Squatting for a long time the knee can not straighten the pain may be due to physiological and pathological reasons. 1. Physiological reasons: when squatting for a long time, due to squatting so that the blood back to the heart increased, and squatting for a long time so that the lower limbs of the local blood circulation is not smooth, resulting in a short period of insufficient blood supply, so it will result in squatting for a long time the knee can not straighten the pain of the situation. 2. Pathological reasons: when patients due to various reasons lead to meniscus injury, ligament injury, will lead to cartilage wear and tear caused by aseptic inflammation, so there will be pain symptoms, meniscus, ligament damage at the same time make the knee joint activity obstacles, leading to squatting for a long time the knee can not straighten. Therefore, when there is meniscus injury, ligament injury and other pathological conditions, there will be squatting for a long time the knee can not straighten the pain. When squatting for a long time the knee can not straighten the pain, the patient should go to the orthopedic department for examination, under the guidance of professional doctors for examination and treatment.