Pain in the knee during leg extension may be caused by the following diseases: 1, may be involved in patellofemoral pathology of the knee joint, this condition may occur because doing too much strenuous exercise or weight-bearing work, such as climbing stairs, prolonged weight-bearing and other activities, the patella or knee joint joint surface friction caused by chronic damage to the articular surface cartilage, there will be pain in the knee during leg extension. 2, may also be congenital factors The patellar instability caused by this type of common in adolescents or children, mostly due to congenital factors or trauma, resulting in instability of the knee joint, prone to wear and tear. The pain may also be caused by suprapatellar bursitis, which is likely to be caused by cold or trauma to the lower extremities, and pain caused by the patella touching the inflamed position when the knee is straightened. 4.