If runners were superheroes, elastic recoil would be our superpower. Elastic recoil occurs when you convert energy temporarily stored in tendons and fascia into a free push. Why free? Because it’s fueled by the impact force of each foot strike. How big a push? The recoil generated by your Achilles tendons, plantar fascia, and the fascia associated with your muscles provides up to 50 percent of the propulsive force for each stride. Best of all, it’s trainable.
Tendons are the major driver of recoil. At rest, their tough collagen fibers line up in parallel wavy lines. Under tension, the wavy patterns straighten, allowing a stretch of 4 to 6 percent. The stiffer your tendons, the more energy it takes to stretch them. This energy is momentarily stored in your tendons, then released when your muscles contract.
When the impact force from each foot strike stretches your Achilles tendon, it is stored as energy. As your calf muscle reaches its maximum safe stretch, your stretch reflex causes your calves to contract. Simultaneously, your Achilles tendon (along with your plantar fascia and other fascia) releases its stored energy, creating a catapult effect–elastic recoil–that adds to the force generated by your muscles. And the faster the release of that recoil force (i.e., the shorter your ground contact time), the stronger the effect.
To appreciate recoil, try a simple exercise. Put your hand on your thigh and then tap your index finger as hard as you can. Next, pull back that index finger to its maximum safe stretch and then release it. Did it thwack your thigh a little harder than muscle-generated tapping alone?
To improve recoil, you need to increase the energy it takes to stretch your tendons by strengthening the fibers that makeup tendons. Keep in mind that stretching a tendon beyond 4 to 6 percent is dangerous; more than 8 percent risks rupture. You’ll also want to practice quick foot strikes to decrease ground contact time. After training your elastic recoil, you’ll feel like you’re running on coiled springs.
BY PETE MAGILL, RUNNER’S WORLD
MAR 18, 2014