The phenomenon of lying down and not being able to straighten the leg upward 90 degrees may be physiological factors, or pathological factors, such as hip and knee joints, lumbar spine diseases, etc.. 1. Physiological factors: mainly caused by the lack of exercise. The tendons of the lower limbs are not stretched, and the stretching of the tendons will be poor, which will lead to the leg not being lifted up. Usually you can do some exercise, such as bending over and touching the heel with your hand, the tendon will be stretched and the problem of not straightening the leg will be solved. 2. Hip and knee joint lesions: due to meniscus and soft tissue damage or hip dysplasia, resulting in restricted range of motion of the hip and knee joints, resulting in the phenomenon of lying down and stretching the leg up to 90 degrees. Such as osteoarthritis of the hip or knee joint. 3. Lumbar spine disease: lumbar spine disease can lead to lumbar spine around soft tissue injury, joint instability and a series of diseases, which can lead to lying leg up 90 degrees not straight. In addition, tumors, herniated lumbar discs and other factors may lead to the situation that the legs cannot be straightened upward by 90 degrees when lying down. After eliminating the physiological factors, you should consult a doctor in time to clarify the cause of the disease and treat the symptoms for early recovery.