Hidden pain in one leg is considered to be possibly related to lactic acid buildup, soft tissue injury, vascular lesions, etc. 1. Lactic acid accumulation: When exercising for a long time, a large amount of lactic acid is produced due to anaerobic respiration of muscles and other cells, which accumulates in the tissues, thus leading to symptoms such as vague pain and soreness in the legs. 2. Soft tissue injuries: Strenuous or sudden exercise can lead to muscle, fascia and other soft tissue injuries due to tension, resulting in hidden pain in the legs, severe pain when the injury is severe, and may be accompanied by swelling, or even restricted movement. 3. Vascular lesions: When there are varicose veins, arterial occlusion, arterial inflammation and other vascular lesions in the blood vessels of the leg, due to the vascular lesions that make the blood supply to the leg impaired, vague pain in the leg can occur, and intermittent claudication, sensory disturbances and other symptoms can also occur. A vague pain in the leg may be caused by a variety of diseases, it is recommended to improve the leg vascular ultrasound, CT, MRI and other examinations to clarify the cause of the disease and then targeted treatment.