Improper mechanics could be an issue with the knee going past the toes. If the foot is turned out, this is the mechanism of a meniscus tear (medial)-knee flexion with exernal rotation. If the knees are without pathology, complete flexion should be no problem.
Having cracked meniscii in both knees (medial in right and medial and corner of lateral in left) using this motion I can attest to the effect-one was doing handsprings as a teen and the other was stepping off a ladder after a weekend of aikido seminar seiza.
ATM has articles on the knee written by both Janet and myself.