In running, speed doesn’t just depend on how strong your muscles are, but on how stiff and reactive your tendons are in absorbing the impact with the ground and then returning it as a “free push.”
- Running doesn’t mean pushing off your muscles with every step, but utilizing a continuous series of bounces to move forward.
- The Achilles tendon acts exactly like a large spring: it stretches when your foot hits the ground and snaps back violently when you push off.
- A “stiff” (i.e., well-trained) tendon is like a tightly wound spring: it reduces the time your foot stays in contact with the asphalt, making you run faster.
- Tendon work is free energy: unlike muscles, tendons don’t consume oxygen or sugars to generate this propulsion.
- Tendons, being made of collagen and receiving little blood flow, strengthen much more slowly than muscles. Patience is crucial to avoid injuries.
Running as a Sequence of Elastic Bounces
When we think about running, we often imagine an engine (the muscles) working continuously to push us forward. In reality, from a biomechanical standpoint, running is much more akin to the movement of a bouncing rubber ball. It is a continuous series of leaps from one foot to the other.
If we had to use exclusively muscular force to lift ourselves off the ground with every single step, we would get tired after just a few hundred yards. The secret to human efficiency lies instead in our ability to harness the energy of impact. In fact, instead of dissipating force when the foot lands, our body is designed to capture it and use it for the next step.
The Structure of the Achilles Tendon: Storing and Releasing Energy
The main actor in this energy-saving mechanism is the Achilles tendon, the “cable” that connects your calf muscles to your heel.
Imagine it as a heavy-duty rubber band or spring. When your foot lands on the ground, your body weight and gravity push it downward. In that moment, the Achilles tendon stretches, storing all the energy of the impact (like drawing back a slingshot). A split second later, during the push-off phase, the tendon snaps back, violently releasing the stored energy and catapulting you forward. It is a true “elastic recoil” that does much of the hard work for you.
Ground Contact Times and Tendon Stiffness
To run truly fast, the most important quality a tendon must possess isn’t flexibility, but “stiffness.” A stiff tendon isn’t a locked tendon, but one capable of resisting deformation and snapping back in a fraction of a second.
If your Achilles tendon is too soft, it will behave like a loose rubber band: it will take too long to stretch, and the energy will dissipate. Consequently, your foot will remain glued to the asphalt for too many milliseconds. If, on the other hand, the tendon is stiff and reactive, ground contact time drops drastically. The less time you spend on the ground, the faster your body is projected into the air toward the next step. Elite marathoners fly over the asphalt precisely because their ground contact times are extremely low.
The Energetic Difference Between Muscular Push and Elastic Recoil
The beauty of tendons is that their work is completely free for your metabolism.
When a muscle contracts, it consumes “fuel” (sugars) and oxygen, producing fatigue and lactic acid. The tendon, however, is a mechanical element: it doesn’t burn calories to stretch and snap. Optimizing the elastic recoil of your tendons means being able to maintain much faster running paces at the same heart rate and breathing effort. Essentially, you are teaching your body to use its springs instead of squeezing its engines.
The Biological Limit of Collagen Adaptation
If tendons are so fantastic, why can’t we make them super strong in just a few weeks? The answer lies in their biological structure. Muscles are rich in blood vessels, receive plenty of nourishment, and can grow and adapt within a few weeks of training.
Tendons, on the other hand, are made primarily of collagen and are poorly vascularized tissues. Their metabolism is very slow. If you increase your mileage or speed too quickly, your muscles might be ready, but your tendons are not. This imbalance is the primary cause of inflammation (like tendinitis). Strengthening and “stiffening” tendons requires time, consistency, and the gradual introduction of specific workouts like static exercises against a wall or jumping, always respecting the body’s slow recovery times.