Listen to most modern day musings about the hip flexors and you'd think they were red headed step children - Nothing but irritating muscles that need to be stretched and contribute nothing of importance. Afterall, tight hip flexors contribute to back pain, gluteal amnesia, and they get in the way of your ab work when you're trying to strengthen your abs or develop a sexy midsection, why would you want to focus on strengthening them?
**For any anatomy newbies out there the hip flexors are the muscles responsible for lifting your knees and feet up off the ground when you run or walk. The higher you lift your knees, the greater the degree of hip flexor activation you get.
Personally I'd always believed the hip flexors, collectively known as the iliopsoas muscles, weren't much worth worrying about and prescribed only a modicum of remedial work for them for most athletes. It wasn't until I had a personal experience with an injured hip several years ago that my opinions began to change. Before I get into that let's look at some rather surprising science:
The Science Might Surprise You
One fairly recent study showed that a 12.2% increase in hip flexor strength improved 40 yd dash and shuttle run times by 3.8% and 9.0% respectively (1) Another paper demonstrated a link between larger relative psoas strength (the primary hip flexor) and 100 meter speed.(2) Another paper found that hip flexor strenth more accurately correlated with 20 & 50 meter sprints then squat strength - the association was particularly strong for 20 meter sprints.(3) Interestingly enough, another paper demonstrated that the size of the psoas muscle in male african americans was relatively more than 3 times greater compared to whites.(4) Based on this it seems people that are fast naturally have bigger and stronger psoas muscles AND strengthening the hip flexors might be beneficial for speed seeking athletes.
However, there is a bit of a discord on exactly what role the hip flexors play in the sprints. The common belief is the stronger your hip flexors the more powerful and faster you can lift your knees and this might contribute to making you faster. However, in my opinion this view is not entirely accurate. Let me explain:
The Weyand Study
Based upon the now infamous Weyand study on sprints we know that what happens to the legs once the foot comes off the ground plays no direct role in running speed. This study showed that elite sprinters ran faster solely due to the power they put INTO the ground with each stride. It also showed that they took more time in the recovery phase than slower sprinters. The recovery phase is the point where you bring the foot off the ground in preparation for the next stride, or the phase where you lift your knees up.
So, the harder you push against the ground, the faster you go, the speed that you lift your knees and the speed that you rotate your feet thru the air aren't limiting factors.
So, what's the deal with having strong hip flexors? Well, in my opinion the hip flexors are the ultimate "setup" muscles. In order to explain fully let me go back to my hip injury and talk about what I experienced:
Several years back I had been dealing with an incredibly painful hip and none of the medical practitioners or therapists I knew seemed to be able to help me with it. So, I spent many months on my own researching everything I could on hip conditions trying to figure out how to deal with it. Shirley Sahrmanns book "Diagnosis and Treatment of Movement Impairment Syndromes" was particularly helpful. What I discovered was in MOST hip issues one of the most important things to do was STRENGTHEN the hip flexors to help establish proper femoral control. I've touched on this before but Femoral control means the muscles that attach on the upper thigh bone from up around the waist (the psoas and glutes) should be fully in control of the thigh bone, rather than those that attach lower, such as the TFL. This correctly pulls the head of the femur tight into the hip socket preventing excessive movement. It's that excessive movement of the femur head in the socket that can create hip pain. Many athletes in general struggle with this, particularly those with hip pain. I had a MAT chiropractor test my hip flexor strength and the psoas in the injured hip was a little weak in the short range, which would be the very, very top of a hanging knee raise. Whether it was weak as a result of the injury or the injury came as a result of the impairment I'm not sure, but it was something to pay attention to.
