Pay attention to alignment.
Keep your ankle in line with your sitbone and your femur. Keep your second toe and second metatarsal in line with your tibia. Flexing your ankle while keeping your second toe in line will help set the angle, as well.
Maybe once you grab your foot or ankle, before you begin the stretch in earnest, contract your quads once to set the angle. If you're in good alignment, nothing will move; if it does, line up there and re-check.
(And hopefully Knott will chime in...)
"Chondromalacia," aka patellofemoral syndrome, is a kneecap tracking issue that has everything to do with alignment. A good yoga teacher will give you all this...
The other half of it is that in most Americans, if quads are tight, they're not the half of it. Hip flexors are harder to get into, but obviously they don't involve the knee, and if you can get a good hip flexor stretch, they're all involved with the tops of the quads, too.
ETA: The "Sir Galahad" that FunSize described is a hip flexor stretch. Since the rectus femoris (the big central quad muscle) is a hip flexor as well as a knee extensor, you will get some there.
What goes hand in hand with that is remembering not to rotate your pelvis anteriorly when you stretch your quads (which can be a challenge for those of us with flattened lumbar curves and tight hip flexors). If you're not focusing on lengthening the hip flexors, you won't get a complete stretch of the rectus. A lot of people "cheat" by rotating their pelvis, to get their heel closer to their butt in a standing or side-lying quad stretch - which obviously defeats the purpose. If you can't reach your foot or heel in good posture, use a strap.
Last edited by OakLeaf; 02-22-2011 at 04:36 AM.
Speed comes from what you put behind you. - Judi Ketteler