Stand up & notice your knees. From the side, your hip, knee, and ankle should be in one long line. If your knees are hyper-extended or bent backwards of that side line, the ligaments and tendons of the knee are getting overstretched and more unstable. This hyper mobility increases your risk for knee pain, arthritis, & injury.
This bent backwards position is especially damaging during exercise, where the joint, rather than the muscle, takes on extra work loads or force absorption.
Even worse, this hyper-extensive knee posture usually forces the pelvis to shift forward, the torso to shift back, the head to jut forward, and the foot arches to drop. All of these compensations can create chronic low back, neck, and foot pain.
To correct this habit, gently flex or bend the knee by engaging the quadriceps, the muscles above the knee cap. The correct position of the knees is that the knee is straight, but not bending forward.