When you exercise, your knees bear considerable force every time they flex or extend. If your knees are misaligned, it can result in improper tracking and damage to the cartilage under your kneecaps. Because your knee joint loses its natural cushion, your bones rub against each other, causing a grinding sound.
Disintegration of Connective Tissue
If you flex or extend your knees while exercising and your knees are properly aligned, your kneecap should slide with ease across your thigh bone, or femur. Any misalignment of your knee can result in your kneecap grating across the femur. This grinding movement results in the softening and deterioration of the cartilage under your kneecap. If you put your hand on your kneecap and then flex or extend your knee, you can feel this grinding movement, or crepitation, just below the patella.
A Common Condition: Runner's Knee
The cartilage surrounding your kneecap is thicker than the connective tissue that cushions your other joints. After age 15, your knee cartilage starts to wear down. Certain physical activities, such as running or hiking, as well as particular movements like lunges and squats can increase the load on your knees by as much as six times your body weight, according to the Palo Alto Medical Foundation. If you're regularly subjecting your knee to these high forces, your knee cartilage gradually deteriorates. Although cartilage doesn't have any nerve endings, the bones and other connective tissue that form your knee joints do have nerve endings. When these bones rub together without enough cushioning, it often results in inflammation and pain.
The Impact of Aging
Another condition, osteoarthritis, can cause grinding knees when you exercise. In the elderly, the cartilage of the weight-bearing joints starts to break down due to normal wear and tear. If you're ascending or descending stairs, you may feel your kneecap and thigh bones grinding together. As the cartilage disintegrates, this condition grows progressively worse, leading to the swelling and inflammation of your knees. By strengthening your quads and hamstrings, you can provide muscular support for your knee joints during workouts.
Check Tracking and Correct
Assess the alignment of your knees and exercise mechanics during a workout to determine the cause of the knee grinding. For example, have a partner observe your form while exercising. If you're pronating (turning in) your foot, knee or leg, it'll warp the way in which your knees track over your feet. When one leg is shorter than the other, it can cause the short leg and knee to rotate away from your midline, resulting in a toes-out foot position. A wide pelvis can encourage your thighs and knees to rotate inward. All of these imbalances can pull your kneecap off its tracks.