Connecting the femur to the tibia, the knees receive considerable stress from above and below, making their stabilizing muscles and especially ligaments among the most frequently strained in physical yoga practices. Athletes, runners, even committed sitting meditators discover that the stress created in the knees from their athletic or spiritual avocation can lead to debilitating injury, especially when lacking the beneficial effects of a balanced, appropriate practice of physical asanas. Even in a balanced yoga practice, the knee still has to handle considerable forces, primarily from bearing weight but also due to twisting forces exerted from above and below. In more strenuous yoga practices, the knee has to handle very powerful physical forces. Primarily a hinge joint capable of extension and flexion, with minor capacity to rotate when flexed to about ninety degrees, sudden or excessive movement in any of these motions can tear one of the supporting ligaments or cartilage. Understanding and honoring the knees is one of the keys to guiding a sustainable yoga practice. Let’s take a closer look the knee, which is actually two joints:
• the femorotibial joint, which links the femur and tibia;
• the femoropatellar joint, where the patella is situated within the anterior thigh muscle and a groove it slides through on the front of the femur.
The distal femur and proximal tibia are expanded into condyles that increase their weight-bearing capacity and offer larger points of attachment for supporting ligaments. The convex shape of the femoral condyles articulates with the concave tibial condyles. The joint is cushioned by articular cartilage that covers the ends of the tibia and femur as well as the underside of the patella. The medial meniscus and lateral meniscus are C-shaped intra-articular pads made of fibrocartilage that further cushion the joint, functioning as shock absorbers between the bones and preventing the bones from rubbing each other. Tears in the medial meniscus are common in yoga, whether originally injured during an asana or exacerbated in an asana such as Padmasana or others where forced rotation at the hip joint can transfer stress into the knee when the foot is held in place by the floor or another part of the body. With little or no blood supply, they heal slowly—if at all. A set of ligaments, all of which are in the fully stretched position when the knee is extended (leg straight), help to stabilize the knee. When the knee is flexed, the ligaments are softened (shortened), allowing rotation in poses such as Padmasana.
The medial and lateral collateral ligaments (MCL and LCL) run along the sides of the knee and limit sideways motion. The MCL, extending vertically from the femur to the tibia, protects the medial side of the knee from being bent open by force applied to the outside of the knee, such as when a student presses down on the outside of the knee of the back leg in Parsva Dhanurasana. The LCL protects the lateral side from an inside bending force, such as when a student inappropriately places the heel of the right foot against the inside of the left knee in Vrksasana. Both of these ligaments are supported by muscles that run outside them.
Inside the knee joint are two cruciate ligaments. The anterior cruciate ligament, or ACL, connects the tibia to the femur at the center of the knee. Its function is to limit rotation and forward motion of the tibia away from the femur; without it the femur would slide forward off the knee. We will revisit this when looking at a variety of poses, especially lunges such as Warrior I or II, where the ACL is both a crucial source of stability and at considerable risk if the knee is not properly aligned. The posterior cruciate ligament, or PCL, located just behind the ACL, limits excessive hyperextension (backward motion) of the knee joint. Injury to the PCL is rare, especially in yoga, where there are no asanas that place great force on this ligament. The patella ligament is sometimes called the patellar tendon because there is no definite separation between the quadriceps tendon, which surrounds the patella, and the area connecting the patella to the tibia. This very strong ligament helps give the patella its mechanical leverage and functions as a cap for the condyles of the femurs.
The muscles acting on the knee from above—the abductors (primarily the glutei and tensor fascia latae, acting through their attachment to the iliotibial band), adductors (primarily the gracilis), the quadriceps (for extension), the hamstrings (for flexion), and the sartorius (a synergist in flexion and lateral rotation)—help the ligaments to stabilize the knee when contracting from their various origins on the front, back, and bottom of the pelvis. The gluteus and tensor fascia latae attach to the iliotibial band, which in turn attaches to the lateral tibial condyle below the knee, contributing to lateral stability. The medial side of the knee is given more balanced stability through the actions of the gracilis, sartorius, and the semitendinosus (one of the hamstrings) as they pull up and in from their attachments on the medial tibia just below the knee: the gracilis from the pubic ramus at the bottom of the pelvis, the sartorius (the longest muscle in the body) from its origin at the anterior superior iliac spine (ASIS), and the semitendinosus as it runs up the back of the leg to its origins in the ischial tuberosity (most commonly known as the sitting bones). These medial and lateral stabilizers also play a small part in the rotation of the tibia on the femur when the knee is flexed and the foot drawn toward the hip in poses such as Vrksasana or Padmasana.
While lending stability to the knee, the quadriceps and hamstrings are the most powerful muscles involved in knee extension and flexion. The most powerful muscle in the body, the quadriceps (so named in Latin for its “four-headed” origins) has just one foot as the four parts combine to form the quadriceps tendon, which extends across the front of the knee to become the patellar tendon and inserts on the proximal edge of the patella, which then transfers their action via the patellar tendon to the tibia. Three of the four—vastus medialis, vastus lateralis, and vastus intermedius—originate from the femoral shaft, while the rectus femoris arises from the top front of the pelvis, giving the rectus femoris a strong role in hip flexion as well as knee extension. This combined action is involved in Utthita Hasta Padangusthasana. Their collective power in knee extension is increased through the fulcrum-like structure of the patella. Their concentric or isometric contraction extends or holds the knee in extension to stretch the hamstrings in a variety of standing and seated poses and contributes to lifting the body through eccentric contraction in back-bends such as Setu Bandha Sarvangasana and Urdhva Dhanurasana.