Chondromalacia patella, also known as runner's knee, is a knee condition that can cause pain and grinding when the knee is flexed. It's due to the breakdown in the cartilage on the undersurface of the kneecap (patella) that causes bones to rub against each other. Overuse (especially from running or cycling), trauma, such as a fracture, or a misalignment of the kneecap and femur can all cause chondromalacia.
Source: Verywell Health