In the book by Martin Koban "Beating Patellar Tendonitis" the correct form for eccentric squats on a slant board for treating patellar tendonitis involves having the shins as vertical as possible with little to no forward movement. However, all of the videos of this exercise I can find on the internet show quite a bit of forward knee motion, for example: https://www.youtube.com/watch?v=a-4tQxO8_QU
In the correct form for eccentric squats on a slant board for patellar tendonitis, should the shin bone remain vertical or does it not really matter?