The sound of a clicking knee during movement, known medically as crepitus, is common, especially when performing deep flexion exercises like the squat. This noise can range from a soft crackle to a distinct pop. While it may sound alarming, it is frequently a benign occurrence that does not indicate joint damage. Understanding the origins of these sounds helps distinguish between normal biomechanical function and a potential issue. If the noise occurs without accompanying discomfort, it is usually just a byproduct of joint mechanics and not a cause for concern.
The Sound of Air Bubbles Escaping
One of the most common causes of a popping sound is joint cavitation. This process occurs in the synovial fluid, a thick, lubricating substance that fills the joint capsule and contains dissolved gases. As the knee joint is rapidly flexed during a squat, the volume inside the joint capsule temporarily increases.
This rapid expansion causes a sudden drop in pressure within the fluid. The reduced pressure forces the dissolved gases to rapidly come out of solution, forming small bubbles or a larger cavity—a process known as tribonucleation. The audible click or pop occurs when this gas cavity forms or rapidly collapses, releasing a sound wave. This mechanism is similar to the sound produced when cracking knuckles and is not related to the development of arthritis.
Clicking Caused by Snapping Tendons
Another cause of knee clicking involves soft tissues moving over the joint’s bony structures. The knee is surrounded by tendons and ligaments that guide motion and provide stability. During the deep range of motion required for a squat, a tight tendon or ligament may momentarily catch on a prominent piece of bone.
When the tissue tension overcomes the resistance, the tendon snaps back into place, creating an audible click. The iliotibial (IT) band is a frequent culprit, as this thick band of fascia can snap over the lateral aspect of the femur. The quadriceps or hamstring tendons may also be involved in this mechanical friction. This type of clicking is painless, representing a momentary friction point during the change in joint angle.
When Knee Clicking Indicates a Problem
While most knee clicking is harmless, the noise becomes concerning when consistently accompanied by other symptoms. The differentiator between benign and problematic crepitus is the presence of pain, swelling, or functional impairment. A painful click or a deep, grinding sensation beneath the kneecap may suggest an issue with the joint surfaces or internal structures.
A sharp clicking or popping sensation accompanied by pain, especially after a twisting movement, can signal a meniscal tear. The meniscus is a C-shaped cartilage shock absorber; a tear can cause fragments to get caught between the bones, resulting in a painful mechanical lock or catch. A persistent, gritty grinding sound, often felt when descending or ascending stairs, may indicate patellofemoral pain syndrome. This condition involves poor tracking of the kneecap within the groove of the thigh bone, leading to friction and irritation of the underlying cartilage.
A painful crunching sound could also be a symptom of cartilage wear, which occurs in conditions like osteoarthritis. When the smooth cartilage breaks down, the roughened surfaces rub together, producing a grinding noise. If the clicking causes the knee to lock up, give way, or is paired with visible swelling or sharp pain, stop squatting and seek a medical evaluation.
Adjusting Your Squatting Technique
For non-painful clicking, adjustments to movement preparation and technique can reduce the noise. A proper warm-up is important, as it increases the temperature of the synovial fluid, making it less viscous and improving tissue elasticity before placing the joint under load. Dynamic stretching and light movement, rather than static holds, prepare the tendons and ligaments for the exercise.
Focusing on correct squat mechanics ensures the kneecap tracks smoothly. This involves consciously pushing the knees outward so they track in line with the middle of the foot, preventing the knees from caving inward. Maintaining core tension throughout the movement also helps stabilize the pelvis, which controls the alignment of the thigh bone and kneecap.
Targeted strengthening of the supporting musculature is a long-term strategy to reduce mechanical clicking. Strengthening the gluteal muscles and the vastus medialis oblique (VMO) helps control the dynamic stability of the knee. Stronger glutes prevent the thigh bone from rotating inward, and a balanced quadriceps complex ensures the kneecap glides correctly in its femoral groove, minimizing soft tissue friction.