Muscular endurance linked to running economy

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I blogged a few weeks ago about a study on strength training and cycling efficiency, and a commenter asked why so many of these studies are done on cyclists rather than runners. In response… here’s a interesting running study, just posted online at the Journal of Strength and Conditioning Research, that looks at muscular endurance and running economy.

The question they set out to ask was: does having better muscular endurance allow you to maintain better running economy (i.e. burn less energy while running at a given pace) as you get tired? To test it, they asked 10 well-trained runners to do two 30-minute runs at a moderate pace. In the middle of one of the runs, the runners had to speed up to VO2max pace for four minutes, then slow back down — enough to tire them out a bit without exhausting them. As expected, their running economy got worse after the four-minute surge by 3.0%. This is typical: as runners get tired, their running economy gets worse.

What remains hotly debated is why, exactly, running economy gets worse with fatigue. I’m not going to delve into the details of all the various mechanisms that have been proposed to explain this — it’s almost certainly caused by a mix of many different factors. One possibility relates to your knee flexors (a.k.a. hamstrings and surrounding muscles on the back of your leg above the knee) [UPDATED 6/27: had mistakenly written knee extensors], which contract eccentrically to act as a “brake” during each stride. There’s some evidence that eccentric contractions decline more quickly than concentric contractions during exercise — so as that braking action gets less effective as you fatigue, your stride gets less efficient.

Okay, now we finally get to the point. The researchers also tested the eccentric muscle endurance of the knee and hip flexors and extensors of all their subjects, then looked for correlations with the running economy results. Sure enough, they found that eccentric knee flexor endurance was “strongly related” to how much running economy worsened after the fast section of the run. Bingo!

So what does it mean? Well, there’s a big chasm between saying “hamstring quad endurance and running economy changes are linked” and concluding “therefore, you should do X, Y and Z in training.” However, it’s not crazy to see this as a good argument for some lower-body strength training and plyometrics. Here’s what the authors conclude:

Our results suggest that coaches and athletes could effectively implement conditioning strategies that challenge eccentric muscle actions. These strategies include plyometrics, resistance training with an emphasis on eccentric portion of repetitions, down-hill running and over-speed training.