My Hip Rehab
The focus of my hip rehab was on re-establishing optimal femoral control, and I did a lot of isolated hip flexor exercises for about 2 weeks without any other lower body movements. After a couple of weeks of this my hip was feeling better so one day I went out and did some light movement work on a basketball court. I immediately noticed the perceived activation level of my glutes and hamstrings had improved immensely. For the first time in years I could actually feel my hamstrings activate when I moved and the next day my glutes were so sore I'd felt like I'd done 10 sets of squats. So, doing hip flexor exercises seemed to improve my ability to get power from my posterior chain. Others I've trained have noticed the same thing and this is the reason I believe strong hip flexors are beneficial for athletes such as sprinters:
The Set Up
See, one of the side effects of having good femur control is the primary drivers (the glutes) are more easily primed for action on the downstroke when you can efficiently lift your knee on the upstroke. Basically, if the hip flexors are weak the glutes can't optimally do their job becuase the posture of the pelvis isn't set in a way that primes the glutes optimally when the foot comes off the ground in walking or running. It's extremely common in people with a naturally swayback posture. Strengthen their hip flexors and all the sudden the posture of the hips changes and the glutes are more easily engaged in every day life and back/hip pain tends to dissipate.
Having an optimal posture in the sprints (repositioning your legs correctly during the recovery phase and having the strength to do so) allows you to set up in a way that allows you to more efficiently exert more force when your foot does hit the ground and you push off....you get a better pre-activation, better leverage, and a greater pre-stretch from the contributing musculature (the muscles that contribute when your foot makes ground contact). It is the same basic principle in which improving upper and mid-back strength can improve bench press strength.
So, it's not so much that having a strong knee drive makes you fast and that being able to rapidly drive your knee off the ground is important, it just promotes more leverage when your foot strikes the ground and you push off.
So, how do you identify whether your hip flexors are weak or not? Well, first look at some assessments.
Exercises and Assessments
Look at the sprinters posture then try to duplicate. In my experience the hip flexors are one of the few muscles that tend to be weak in the shortened range (the very top of a full knee or leg raise) rather than the stretch range. You want to focus on exercises where you raise your knee up to where your thigh is above 90 degrees. Here is an exercise to test strenght, and also a good exercise in itself. Keep the natural arch in your spine and don't lean back.
A lot of people won't be able to lift their knee an inch without squirming around all over the place. You should be able to come up several inches. The further you lean forward the harder the exercise is. I'd say if you lean forward about 45 degrees and can't get your foot off the ground at all you could probably use some work.
I recommend doing a couple of sets of 8-10 with a 2-3 second hold at top on that exercise 2-3 x per week Use bands for extra resistance. Fortunately, the hip flexors tend to respond very quickly and once a modicum of strength is developed it is maintained rather easily.
Another good exercise is doing the same thing standing up, which also happens to hit the glute of the plant leg:
Standing Hip Flexor Drill (keep the plant leg straight, stand up straight, and don't let the knee bend)
The top half of a hanging knee raise or hanging leg raise also hits the psoas hard. For the ultimate, look into pike variations:
Watch Your Running Form
Also, watch yourself when you run from the front and the back. If the hip flexors are weak people will tend to shorten their strides the knees will often cross in across the midline of the body when viewed from the front. From the back the feet will flail sideways, or to the outside. When you do an exercise like a-skips (high knee skips) your knees should reflexively and involuntarily "pop" up high off the ground. If you have to lean back or force your knees up that's an indication you could use some dedicated strength work in that area.
Now, what about stretching the hip flexors?
Well, the psoas is a muscle that is often tight AND weak, so it's still important to do hip flexor stretches, just don't necessarily neglect all strength training for them, especially for beginners.
Anyway, don't take this article to imply that you should begin spending 90% of your time on training the hip flexors - they need not be a main focus and their contribution still pales in comparison to your prime lower body movers (quads, glutes, hamstrings) but, if in doubt, spend 15 minutes a week on some regular hip flexor work for a month or so and see what you notice!
1. Effects of Hip Flexor Training on Sprint, Shuttle Run, and Vertical Jump Performance The Journal of Strength & Conditioning Research. 19(3):615-621, August 2005.
2. Influence of the psoas major and thigh muscularity on 100-m times in junior sprinters. Med Sci Sports Exerc. 2006 Dec;38(12):2138-43.
3. Predicting sprint running times from isokinetic and squat lift tests: a regression analysis. The Journal of Strength & Conditioning Research (1998). Volume: 12, Issue: 2, Pages: 101-103
4. Anatomical differences in the psoas muscles in young black and white men. Hanson, Magnusson, Sorensen, Simonsen.