The Achilles Tendon as a Spring
Written By Sam Bugeja, April 2023
The achilles tendon is a magnificent structure. It is the key to the success of any runner. Like so many amazing structures in the human body, the achilles is taken for granted until something goes wrong with it.
Tendons are like ropes which attach muscles to bone. When muscles contract and shorten they pull on the bones and produce movement at our joints. At least that is the way most people think of tendons if they think about them at all. However, tendons and muscles are far more complicated and don’t really function in this simplistic way.
Tendons are actually more like springs. They are able to store and release mechanical energy. In other words they bounce. Think of the achilles tendon which attaches our calf muscles to our heel. It is like a spring not a rope. Each time we hit the ground as we run the achilles “spring” absorbs some of the energy from us hitting the ground and then returns that energy bouncing us up and forwards. Tendons are able to return up to 93% of the mechanical energy. This is amazingly efficient. This spring-like action is the main reason why humans are able to run great distances. If we relied only on our muscles to run without the energy return we get from tendons then we would either be very slow or only able to run very short distances before being completely gassed.
Another great way to imagine the action of the achilles tendon is to think of a superball. If you drop a superball then it will bounce back to almost the same height. Awesome energy return! Tendons, springs, superballs are all viscoelastic. In other words they exhibit viscous and elastic behaviour when deformed. The elastic behaviour is the return of mechanical energy that I mentioned. The viscous behaviour is where some of the mechanical energy is converted to heat energy. You can feel this heat energy if you get a stiff rubber band and stretch it a hundred times. You will notice the rubber band starts to get warm. The same thing happens in your tendons as you run, they get warm. So up to 93% mechanical energy return, the other 7% converted to heat.
In order for our achilles tendon to store and release mechanical energy like this we need for the calf muscle to be stiff. If it was relaxed and loose then the tendon couldn’t work like a spring. So just prior to our foot hitting the ground our calf muscles need to contract and become stiff to act like a strut. There are different types of muscle contractions that can occur. For example they can:
Contract and shorten to produce force and move objects. In this example the muscle is acting as an engine. It is producing work, the energy transferred to or from an object via the application of force along a displacement. This is called a concentric muscle contraction.
Contract and lengthen to act as a brake. The muscle is producing force to slow or stop a movement. This is an eccentric muscle contraction.
Act as a strut. Contracting and stiffening to prevent any change in length. This is called an isometric contraction.
So in order to run efficiently we need our calf muscle to contract isometrically to become stiff, this allows our achilles tendon to act as a strong spring. Of course running is far more complicated than this but the action of the calf and achilles that I have described are a key component to efficient running.