The appearance of knees that “bend backwards” is medically known as knee hyperextension or genu recurvatum. This occurs when the knee joint extends beyond the typical straight alignment of the leg, causing the lower leg to move excessively backward relative to the thigh bone. While acute injury can cause it, for many people, it is a chronic condition rooted in innate anatomy or functional habits.
Understanding Normal Knee Movement
The knee functions primarily as a hinge joint, facilitating two main movements: bending (flexion) and straightening (extension). In a fully straightened position, the joint is at zero degrees of extension, which is the functional end point for most people during standing and walking.
The joint structure, including the bony surfaces of the femur and tibia and surrounding soft tissues, prevents the knee from moving much past this straight line. However, a slight hyperextension of up to five degrees is often considered a normal, physiological variation, particularly in women and children. Strong collateral and cruciate ligaments, along with the posterior joint capsule, limit the backward motion of the tibia and maintain stability.
This natural limit is necessary because the knee is subjected to high forces daily. Movement past the physiological limit places increased strain on passive restraints, such as the anterior cruciate ligament (ACL) and the posterior capsule. The surrounding muscles also work actively to ensure the knee stops precisely at the neutral, straight position.
Anatomical Reasons for Hyperextension
For many individuals, the tendency for the knee to bend backward is a result of inherited structural characteristics. One of the most common anatomical causes is generalized ligamentous laxity, meaning the connective tissues throughout the body have more elasticity than average. This inherent looseness allows the ligaments that stabilize the knee to stretch further, permitting easy hyperextension.
This laxity often has a genetic component and can be part of joint hypermobility, where flexibility is increased across multiple joints. Specific connective tissue disorders, such as Ehlers-Danlos syndrome, are characterized by this excessive flexibility and frequently involve chronic genu recurvatum. These structural differences are not necessarily problematic unless they lead to pain or instability.
Bony structure also plays a role, as some people possess a unique contour of the joint surfaces. Variations in the shape of the tibial plateau, such as an increased anterior tibial slope, can naturally predispose the lower leg to slide further backward under load. These differences in the alignment of the femur and tibia provide less bony block to extension, allowing for a greater degree of backward movement. This variation is often present from birth.
Acquired Causes and Associated Risks
While innate structure is a factor, hyperextension can also develop or worsen due to acquired causes, often related to muscle function or past trauma. Muscle imbalances are a frequent contributor, particularly weakness in the quadriceps or hamstrings, which are the main dynamic stabilizers of the knee. If these muscles do not engage properly, the body may rely on passive structures, like the ligaments, to “lock” the knee into hyperextension for stability while standing.
Neurological conditions affecting muscle control, such as post-polio syndrome, stroke, or cerebral palsy, can also lead to acquired hyperextension. When lower limb muscles are weak or have impaired signaling, the knee joint may buckle backward during the gait cycle. This reliance on passive restraints rather than active muscular control creates a movement pattern known as an extension gait.
A previous significant knee injury, particularly those involving the ligaments, can destabilize the joint and lead to genu recurvatum. Damage or stretching of the anterior cruciate ligament (ACL) or the ligaments on the back (posterior) and side of the knee can remove the joint’s natural restraints. Even after healing, the resulting instability may cause the knee to chronically hyperextend.
Walking or standing with the knee in hyperextension increases mechanical stress on the joint. This repeated stress can strain the posterior joint capsule and ligaments, potentially causing chronic pain in the back of the knee. Over time, uneven loading of the joint surfaces may accelerate the wear of articular cartilage, contributing to premature osteoarthritis. If hyperextension is sudden, painful, or causes the leg to feel unstable or “give out,” a medical professional should evaluate the knee to rule out acute ligament damage.