The most common causes of thigh pain after exercise are as follows: 1. It may be due to the production of lactic acid after strenuous exercise, which accumulates near the muscles at the root of the thigh, thus stimulating the muscles and causing local soreness. 2. If you have pain at the root of your thigh after exercise, along with the usual pain at the root of your thigh, you must pay attention to the possibility of hip arthritis or even aseptic necrosis of the femoral head. This situation generally must go to the hospital to check the local X-ray or MRI, basically can confirm the diagnosis.