The hip flexors help balance the posterior pelvic muscles. Three key muscles often become tight and shortened as a result of activities of daily living. These are the iliacus, psoas major, and the rectus femoris. The iliacus and the psoas major are often referred to as the iliopsoas because they share the same insertion at the lesser trochanter of the femur. The psoas minor inserts on the superior ramus of the pubis bone and mainly supports the natural lordotic curvature of the spine, but is only found in about 40% of the population. The psoas major originates on the anterior surface of the lumbar vertebrae and runs over the pubis bone and inserts into the lesser trochanter of the femur. This muscle not only helps to flex the hip, but also has an effect on the lordotic curvature of the lumbar vertebrae. The rectus femoris has a proximal attachment at the acetabulum and inserts into the tibial tuberosity. This long muscle plays a role in both hip flexion and leg extension (Figure 9-4).

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.
Located deep in the front of the hip and connecting the leg, pelvis, and abdomen, the hip flexors— surprise, surprise— flex the hip. But despite being some of the most powerful muscles in our bodies (with a clearly important role), it’s easy to neglect our poor hip flexors— often without even knowing it. It turns out just working at a desk all day (guilty!) can really weaken hip flexors since they tend to shorten up while in a seated position. This tightness disrupts good posture and is a common cause of lower back pain. Weakened hip flexors can also increase the risk of foot, ankle, and knee injuries (especially among runners) Hip muscle weakness and overuse injuries in recreational runners. Niemuth, P.E., Johnson, R.J., Myers, M.J., et al. Rocky Mountain University of Health Professions, Provo, VT. Clinical Journal of Sport Medicine, 2005 Jan; 15 (1): 14-21.. So be sure to get up, stand up every hour or so! And giving the hip flexors some extra attention is not just about injury prevention. Adding power to workouts, working toward greater flexibility, and getting speedier while running is also, as they say, all in the hips The effect of walking speed on muscle function and mechanical energetics. Neptune, R.R., Sasaki, K., and Kautz, S.A. Department of Mechanical Engineering, The University of Texas, Austin, TX. Gait & Posture, 2008 Jul; 28 (1): 135-43..

The more than 20 muscles that make up your hips are responsible for stabilizing your pelvis, moving your legs from side to side, and shortening to draw your knees toward your chest every time you sit down, run, jump or pedal, explains Kelly Moore, a certified yoga instructor and co-founder of Mindfuel Wellness, which brings health and wellness initiatives to companies throughout Chicago.
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