Originally Posted by
Jimbo
The reason your support knee bends is because that is natural body mechanics. If you were talking about a straight-leg front stretch kick, then it is best to keep the base leg straight as well. But in a front kick that fires from a chambered (bent knee) position, the base knee must have some bend to it.
YKW is right, traditional kung fu IS for fighting. Performance wushu keeps the base leg straight on front kicks. Try to kick a heavy bag, either the bottom of it with your instep like a groin kick, or into the body of the bag w/ball of foot. Either way, you need a slight bend in the support knee. Otherwise your kick will be floaty/lack penetration, and you will fall back.