That being said, that is, angling of feet or changing grips for triceps may either compromise the knee or not result in full contraction or extension of every head of a muscle group, i have found that i am able to focus on a certain head more effectively by doing so. However, i dont limit myself to one position or another, i do use positions that do allow and promote full ROM for the particular group i am working.
When i work on a certain machine such as leg press with a toes pointed in, the technique changes so that the focus is on the negative as much as the contractual portions of the movement. In this manner, i feel that my knee joint isnt compromised and i can focus the contraction vastus medalis. For me, i get a great pump and lactic acd increase by doing so. Perhaps the positioning affords a means for a greater mind-muscle connection. And, by no means am I saying that the other heads of the quadriceps (even the hamstrings) are not being engaged, i simply feel a greater response in the VM.
Often, i think of the quadriceps, or any other muscle groups as a jazz band, the other instrumentalists do not stop playing when say the guitarist takes the lead, then when its the pianists turn to take off and jam, the others still keep going. The mind is like Miles Davis, the executive conductor, whether Pete Cosey, or John McLaughlin is the guitarist in a given line up.