Strong ligaments and muscles support the SI joints. There is a very small amount of motion in the joint for normal body flexibility. As we age our bones become arthritic and ligaments stiffen. When the cartilage wears down, the bones may rub together causing pain (Fig. 1). The SI joint is a synovial joint filled with fluid. This type of joint has free nerve endings that can cause chronic pain if the joint degenerates or does not move properly.
You can strain or tear one or more of your hip flexors when you make sudden movements such as changing directions while running or kicking. Sports and athletic activities where this is likely to occur include running, football, soccer, martial arts, dancing, and hockey. In everyday life, you can strain a hip flexor when you slip and fall, for example.