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Tendon length, joint flexibility and running economy

July 22nd, 2011

We already know that pre-exercise stretching makes you less efficient at running and cycling. But is it the stretching that’s bad (e.g. by temporarily interfering with the signals from your brain to your muscles), or is flexibility itself a potential problem? For running (though not cycling), your legs function like springs, storing energy with each stride then releasing it in the next stride. If you’re too flexible, the thinking goes, those floppy springs will be less efficient at storing energy.

A new study from the University of Alabama at Birmingham in next month’s issue of Medicine & Science in Sports & Exercise takes a look at this question. They took 21 male recreational runners and measured a bunch of physical traits, then measured their running economy to look for correlations. The key traits they measured were tendon length (Achilles, patellar and quadriceps) and joint flexibility (knee and ankle). They expected that longer tendons would be a good thing: they can store more elastic energy simply because they’re longer. On the other hand, they expected greater joint flexibility to be a bad thing (as far as running economy goes). And that’s exactly what they found:

In conclusion, lower limb tendon length, especially Achilles tendon length, is associated with improved running economy in male recreational distance runners. In addition, plantar flexion and knee extension flexibility are negatively related to running economy.

I don’t think the finding about longer tendons being more efficient means that we should aim to do lots of stretching (after workouts, perhaps) to lengthen tendons. Instead, I think that’s likely just something that’s genetic: some people (including a lot of Kenyans, it seems) have long, thin tendons, and they’re more likely to be efficient runners. But I’m not sure if that’s the correct conclusion. We don’t know whether a couple of years of dedicated stretching to lengthen tendons would have increased or decreased economy, and this study has nothing to say about the question. One problem is that stretching to lengthen tendons will also increased plantar flexion and knee extension, which hurt running economy — so it may be a case of one step forward, two steps back.

I’d also like to point out the contrast between these results and the cycling study I blogged about last month. The two studies seem to point to completely different mechanisms by which stretching could impact endurance performance. This study suggests that stiff musculo-tendon complexes are good for storing energy; the cycling study (which doesn’t involve this stored-energy effect) suggests that stretching does something to the muscle fibres or neuromuscular signalling. So it’s not a simple picture.

Finally, we should bear in mind that running (or cycling) economy isn’t the whole picture. Many athletes — particularly recreational ones — would happily accept a 0.5% decline in economy if it reduced their chances of injury by 50%. I personally don’t think that the evidence for stretching and injury prevention is convincing, but it’s important to remember that we’re balancing different desired outcomes in making training decisions.

  1. Ben
    July 22nd, 2011 at 04:29 | #1

    On a slightly related note, did you see this study in PLoS ONE: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0020601 that suggests exercise may have been a driving force in the increase in human brain size compared to other primates? Interesting ideas there, but humans don’t quite fit the rest of the data…

  2. John Lofranco
    July 22nd, 2011 at 16:27 | #2

    There must be a “green zone” of tendon length though, where you want to have AT LEAST a certain amount of flexibility? I ask because one of the women I coach has VERY short Achilles tendons. Like she walks around like she’s in heels, even when in bare feet. I figure it would probably be beneficial for her to do some lengthening, just so she can walk with her heels touching the ground, at least a little.

  3. Colm Colohan
    July 22nd, 2011 at 17:52 | #3

    Interesting article. But Increased ankle flexibility might help avoid a heel-strike and a relaxed ankle will help the foot land flat upon contact with the running surface

  4. alex
    July 25th, 2011 at 00:16 | #4

    @Ben: Hadn’t seen that study — cool stuff! Maybe worth a separate blog post. Thanks for the heads-up.

    @John: Intuitively, I agree with you: there must be a point where some lengthening is beneficial. We don’t know exactly where that point is, but I’d certainly agree that it sounds like your athlete could benefit from some Achilles stretching (though not right before workouts or races!). I think we were stuck in a “more flexibility is always better” mode for several decades. Now we’re temporarily swinging the other way, wondering whether ALL stretching is bad. Presumably we’ll find the right balance eventually — stretching to address areas of abnormal tightness, but not stretching just for the hell of it.

    @Colm: Interesting thought. But if you need extra ankle flexibility to avoid heel striking, doesn’t that suggest that you’re overstriding too?

  1. July 22nd, 2011 at 17:49 | #1