Your hip flexors and psoas muscles help drive your legs up when you are running. To stretch these muscles, kneel on the floor and then step out forward with your left leg so that your left foot is on the floor. With your back straight, lean forward so that your weight is on the front foot and you feel a stretch in the hip of the back leg. Perform the stretch on both sides.
Even though it seems like your legs are moving forward and backward when you’re running, in reality, the femur (your thigh bone) both rotates and tilts in the hip socket, Kalika explains. It’s the hip adductors—most notably the gluteus medius—that keeps the femur sitting in the socket as designed. (The hip adductors are the muscles that move your legs inward.) Any weaknesses make the joint unstable, and can contribute to poor running mechanics, hip drop (when the pelvis drops to one side), too-narrow stances, and aggravated tissues throughout the entire body, Sauer says.
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.