When you watch world-class runners like Berhane Adere and Kenenisa Bekele in action, the last word you might conjure up to describe their running style is "stiff." These runners look smooth and fluid, not stiff. It's the back-of-the-pack runners shuffling along in their lock-kneed manner who look stiff.
Nevertheless, a certain type of stiffness is actually a hallmark characteristic of the best runners' strides. Elite runners like Adere and Bekele have the most of it, while lesser runners like us could use a lot more of it. The type of stiffness I'm referring to is the type that physicists talk about in relation to springs.
The human body does in fact function as a sort of spring during running, and just as a spring with adequate stiffness will bounce more efficiently than a spring that's too loose, a runner who exhibits sufficient muscular stiffness when his or her foot strikes the ground will run more efficiently than a runner whose muscles are too loose on impact.
A spring works by reusing energy. When it falls to the ground from a given height it compresses, which converts the "kinetic" energy of the fall into "potential" energy stored in the form of tension in the spring. As the spring returns to its natural length it converts this potential energy back into kinetic energy in the form of a directional force directed into the ground. As a result, the spring bounces back up into the air.
The Spring Effect
Something very similar happens when we run. As the foot makes contact with the ground, tendons and elastic components of certain muscles stretch beyond their natural length, thereby capturing and storing energy from the impact.
As these tissues return to their natural length, this energy is released. Exquisitely timed and coordinated muscle actions direct the energy back into the ground, sending the runner's body upward and forward.
Few runners realize just how much energy they are able to reuse thanks to this spring effect. Research has shown that runners consume oxygen at a rate that is sufficient to produce only about half of the energy that is needed to run at any given speed. The other half is provided by the spring effect.
A stiffer spring is able to reuse more energy than a looser spring because it returns energy to the ground faster. A looser spring stretches and compresses too slowly, allowing more stored energy to dissipate as heat or friction. Top runners spend less time with their foot on the ground than lesser runners, in part because their superior muscle stiffness allows them to return more energy to the ground faster.
Ironically, this is one reason why the elite runner's stride looks smoother and more fluid. These runners spend more time "floating" in the air and get more of their energy for free, so they don't have to produce as much energy through muscle contractions to run at any given speed.
Muscle Stiffness
What contributes to muscle stiffness? Part of it comes from the actual elastic properties of the muscles and tendons themselves. These can be enhanced through proper training.
The other part comes from running technique. A runner with excellent technique is able to coordinate his or her muscle actions in a way that enables them to reuse more energy for forward thrust.
For example, top runners do a better job of pre-stretching and stiffening certain muscles (particularly the hamstrings) just before the foot makes contact with the ground, which enables the runner to capture more energy in the muscles and tendons, return it to the ground more quickly, and direct more energy backwards, resulting in more forward thrust.