Soreness in the left buttock may be caused by lactic acid buildup, hip joint pathology, pyriformis syndrome, gluteal fasciitis, and lumbar disc herniation. 1. Lactic acid accumulation: left buttock soreness after a lot of activities may be caused by lactic acid accumulation. Generally, it can be relieved after resting, and you can also carry out local massage and hot compress to promote the recovery of the soreness. 2. Hip joint lesions: left hip joint lesions such as osteoarthritis, necrosis of the left femoral head and synovitis of the hip joint may cause left buttock soreness. 3. Piriformis syndrome: mainly manifested as radiating pain in the lower limbs, hip pain, etc. It is mainly caused by the compression of sciatic nerve by the tension of the piriformis muscle, and the left buttock pain may be caused by the compression of the left sciatic nerve. 4. gluteal fasciitis: it is a kind of aseptic inflammation of the fascia, which may be related to overwork, moisture and other factors, and may be manifested as muscle spasm, local pain, and so on, and the left buttock soreness may also appear. 5. Lumbar disc herniation: lumbar disc herniation may compress the sacral and lumbar nerves, resulting in pain and numbness at the control point of the compressed nerves, such as lower limb pain and left buttock soreness. Left buttock pain may also be caused by other reasons, pain symptoms for a long time can not be improved, it is recommended to seek medical treatment as soon as possible, so as not to delay treatment.