Soreness in the side muscles of the buttocks may be caused by strenuous exercise, hip arthritis, strained gluteus maximus and other causes. 1. Strenuous exercise: Long-term walking and strenuous exercise can lead to anaerobic metabolism in the hip muscles, resulting in the accumulation of lactic acid, which can lead to muscle soreness on the side of the buttocks. 2. Hip arthritis: when the hip joint is inflamed, the stimulation of inflammation will cause pain in the muscle joints, which will cause muscle pain in the hip. 3. Hip major muscle strain: under the influence of various violence, trauma and other factors, the hip major muscle will appear localized swelling, pain, activity limitation and pain will be more serious when it is injured. If the symptoms of muscle soreness on the side of the buttocks persist without relief, you need to seek medical treatment in time.