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.
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Grade III (severe): A complete tear in your muscle that causes severe pain and swelling and you can't bear weight on that leg, making it difficult to walk. You've also lost more than 50 percent of your muscle function. These injuries are less common and may need surgery to repair the torn muscle. They can take several months or more to completely heal.