“Place your foot either below your knee or above your knee, but to avoid injury, do not place it directly on your knee.”
Raise your hand if you’ve heard this alignment instruction given in yoga’s tree pose (vrksasana)! I know that I certainly have, and my guess is that 99.9% of you (rough estimate!) have as well.
As common as this yoga rule is, does it actually hold up to scrutiny when we apply the science-based tools of anatomy & biomechanics to it? Today I’ll use a question-and-answer format (based on real conversations I’ve had about this issue) to investigate this inquiry.
If you find this discussion about the “no foot on the knee in tree pose” rule to be helpful, please feel encouraged to share it with your networks so that we can raise awareness about the value of critical thinking and questioning what we’ve traditionally been taught in the yoga world. Thank you for reading!
QUESTION 1: The knee joint is a hinge joint that is only meant to move forward and back. So any pressure from the side such as placing the foot on the knee in tree pose is inherently damaging for the knee joint, right?
ANSWER: The knee joint is often described as only moving in the sagittal plane (flexion and extension), but it also has movement available to it in all three planes of motion (i.e. varus/valgus rotation in the frontal plane and medial/lateral rotation in the transverse plane).
Therefore, although the knee has its greatest and most obvious range of motion in flexion and extension, this joint can also move and receive load in a wide variety of other directions. It’s not the case that sagittal plane loads are the only loads that this area of the body can tolerate. In fact, for the knee to be a truly resilient joint, it needs to be able to withstand multi-directional loads.
QUESTION 2: I didn’t realize that the knee joint was capable of moving in so many other directions too. I guess it’s not accurate to describe it as only moving forward and back. However, isn’t it still a lot of load to place the foot on the inner knee in tree pose? Doesn’t this apply enough force to be inherently injurious?
ANSWER: We might not realize it, but our knee is regularly subjected to lateral forces that are higher than foot-on-the-knee in tree pose, and it’s just fine when exposed to these loads! Think about standing in the ocean and having waves hit you from the side, or standing on a subway facing sideways and absorbing the lurching forces as the subway starts and stops. We don’t caution about injury from those loads because we know that our knees have no problem handling them.
Or what about jumping jacks? Jumping jacks are a benign exercise practiced as a warm-up by thousands (millions?) of people every day all over the fitness world. With this movement, we load our knees laterally when we jump our legs out to the sides over and over (and over!) again. And because of the greater impact involved with jumping, the forces applied to our knees are higher than when we are simply standing still in tree pose with our foot resting on our knee. Yet no one cautions against the inherent dangers to our knees in jumping jacks.
Or within the context of yoga itself, what about side plank pose (vasisthasana)? In side plank, we’re facing sideways and we are balanced on the outer edge of our bottom foot. This is directing a significant amount of force through the knee joint laterally. Or what about the back leg in warrior 2 (virabhadrasana 2)? That leg is arranged at an oblique angle in this pose, which means that knee is being loaded laterally. Yet we don’t hear widespread cautions about the inherent danger to the knees in side plank or warrior 2. Why is that?
The forces involved when we place our foot on our knee in tree pose are low and are quite safe. Perhaps it’s tempting to imagine that this so-called “misalignment” in tree pose is injurious because the lateral loading here is more obvious to our eyes than it is in many of these other examples I’ve listed. But whether we realize it or not, our knee is exposed to lateral forces all the time as we move through our lives. These are natural loads that this joint can tolerate.
QUESTION 3: What about the risk of creating hypermobility in the ligaments of the knee? Ligaments aren’t meant to be stretched because they aren’t elastic. When they are pulled on, they can become lax and not return to their original length. By placing our foot on our knee in tree pose, aren’t we at risk for creating lax ligaments and de-stabilizing our knee?
ANSWER: Despite popular belief, we don’t actually have evidence to support the idea that our ligaments become lax as the result of being stretched. Ligaments are a tensile soft connective tissue, which means that they’re designed to respond well to tensile forces, which are “pulling” or “stretching” forces. When ligaments are pulled on, the cells that live there respond by laying down more collagen to build stronger tissue that is better able to handle tensile (pulling) loads [Ref], [Ref].
And although it’s often claimed that ligaments are not elastic and do not return to their original length after being stretched, this is simply inaccurate. Ligaments are viscoelastic tissues, which means that elasticity is one of their inherent properties [Ref]. A material that is elastic returns to its original length after being stretched.
Now ligaments can of course be injured, but this is different from what most people are picturing when they warn about ligament “laxity” and hypermobility resulting from yoga poses. In a ligament injury, tissue has been partially or fully torn (ruptured).
Actual knee ligament injuries are generally the result of dynamic movements involving high-magnitude, high-rate forces, such as in skiing accidents, fast changes of direction when running (as in soccer, basketball, and other field & court sports), and other athletic-type accidents [Ref], [Ref]. By contrast, tree pose is a static asana involving low loads.
We are therefore not inherently at risk of injuring ligaments or of creating hypermobility by placing our foot on our knee in tree pose.
QUESTION 4: What if someone has a knee injury? Would you still recommend that he or she place their foot on their injured knee in tree pose?
ANSWER: I wouldn’t recommend that someone with an acute knee injury place their foot on their knee. But I wouldn’t recommend that any yoga student load any joint that has an acute injury – the knee isn’t special in this regard. If we were worried about everyone’s possible injuries they could conceivably have at all joints when we teach yoga, then we wouldn’t teach any movement at all!
Yoga rules are meant to be general and are designed for asymptomatic, healthy populations. We then tailor a yoga practice to any issues that might be presenting for a specific student in her or his individual body.
Placing the foot on the knee in tree pose is a biomechanically benign alignment that asymptomatic yoga students do not need to go out of their way to avoid. Rather than specifically warning yoga students to keep their foot off their knee, a more preferable approach would be to simply instruct students to “place your foot at some point along your inner leg”, and to leave it at that. As with many cues in yoga, the less we say as yoga instructors, the more effective our teaching will become.