There are more reasons that cause a tendon in the back of the thigh to hurt when it is straightened, such as muscle strain, lumbar disc herniation and calf triceps injury, etc. The patient should be cared for and treated according to the different reasons. 1. Muscle strain: muscle strain caused by strenuous exercise, long walking, in this case, the thigh will be straightened because of muscle pulling to cause pain in the back of the thigh. Patients should reduce activities, pay more attention to rest, and elevate the affected limbs when sleeping at night. 2. Herniated lumbar intervertebral disc: The herniated intervertebral disc will compress the nerves and cause radiating pain in the innervated area, leading to pain in the back of the thigh when a tendon is straightened. Patients should rest in bed, wear lumbar protection, and at the same time can follow the doctor’s instructions to take drugs such as ibuprofen, diclofenac, meloxicam and other anti-inflammatory and analgesic treatment. 3. Calf triceps injury: due to sports, cold, fatigue and other factors, may cause acute and chronic inflammation of the calf triceps muscle, resulting in a tendon at the back of the thigh hurts when it is straightened. Patients should pay attention to rest, avoid exertion, do a good job of warm measures, you can reduce the symptoms through hot compresses, massage, physiotherapy and other ways. If there is a tendon in the back of the thigh a straight pain in the symptoms continue not to ease, the need for timely diagnosis and treatment, clear cause, so as not to delay the treatment.