Full crescent lunge is one of the best stretches and strengtheners for your hips and legs. But, there are quite a few intricacies to be aware of.
TO GET THE PROPER STRETCH
Upon first glance, the photo above may look "graceful", but this alignment won't get you the stretch you need. Let me outline why. Literally.
See how the hips are pitched forward and the low back has a huge curve? Now, the normal shape of the lower spine IS indeed concave (to speak in terminology that is more familiar to the everyone), but certainly not as concave as this picture! It's quite easy for the hips and gluten to move back in space while the back leg moves into this motion.
Take one hand to your low back/sacrum area, and one hand to your low belly. Bend the back knee and tuck the tailbone/hips up underneath you, aiming for a rather vertical line (again, there will still be a little bit of that natural low back curve left and in general, a small "S" shape in the spine from tailbone to neck). You will immediately feel more stretch through the front of the thigh, hips, and low belly. This is great! These muscles (primarily the hip flexors) get very short from actions like sitting and excessive forward folding (for yogis). So, they usually need to be stretched to gain back some of their original length and bring more balance to the body system.
Precautions: If performing this version of a lunge hurts either your back knee or back toes, you can alternatively come out of the bend in the front thigh a bit until you can scoop the hips underneath you rather than bending the back knee. That should help. That is what has helped me in the past.
Once you've performed the double bent knee version, try to maintain the "vertically stacked" and level hips as you work the back leg towards straight again. This requires a cinching in (and up- if you can make sense of that) of the low beIly and a drawing in of the ribs towards each other so that the heart and ribs aren't splaying open and forward.That back leg might not make it to straight. That's okay! Someday it will. Keep practicing. Another great way to tell if your hips are tilted is to look at the top edge of your pants (if they aren't high-waisted). If the front dips down lower than the back, your hips are tilted forward and you need to repeat the exercise.
HAVE TROUBLE WITH BALANCE IN YOUR LUNGE?
Crescent lunge can be tricky for balance. The first thing you should try (besides breathing slowly and finding a focal point that is eye level) is to step the feet wider apart as if they are on train tracks instead of a tight rope. This gives you a broader base of support. Look at how much space is between my inner thighs in the first photo versus the second.
If you are losing your balance, it's tempting to set the back heel down. If you need to do that to regain balance, do it! But then try to swivel the back heel up not only off of the ground but towards the sky as pictured below!
That's better! The heel does not have to be directly over the toes as pictured, but it should be pointing up and the toes and kneecap and hip should be facing fully forward in order to get the stretch in the area that needs it the most!!! You can place your hands on either side of your hips to give yourself an adjustment. Use your hands to steer the hips back on the same line so that one hip isn't back farther than the other.
PROPER ALIGNMENT TO SAVE YOUR KNEES!!
In general, your knee should not go past your toes. Some people say it shouldn't even go past your ankle, so as not to put stress on the knee. To be safe, you can try to line up the front knee with the the ankle so they make a straight line as pictured.
ALIGN THE TRUNK
Observe the area of the belly and ribcage here. I placed my hands on the sides to show how shortened the belly is as a result of letting the ribcage sink down towards the hips with gravity.
I love giving myself adjustments. Once you learn them, it passes the time so you don't even think about how long you are lunging and how hard the thighs are working. Wrap your hands around your ribcage, below your chest, and grab on firmly. Then, pull the ribs up away from the hips. Ahhh! So much space. It should feel ahhhhhmazing for your low back!
The shoulders commonly come up by the ears after the last adjustment (lifting the ribcage). Take a deep inhale and, on the exhale, sigh and drop the shoulders away from your ears.
I have a neck after all!!!