There are three major joints to consider when teaching a Squat: the hip, the knee, and the ankle. If any one of these three joints is limited in its range of motion (ROM), then any of the squatting poses will be awkward and uncomfortable. You can do some simple ROM tests with your students who are struggling with these poses.
The first and easiest joint to test is the hip. Pavanamuktasana, or Leg Cradle, is a simple exercise that can help you assess hip ROM.
The student should lie on her back, bend her right knee, and use her hands to try to hug her right thigh to her ribs. She should test this on each side, and then hug both knees to the ribs at the same time. If she can do this, then her hips have sufficient ROM to do a Squat. In fact, if our student could hug her knees this way and we were able to roll her up off her back and onto her feet, she would actually be in the Squat.
The next joint to consider is the knee. The pose that tests its ROM is a simple lunge, called Crescent Pose, or Anjaneyasana. In Taoist Yoga, it is called Dragon Pose.
The student first kneels with his right foot in front and his left knee on the floor. Placing his hands on the floor for balance, he should slowly bend his right knee to lower himself closer to the floor. At the same time, he should lean forward and press his ribs to his right thigh to help push deeper into the lunge. His arms should be on each side of his right leg for balance. He should keep leaning, bending his knee and leaning forward until the back of his right thigh (his hamstrings) press against his right calf. If he can do this, then his knee has the ROM for a Squat. In fact, he is already doing a Squat with his front leg. If we could bring his left leg forward into the same position, he would be squatting. Help your student test both sides.
Please note that it is okay for the heel of the front foot to come off the ground in this test. We are testing the ROM of the knee, not the ankle.
The final joint to consider, and the one most likely cause difficulties, is the ankle. Specifically, we need to test how far the ankles can dorsiflex, or bend upward.
The test for this is also the Crescent Pose, the same pose we used to test the knee. The student kneels with the right foot forward and left knee on the floor. But this time when he bends his knee leans forward; we want him to stop at just the point where the front heel starts to raise off of the floor. This is the limit of his ankle flexibility.
If a student's heel starts to come off of the floor before the back of the upper leg (hamstrings) is pressing the calf muscle, then squatting will be difficult or impossible to do with the heels down.
The Principle of Counterbalance
Why must the ankle be flexible to do a flat-footed Squat? Couldn't a student compensate with loose hip or knee? No. Most yoga students have adequate flexibility in the hips and knees to do a Squat, but many can't do it because of the ankles. The reason for this is the principle of counterbalance.
This principle is most easily demonstrated by standing with legs straight and bending forward with a flat back. Doing this is the middle of the room is no problem for most people. One needn't be very flexible, just flexible enough to hinge forward a little below horizontal. But here is the trick: Try to do this while standing with your back against a wall, legs straight, and heels touching the wall. You will find this impossible to do without falling forward. Why?
When we stand straight with feet together, we unconsciously balance equal amounts of weight on the front and back of our feet. When we lean forward in the middle of the room, we bring much more weight in front, but we unconsciously move our hips backward just enough to counterbalance the weight of the torso. This is why we don't fall forward even when we hold our spines parallel to the floor. Our hips shift backward just enough to counterbalance the torso. When we are standing against the wall, it is impossible for us to shift our hips back. So when we lean our torsos forward, we fall.
What has this to do with the Squat? When we bend our knees and sink into a Squat, our hips shift backward. This must be counterbalanced by bending the knees forward. But we will only be able to bend our knees forward until the ankle has run out of ROM. After that, the only way to bend the knees forward is to allow the heels to lift.
You can demonstrate how the knees need to bend forward to counterbalance the hips by trying to perform a Squat while facing a wall. Stand facing a wall with your knees about six inches away from the wall. Now try to squat. Your knees will bend until they contact the wall. Then, if you try to squat further, you will fall backward because you have no forward counterbalance.
Knees or Torso
When teaching Uktasana, look around the room and observe who is able to hold the spine nearly vertical and who is leaning awkwardly. You should observe that those who are nearly vertical have more ankle flexion, and those who are leaning forward have less. The less flexible their ankles, the more a student has to lean the torso forward to counterbalance her hips.
Shape of the Bones Limit the Ankle
The most important factor in determining how far an ankle flexes is the shape of the bones. When doing the Crescent Pose as a test for ankle ROM, ask your student to focus on the feeling at the top of the foot, the joint where the shinbone meets the foot. When these bones make contact, they are in a state of compression, and this is the end of the ankle's ROM.
What to Do?
The best help to offer a student with limited ankle mobility is to have them stand with their heels on a slight elevation, such as a thin block or rolled towel, when it is time for squatting poses. This adds ROM because they are starting from a slightly pointed foot position.
You might also teach variations of the Squat done while balanced on the balls of the feet. This takes more strength and balance, but many students find it a pleasant alternative to doing Squats with the heels down.
Another option is to allow students to turn their feet out and open their knees wide. This makes it easier to lean the torso forward between the legs. This helps counterbalance the hips.
The final option is to just leave your students alone and encourage them to do the best they can, even if they are always going to lean forward when squatting.
Paul Grilley has been studying and teaching yoga since 1979.