Very often, in training discussions online and in books—even sports science textbooks—you encounter the claim that, during running, tendons stretch out and store up energy on impact with the ground, releasing that energy later when you push off the ground. This energy storage improves your running economy, because without it, you’d have to produce that same force via the active contraction of your muscles.
The “tendons store energy for free” claim is used to justify doing plyometric training, heavy load weightlifting, or hill sprints, with the goal of increasing this energy storage and improving your running economy.
However, this claim is wrong—tendons do store energy, and a stiff, strong tendon does improve running economy, but this energy storage does not happen “for free.” The reasoning is quite obvious when you analyze the biomechanics behind tendon energy storage. In this post, we’ll consider the case of the Achilles tendon, but the argument also applies to most of the other major tendons of the lower body.
The biomechanics of muscle and tendon in running
The basic biomechanics of the situation are as follows: a tendon connects a muscle to a bone. In the case of the Achilles tendon, it connects the calf muscles (both heads of the gastrocnemius muscle and the deeper, more powerful soleus muscle) to the base of the heel, at the calcaneus bone. Now, mechanically speaking, the tendon is said to be in series with the muscle. And if the tendon and the muscle are in series—physically connected to one another—the only way they can transmit a tensile load is if the force in the muscle is in equilibrium with the force in the tendon.[1]
To visualize why this must be the case, imagine a rubber band attached to a rope, which is tied to something solid. Even though the rope is rigid, and the rubber band is stretchy, no matter how hard you pull on the rubber band, you’ll always be generating an equal amount of force in the rope and the rubber band. There’s no way to get out of this equal-force equilibrium.
The difference between this rubber-band + rope example and a tendon-muscle complex is that the rope is always rigid no matter what. A muscle, in contrast, can only act as a rigid force-transmitting structure when it is actively producing muscular force. And there’s a one-to-one relationship between how much force the muscle produces and how much tensile load is transmitted to the tendon.
So, if a tendon stretches out and stores up a certain amount of elastic energy, the muscle attached to that tendon must be producing exactly the right amount of force to maintain the tendon at that given level of “stretched-out-ness.”[2]
And yet…we know that there’s a correlation between having a stiff Achilles tendon and better running economy. And we even know that chimpanzees—who have an extremely short Achilles tendon—are much less efficient (bipedal) walkers and runners than humans. So, what’s going on? Why do tendons really help with running?
Why stiff tendons help reduce the energetic cost of running
Here’s the key: when we talk about the energetic cost of running, we’re not talking about mechanical energy. We’re talking about metabolic energy—food calories. Muscles are essentially “engines” for converting metabolic energy into mechanical energy, and like all real-world engines, they operate at different efficiency levels depending on their configuration.
As such, just knowing how much force a muscle is producing does not tell you about the metabolic cost of that amount of force production.
For one thing, muscles are more efficient—in an ATP-turnover to force-production sense—when the muscle fibers are neither too stretched out nor too compressed. In other words, there’s an optimal “length” for the muscle fibers, in terms of how stretched out the muscle (not the tendon) is at any given point.[3]
Additionally, the energetic efficiency of a muscle also depends on whether or not the fibers are moving—i.e. whether you are doing a concentric, eccentric, or isometric muscle contraction. In terms of energy cost for a given amount of force, concentric is more costly than isometric, and isometric is more costly than eccentric.
So, just knowing that the calf muscles are producing, say, 400 pounds of force doesn’t tell you how much (chemical) energy that’s going to cost. What matters is the configuration of the muscle—its length and contractile velocity.
In an efficient muscle-tendon complex, the tendon does the stretching, not the muscle
The final piece of the puzzle comes by examining what actually happens in a muscle and tendon during real running. Even though the muscle has an optimal length (and an optimal velocity) for efficiently producing force, the biomechanical constraints of running put some hard requirements on how you configure your body.
For example, if your foot is at an angle of 90 degrees relative to your shin, there’s a pretty strict limit on the length of your calf-Achilles complex. But—that’s where your body has wiggle room. The biomechanics of running dictate the length of the muscle-tendon complex, but the stiffness of the tendon dictates how your body proportions the change in length between a change in muscle length and a change in tendon length.
