Muscle strength and running economy — a “chicken or the egg” problem?

Runners are often told that strength training is integral to improving running speed and running economy. But there might be a little bit of a problem with this advice. I recently posted about a body of research that pointed to the idea that, for a variety of biomechanical reasons, weaker muscles in a trained runner correlated with a greater running economy (specifically at the calf region). The consensus was that running economy increased with achilles tendon loading, and decreased with calf muscle (gastrocnemius and soleus) activity.

More muscle means worse economy. A recent article in Runner’s World confirmed this, citing a study that found that running economy was related to the balance of strength between the anterior and posterior muscles (specifically, the quads and hamstrings). It was not, as most of us suspect, a function of pure muscle strength—overall, competent runners had weaker muscles than novice runners.

This brings up several questions. The first is, of course, how can weaker muscles make you run faster? The answer, I believe, is systemic, and our ability to find it hinges on what we mean by “strength training”—and how usefully we’ve defined it for ourselves. In the most basic terms, the strength of an individual muscle has little to no bearing on how the hip-leg-foot mechanical system will function in practice.

The power of this system—when power refers to how much force the leg can put out per unit of time—is much more a function of how well the parts move together, than how strong any individual part (or indeed, all of its parts) are individually. Someone endowed with extremely strong muscles that are all just slightly out of sync will have a completely rigid leg, not a powerful one.

It’s necessary, therefore, to make sure we all mean the same thing by “strength training.” Strictly speaking, the kind of explosive power (plyometric) training that a lot of runners do, which actually does develop hip and leg power, is “strength training”—but of the entire system. We need to be clear on what we mean by this to know if strength training will actually help us become better runners. Do we mean pure strength, or explosive strength?

The second question is more related to a practical matter, and is a consequence of answering the first. What are our reasons to train pure “muscle strength” in the first place? We’d better have them, given the above evidence that muscle strength correlates with low running economy. If we do prescribe a strength training program to runners, are we potentially hurting their running economy?

I don’t have an answer for this. Most of my training is either isometric or plyometric, and the few strength exercises that I do—such as barbell squats—are for balancing my body out, more than anything.

The third question is a matter of causality: why did the novice runners in the Runner’s World article have stronger muscles? To speculate about this, we have to return to the body of research mentioned above. The reason that weaker muscles correlated with greater running economy has to do with the biomechanics of particular bodies. One of the abovementioned studies looked at the ankle region of highly-trained runners, and found that runners who had longer heels (meaning a greater distance between the ankle and the heel) had poorer running economy and greater muscle power.

None of this is surprising, once you think about it. When the hip-leg-foot system pushes against the ground, it exerts force directly into the ground, at a perpendicular angle. To achieve this, the foot works a lot like a lever: the achilles tendon is connected to the end of the lever arm (the heel bone). When it shortens, the heel raises, meaning that the foot rotates downwards around the ankle—the fulcrum—allowing force to be exerted into the ground. Because every action has an equal and opposite reaction, force also travels in the exact opposite direction: into the calf, parallel to the calf bones.

Achilles-tendon-function

Because of the properties of the muscle-tendon system, this results in a trade-off. If you increase the length of the lever arm—the distance from the ankle to the heel—leverage increases, meaning that the calf muscles have an easier time pulling on the lever and causing the foot to point.

However, this also means that the tendons work more like a rope and less like a spring: The elastic fibers that make up the tendon have to be aligned with the direction of force in order to store that mechanical energy. If the lever is longer, the achilles tendon is at a greater angle to the direction of force, and therefore less capable of storing mechanical energy.

In other words: greater leverage = less energy return. When your skeletal structure compels you to use your muscles more (resulting in stronger muscles), you also have less energy return, which is a critical component of running economy.

The reason that the novice runners in the Runner’s World article have stronger muscles may have less to do with the fact that they’re untrained and more with why they’re untrained. Perhaps one of the reasons is that they are not dimensionally prediposed to train running. Supposing this is the case, you might look at their bodies and find that they are built for leverage, not for energy return.

You might. A longitudinal—long-term—study would confirm this (or not). If the untrained runners started training, would their running economy get better? According to the abovementioned study, not really—or at least not completely: the study estimated that 56% of running economy could be accounted for by heel length alone. In addition, the runners they looked at were all highly trained (and had comparable running performance) and their running economy still varied by 20-30%.

(This also means that while longer heels contribute to a lower running economy, they do not necessarily contribute to lower running performance. The human body has many faculties, each of which contribute differently to performance. Energy return is only one of them).

One thing is clear: as a collective, we need to be a lot more careful with the advice that we give runners. As I mentioned above, what does “strength training” mean, and what exactly are we recommending that runners do, if we make such a suggestion? The skeletal mechanics of the body (let alone the possible interpretations of the phrase “strength training”) means that the same advice given to two different runners can have very different ramifications—or worse yet, none at all.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s