force on the patella (knee) which can lead to injury over time.
Most knee pain occurs when you allow your weight to come onto your toes and then your knees come forward and causes them to strain. It is also very important to have your toes pointed outward at a 45 degree angle. This allows for the knee joint to open properly as you squat down. Keeping your toes pointed directly forward puts an abnormal pressure on the inside of the knee and eventually leads to an injury.
Other reasons your knees are hurting maybe because your Hips & knees move forward while squatting: This is a huge reason most people’s knees hurt squatting and an example of poor movement mechanics or tight hamstrings.
Thighs and lower back muscles doing most of the work: This happens when the core, glutes and hamstrings are weak. When you lack proper core and hip strength muscle imbalances often result in the quadriceps and lower back over working. This is one of the main reasons for knee pain during squatting.
Abs and hips are weak: These muscles need to be properly strengthened in order to eliminate knee and lower back pain.
It’s important to understand that each of the above are caused by muscle imbalances that cause pain and injuries to areas of the body like the knees, back, shoulders, neck, ankles and feet.