When it comes to which elements of your stride you can improve, your hips don’t lie. That’s because hip flexors—the muscles that allow flexion at the hip joint—play a huge role in fluid running, and a set of tight ones can really mess with your mechanics. The iliopsoas is the strongest group of muscles in the
Exercise Instructions: Start in push-up position with your hands about shoulder-width apart and your shins resting on a Swiss ball. Raise your hips upward as you roll the ball in toward your hands. Pause at the top before slowly lowering to the starting position.