Kjær et al. (2009): structural changes and function in human tendon

Tendons are pretty important. They are the connective tissue that help transfer force from the muscle to create movement of the skeleton, or in fact prevent excess movement if the body is subject to external forces that need to be resisted. Tendons withstand huge mechanical loads, and there tends (haha) to be higher tendon injuries in athletes who do a lot of jumping (example used in the article is elite volley-ball players).

The authors (of which there are many) discuss how tendons respond to loading, with a particular focus on collagen. Collagen is the predominant matrix protein found in tendon tissue. Quite simply, acute exercise increases the rate of collagen synthesis. The more collagen, the stronger the tendon right?

That’s where it’s not so simple. It’s not that clear how exactly the collagen synthesis rate is related to increases in tendon size. The authors cite a study from 1987 of connective tissue in the lung to argue there is indirect evidence that at least some newly formed collagen will end up of the insoluble collagen found in the final tendon structure.

Anyway, back to exercise. In a study of athletes who take part in sports where one leg undergoes more loads than the other – such as fencing and badminton – the more habitually loaded tendons of the stronger limb had a greater cross-sectional area compared to the other leg. There was also greater stiffness in the other leg. From my experience, and other reading, stiffness is an important property in being able to transfer force through the body to the ground. If the achilles tendon is slack for example, you can generate as much downward and backward motion of the leg using the quadriceps, hamstrings, glutes and calves, but the force won’t propel the body forward if it can’t travel through a stiff and strong ankle/achilles structure.

What kind of training could achieve this stiffness then? The article doesn’t go into practical details here like you can find in other literature in the field of strength, conditioning and athletic development. Differences in contractions types and their effect on tendons are highlighted though. First, the link between the growth hormone, IGF-1, and collagen synthesis is established. Then some studies are referenced… IGF-1 was increased in the achilles tendon in response to strength training, but no difference was found between contraction types despite eccentric contractions creating greater force production. In the gastrocnemius muscle though, the lengthening contractions led to significantly greater expression of growth factors compared to the shortening contractions. It appears tendon tissue is protected from fast changed in tissue mass, as an unloading periods have led to no decrease in achilles tendon mass.

However, eccentric training has better effects in rehabilitation from injury. In fact, eccentric rehab exercises led to increased collagen synthesis only in injured tendons. Repair activity is associated to migration and matrix synthesis where cells from the epitenon and endotendon migrate to the site of injury and make a new matrix. The authors argue eccentric loading may stimulate this process, and hence be effective in strengthening the tendon post-injury.

A few other things are covered. Flaws in the methodologies used to study tendon structure is flawed. Animal studies are used, where enzymes are measured as an indirect indication of collagen synthesis. Microdialysis techniques and tendon biopsies are also used, and some others, but one problem may exist in that tendon hypertrophy could be commonly overlooked as it actually occurs in the distal and proximal ends of the tendon and not in the middle where it is often examined.

There’s some more detail at a cellular level as well as a brief discussion of gender differences which led to the growth factor section, but the former didn’t mean much to me and the latter isn’t crucial in the information that followed.

This was a little longer than expected! Gonna have to try and keep the following posts a little shorter. Original article reference is below.


Kjær, M., Langberg, H., Heinemeier, K., Bayer, M.L., Hansen, M., Holm, L., Doessing, S., Kongsgaard, M., Krogsgaard, M.R. and Magnusson, S.P., 2009. From mechanical loading to collagen synthesis, structural changes and function in human tendon. Scandinavian journal of medicine & science in sports, 19(4), pp.500-510.

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