How Hip Flexor Stretching Relates to Running Economy

by Erik Bies, DPT, MS

Runners, coaches, and other athletes are always looking for ways to prevent injury and become more efficient and economical while running. In this example, let’s consider our athlete is the weekend warrior with a 40 hour/week desk job or high school student-athlete. This person sits several hours a day, with maybe a 10-minute walking break every hour. Conventional wisdom is that this person will develop a lack of hip extension due to tight/stiff hip flexors. The hypothesis is that stiff hip flexors shortens stride length negatively impacting running economy, defined as steady-state oxygen consumption at a given running speed.

So does improving hip extension range of motion directly affect running economy? According to the evidence, the answer is NO! Though a 20 year old article, this topic has been researched.1 Subjects were young, athletic male college students determined to have “less than normal hip extension” meaning they were unable to passively extend the thigh past 0 degrees. Subjects were divided into a THREE DAYS PER WEEK (yes that is all) hip flexor stretching group and a control group. Despite a statistically significant change in passive hip extension measured using the modified Thomas Test, change in running economy was not statistically significant. The control group (those who did not stretch) actually showed greater improvement in running economy. On average, hip extension improved 9.8 degrees in those who stretched 3 days per week.

What does this mean practically? Improving hip extension through stretching anterior hip structures does not improve running performance at speeds associated with running at paces one could maintain for 10-20 minutes. Could it actually be counter-productive? From both injury and performance perspectives, YES! Consider that running at faster speeds requires sufficient anterior stiffness to withstand the forces generated by some of the strongest torque producers in the body; the gluteals and hamstrings. It has been speculated with good biomechanical evidence that excessive hip extension forces and joint angles are associated with injury to the anterior hip joint.2 Furthermore, improving your stride length is not primarily the result of greater hip joint extension range but rather more distance traveled during the float phase of running. This requires power, the perfect combination of force production and timing. A well-timed and stronger stretch-reflex in the hip flexors generates a more powerful hip flexion moment. Finding the optimal blend of stiffness and mobility at exactly the right time is what is important. Improving economy comes down to practicing a skill and improving timing of force production along with other metabolic processes.

How does this affect you? First, understand the goal of your flexibility exercises. If you are stretching because of hip pain, back off stretching and get assessed by your physical therapist. Stretching could be counterproductive even if you get short-term relief of pain. Are you certain you have limited hip extension? Don’t assume that working at your desk creates short and stiff hip flexors. Videotape yourself from a side view running at fast and slow speeds when you are not fatigued. Even if you notice that your low back is arched and your pelvis is anteriorly tilted, do not assume you have stiff hip flexors. This often is a coordination issue that can be addressed through specific trunk and pelvic girdle movement awareness.

Sources:

1. Godges JJ, McRae PG, Engelke KA. Effects of exercise on hip range of motion, trunk muscle performance, and gait economy. Phys Ther. 1993; 73:468-477.
2. Lewis CL, Sahrmann SA, Moran DW. Effect of hip angle on anterior hip joint force during gait. Gait and Posture. 2010; 32:603-607.