Tendon stiffness: what is it and why is it important for runners?
When standing on the sidelines of a running event, you may have noticed some people run past with very springy, bouncy steps. These people look at ease and often run quite fast. Others run past with heavy, more draggy steps, and they’re often not as fast as the former. Why is this?
Although there are many factors that contribute to why this occurs (Running experience, running technique, height, muscular strength etc), a huge part of how ‘springy’ you run comes down to Tendon Stiffness.
What are Tendons?
Tendons attach muscle to bone. They are responsible for working in conjunction with muscles to store and release energy by stretching and recoiling under force. For example, during Initial Contact when your foot hits the ground in running, the Achilles Tendon has to stretch at the ankle joint to absorb the ground reaction forces, then it will naturally recoil with the calf muscles to produce propulsion off the ground. The recoil function of tendons assists with running economy as it is "free energy" and helps to produce force without active muscular contraction.
What is Tendon Stiffness?
Tendon stiffness “describes the relationship between the force exerted on the tendon and its change in the length” (Kubo et al., 1999). In other words, stiffness refers to the resistance of a tendon to change in length. Compliant tendons change in length a lot when a force is applied. Stiff tendons change in length by only a small amount when the same force is applied.
It sounds confusing- But very easy to understand when you think of tendons as elastic bands. Compliant tendons are like stretchy, thin elastic bands. They stretch a lot even with the slightest pull (force), and will recoil to release energy. But due to their excessive stretch, they will recoil with reduced speed and force such as if you were trying to flick a thin, overstretched elastic band at a mate. Stiff tendons are like thick elastic bands. They stretch less when pulled, and will recoil at faster speed and force like a thick elastic band being flicked across a room.
The relationship between Tendon Stiffness & running
People who run with springy steps will often have stiffer tendons than their less bouncy counterparts. This works in their favour because stiffer tendons stretch and recoil quickly and effectively under high loads, so there is a quicker transference of force from muscle to bone. This results in more "free energy", reduced ground contact time (GCT), improved running economy and running speeds (Arampatzis et al., 2007a; and Fletcher et al., 2010).
Upon impact with the ground, stiff tendons will be able to stretch to a small degree, then recoil quickly and powerfully to get the runner up into the air and move forwards onto the next step. In contrast, those with more compliant tendons will have longer GCTs, as their tendons have to stretch further to absorb impact and the recoil function of those tendons aren’t as efficient. In short, they are required to contract their muscles harder with each step so are generally less economical.
How do you train Tendon Stiffness?
Improving tendon stiffness is not only highly effective for running economy and performance, but will also be proactive in protecting the tendons against tendinopathy injuries. As well as that, age-related changes in the body cause reduction in tendon stiffness, so it becomes even more essential to train the tendons throughout the years.
Heavy resistance training
We all know that all types of resistance training improve muscular strength. However, only HEAVY resistance training improves tendon stiffness as well as muscular strength. Unfortunately, light resistance for higher reps (8+ reps) doesn’t have an effect on tendon stiffness. With this in mind, I would suggest building up to heavier weights of 70% 1RM for 1-5 reps in your strength training.
Eccentric strength refers to the muscle’s strength to resist being stretched. With this in mind, it would make sense that increased eccentric strength will improve overall lower limb stiffness for running.
Most gym exercises have a concentric (shortening) and eccentric (lengthening) phase. The concentric phase is often considered the force-producing or ‘hard’ phase, and the eccentric is the stretching or easy phase. You can train eccentric strength by doing training that focuses on the eccentric portion of these exercises, where you are loading up muscles while lengthening.
For example, the ‘up’ portion of a Squat, Lunge, Deadlift, Calf raise, Nordic Curl, Romanian deadlift, Bulgarian split squat, bicep curl, chest press is the concentric phase, and the down portion is the eccentric phase. The down phase is often the stretching or easy portion as you can utilise gravity to relax and drop the weight, but if you were to control the exercise and SLOWLY lower the weight on the way down, you start to work your eccentric strength. I would suggest a 2-4 second duration for the eccentric portions of these exercises to build eccentric strength.
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Abdelsattar M, Konrad A, Tilp M. (2018) Relationship between Achilles Tendon Stiffness and Ground Contact Time during Drop Jumps. J Sports Sci Med. 2018;17(2):223–228.
Arampatzis A., Karamanidis K., Morey-Klapsing G. (2007a) Mechanical properties of the triceps surae tendon and aponeurosis in relation to intensity of sport activity. Journal of Biomechanics 40(9), 1946-1952.
Fletcher J.R., Esau S.P., MacIntosh B.R. (2010) Changes in tendon stiffness and running economy in highly trained distance runners. European Journal of Applied Physiology 110(5), 1037-1046.
Kubo K., Kawakami Y., Fukunaga T. (1999) Influence of elastic properties of tendon structures on jump performance in humans. Journal of Applied Physiology 87(6), 2090-2096.