The Achilles tendon leverages exactly this wiggle room to reduce the metabolic cost of running. Computational research by Thomas Uchida and others at Stanford University shows that a stiffer Achilles tendon allows your calf muscles to spend more of the gait cycle producing force near optimal muscle fiber length and near optimal-ish muscle fiber velocity.
The consequence is that, although you still have to produce the same amount of force in your calf muscles, you are able to do so more efficiently because of the energy storage—and elastic deformation—of the Achilles tendon.
Interestingly, optimal tendon stiffness increases at faster speeds (right panel). Plot from Uchida et al.
True “free” energy storage and the biomechanical cost of tensile loading
Clearly, the story is a little more subtle than “the tendon stores energy for free.” The energy storage is not free; you still need to produce the requisite amount of force in your muscles. But you’re able to produce that same amount of force with less metabolic cost, which improves your running economy versus not having stiff, elastic tendons.
Now, there’s one interesting case of a tendon-like structure that does store energy “for free,” and that’s the plantar fascia. The plantar fascia is not really a tendon, even though it’s more or less made up of the same “stuff” as tendons (aligned collagen fibers).
Anatomically, the plantar fascia isn’t really a “fascia” either—it’s more like a ligament. The plantar fascia connects bone to bone, not muscle to bone. So, the plantar fascia can (and does) store energy “for free” by stretching out as you put more weight through your foot. But even this energy storage is not really free.
One common thread shared by all of these energy storage structures in the body is that, as economists would say, there’s no free lunch—reducing the metabolic cost of active muscle contraction does not reduce the actual amount of biomechanical load put into the tissue, and even in the case of the plantar fascia, where there’s no muscular force production at all, the high tensile loading causes tissue damage.
In fact, if you rank-order the connective tissues in the body by how much tensile loading they are exposed to during running, you get almost a perfect recapitulation of the most-injured locations in the body! Achilles tendon, patellar tendon, plantar fascia, etc. The same is true, by the way, for tissues exposed to compressive loads: the kneecap, tibia, and metatarsals come out near the top there, and it should be no surprise that those locations are also among the most commonly injured locations.
So, the energy storage is beneficial, but not “free”—at least for tendons, you still need to produce an equal amount of muscular force (albeit in a less-energetically-costly manner), and for all structures in the body, that stored energy comes with a non-negotiable biomechanical stress.
Separately, stiffer and stronger tendons may also help protect against injury, but that’s a consequence of the biomechanics of tissue stress and tissue strain, not anything to do with “free” energy storage.
Recap
In sum, it’s not correct to say that tendons store energy for free. Tendons enable your muscles to work more efficiently and produce the same amount of force with a lower metabolic cost. So, even if you improve your tendon stiffness via strength training or plyometrics or hill sprints, you aren’t necessarily reducing the amount of force your muscles produce.
This distinction has important implications for how to think about biomechanical loading and injury risk: running economy and “efficiency” are a function of metabolic cost, while injury risk is driven by mechanical forces. As the biomechanics of the muscle-tendon complex demonstrate, it’s not always the case that reducing metabolic cost leads to a reduction in mechanical loading.
Learn more about the biomechanics of running
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Footnotes
[1] Technically you also need to assume that the mass of the muscle and the tendon are negligibly small compared to the forces being transmitted by the muscle-tendon complex, but in running and even in walking this is always the case: even large muscles only weigh a couple pounds, and the forces we are talking about are on the order of hundreds of pounds. A computational study by Matthew Millard actually tested this assumption and found that you get no practical difference in estimated tendon biomechanics when you account for the mass of the muscle and tendon, versus assuming they are negligibly light “massless” structures.
[2] If you know your physics, you can actually calculate this amount of force via the relationship between elastic potential energy, Hooke’s law, and the work-energy theorem.
[3] More strictly this metabolic efficiency is related to, but not identical with, the force-length relationship for muscle fibers that you’d encounter in an exercise physiology or biomechanics class.
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So, do you think there is any difference between a program of stiffening the Achilles tendon by doing weighted static calf raises or weighted pogo jumps (assuming the same load performed for each exercise)?
I think the real difference would be in the non-tendon adaptations induced by those two different programs! I suspect the neural and muscular property changes induced by plyometrics are different from those induced by heavy slow strength training...but as for exactly how, I can't yet say!