Biomechanics of tendon adaptation

We can get tendons stronger

What has become clear over the last few years is that tendons are super responsive to loading. They will get stronger if you load them in the right way, just like a muscle. The key factor that has gained attention is exercise intensity which leads to more tendon strain, therefore, more adaptation.

However, there is more to it. There are three key mechanisms I want to highlight, including tensile strain, fluid movement and stress relaxation. You can think of them as a cycle. See the figure.

First, you strain the tendon by applying a tensile load.

Second, you get thinning of the tendon or ‘lateral strain’. This thinning depends on Poisson’s ratio which is lateral strain divided by axial strain. Axial strain is just stretch along the axis. Lateral strain is thinning occurs as you stretch it axially. Just like an elastic band gets thinner as you stretch it. This thinning causes pressure on the viscous or fluid part of the tendon. This pressure forces some of the free and bound water to move – either redistributed around the tendon or pushed out of the tendon. The fluid movement may cause shear stresses that contribute to adaptation.

Third, you get a change in mechanical behaviour over time as the tendon is loaded. One expression of this is called stress relaxation, where the tendon strains more for a given force. This viscoelastic behavior is enabled by fluid movement. That is, fluid movement probably enables greater load on the elastic (collagen) components and more sliding at different levels (between fascicles and between fibrils). This stress relaxation leads to more uniform loading of the tendon, but importantly, it also leads to more strain – so you see where are now back to our first mechanism.

All these loads place forces on tenocytes (tendon cells) which then signal to allow the tendon to get stronger. Here are some tips on maximise each of these mechanisms.

Tensile strain: To maximise tendon strain we use high intensity contractions, but they need to be ramped. Contract quickly and the ‘hydraulic stiffness’ mechanism kicks in. That is, the bound water part stiffens, and the elastic collagen component sees less load.

Fluid movement: Think about this as reducing fluid in the tendon to enable more load to be transferred to the elastic collagen part. This is particularly important in pathology where we see increased water content. Current evidence in humans suggests even with 8sec contractions we get significant fluid movement as long as they are intense and we have sufficient time under tension.

Stress relaxation: This is an increased strain for a given force that occurs whilst a contraction is happening. It won’t happen unless we have some fluid shift, which then enables more sliding at different tendon hierarchical levels. Current thinking is longer contractions may be better, but most of the evidence is from animals or basic science studies.

Thanks for reading. My next instalment in this series will be a post on what type of specific exercise strategies to use to maximise tendon adaptation at different stages of rehab for a tendinopathy.

Acknowledgement: I would like to acknowledge that parts of the figure are from the PhD thesis of my past (and very brilliant) PhD student, Eman Merza.