A lot of people are surprised to hear the hip flexors mentioned as an important muscle for running. So what does this group of muscles do?
If we again delve into Tim Dorn’s 2012 study on the contribution of various muscles in running, we see the hip flexors (iliopsoas) performing close to 2 x bodyweight of work at slower speeds, but with an increased contribution at faster speeds, to the point where at 9 metres per second, the iliopsoas performs the most work of ANY other muscle, at a tremendous 9 times per body weight!!
What function do the hip flexor muscles serve in runners?
The two main muscles that form the “hip flexors” are the iliacus and psoas, with the rectus femoris also assisting.
The hip flexor muscles’ main role in running is to bring the swinging leg forward, to then prepare the leg for another footstrike. As we move from midstance to toe-off, the hip flexor muscles contract to allow the tendons to store energy, which then at toe-off, this stored energy slingshots the leg forward (like letting go of a stretched rubber band). The other function of the hip flexors (especially the psoas) is to stabilize the low back (lumbar spine).
In their 2005 study, Deane et al found that 8 weeks of hip flexor strength training decreased 40 & 10 yard (37m & 9m) sprint time by 0.233 seconds & 0.241 secs respectively, PLUS they also decreased their shuttle run time by 9%. Based on this improvement, the authors concluded that the hip flexors play an important role in acceleration.
What are the consequences of weak hip flexor muscles?
There are a few potential issues that could arise with hip flexor weakness. This includes:
Hip pain & stiffness during and after a run
Hip flexor tendinopathy
Decreased ability to accelerate
Decreased ability to run at faster speeds: Higher speeds require greater demands on hip flexor strength, so if there is insufficient strength, higher speeds may be unattainable.
So there you have it, a bit about the muscle group often not considered in running. In our next blog, we will discuss how to test the strength of the hip flexor muscles, and how a recurrent feeling of “tightness” may actually indicate weakness in that area.
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References
Deane, R. S., et al. (2005). "Effects of hip flexor training on sprint, shuttle run, and vertical jump performance." J Strength Cond Res 19(3): 615-621.
Dorn, T. W., et al. (2012). "Muscular strategy shift in human running: dependence of running speed on hip and ankle muscle performance." J Exp Biol 215(Pt 11): 1944-1956.
Silva, W. A., et al. (2018). "Hip muscular strength balance is associated with running economy in recreationally-trained endurance runners." PeerJ 6: e5219-e5219.