Tendon length, joint flexibility and running economy
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.