A common instruction in Virabhadrasana 1 is to ground the outer edge of the back foot. However, if this instruction is coupled with the cue to wrap the outer back thigh forward it can create rotational torque at the knee joint and stress the ACL (anterior cruciate ligament). The ACL attaches from the underside of the femur (posteriorly) to the topside of the tibia (anteriorly), and has two main functions: 1) to prevent hyperextension of the knee, and 2) to prevent excessive rotation between the femur and tibia when the knee is flexed. It's important for a flexed knee to be able to rotate a little bit, as this is what allows a person to pivot and change direction when walking. However, when the knee is straight the ACL becomes taught to prevent hyperextension. If a rotational force is applied to a straight knee, the already taught ACL will be overly stressed, potentially leading to pain within the knee and over-stretching of the ligament that causes subsequent instability in the joint.
In the above video we offer alternative cues for the back foot, leg and thigh that help prevent rotational torque and keep the knee safe.