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Muscle Tendon Springs

Springing Into Action

In actuality, running is a series of leaps and our tendons act as springs.

The ancient Romans understood the elasticity of tendons and hurled rocks on catapults with twisted tendon of animals.

When we run we want our muscles to contract isometrically, that is not change their length upon landing.  We then spring into action.  Tendons are similar to  steel springs that store energy and return energy, (not produce energy) and once stretched propel us forward.

Like all biological tissue tendons can be stretched.  When stretch occurs the tissue resists and exerts an elastic force on the skeletal system.  Though more complex we can simply look at this phenomena as:

The elastic force = stiffness x amount of stretch

For high force eccentric contractions of the calf muscles (tricep surae) during running, the Achilles tendon may be stretched as much as 10% of its resting length.

As mentioned, a muscle and a tendon, like a spring, store and return energy.  The stiffer the muscle-spring, the greater the energy stored.  This translates into better speed and economy of effort.


Strength training along with sprint training increase the chemical cross-links between collagen molecules in the tendon.  The more cross-links the greater the passive stiffness of the tendon.

When running, muscles start to contract before contact with the ground.  This increases the stiffness of our tendons.  This also maximizes the amount of energy we can return to the track.  The timing of this event has everything to do with how the foot is placed to absorb energy and utilize our muscle tendon springs.

The isometrically contracted calf muscle absorbs as much energy as possible when the foot is on the ground.  The foot then goes from flexed to extended during this time due to the tendon being stretched not the muscle being shortened.

The good news is that you do not have to worry about quickly   contracting the muscle during strength training as you only need to build up the passive stiffness of the muscle tendon spring.

Since the stiffness of the muscle tendon spring is controlled by the precise timing of muscle isometric contraction upon landing, practicing your running skill maximizes the stiffness of the passive tendon that you achieved through strength training and skill development.


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