And why your squat sucks or isn’t as good as it can be. Ankles cause knee pain. Not all knee pain. But healthy loaded dorsiflexion is about 35 degrees off of neutral (effectively a 55 degree angle). Most ankles don’t do this, which makes the sliding joint of the knee excessively pressured. With an ankle which won’t readily move into full dorsiflexion, the force at the knee cannot be distributed across as much tissue or surface area. Thus, the same few structures keep getting hammered, and it hurts. This keeps athletes from achieving top speed, lifters from optimal squats (and lunges,cleans,snatches,etc.), and laypeople from painlessness.
There is a quick way to assess your ankles (video here: https://www.instagram.com/p/CGnF348nRlU/). Put your big toe 5 inches away from the wall. Press the heel strictly in contact with the floor; and work your knee toward the wall. Touch is pass. Miss is fail.
The mobility drill showcased has a very firm tether across the talus, which allows you (along with manual downward pressure at the heel) to more readily train proper range via reps or long holds. Contract anterior tibialis in order to actively draw yourself into optimal dorsiflexion.
A more accurate method of prescribed distance from the wall (because, please do remember anthropometry: adult tibia lengths range from 11 to 19 inches) involves measuring the distance from medial malleolus to medial condyle (hypotenuse), some simple geometry (90/55/35 degrees), and then a variant metric from medial malleolus to wall (bottom leg of triangle). To wit, 40 degrees or more is possible.
For clarity on the math or more tips on fixing knees, both for pain management or maximal athleticism, feel free to contact through the website.