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British researchers who repeatedly stretched horses tendons to assess their elasticity and ability to recover have gained fresh insights into the mechanisms behind the stiffening that comes with aging.
The scientists found that the material between tendon fibre bundles stiffens as it gets older. This, they said, was responsible for older people being more susceptible to tendon injuries.
Researchers from Queen Mary University of London, the University of East Anglia, University College London, and the University of Liverpool used horse tendons for their study, which they said was very similar to human tendons.
Experiments in the past have shown that stiffening in aging tendons contributes to increased injuries in older tendons. This new research shows that it is specifically the stiffening and decreased resistance to repetitive loading of the tissue which holds tendon fibre bundles in place that is responsible.
Tendon fibre bundles are surrounded by what is known as the interfascicular matrix (IFM), made up of tissue which enables the fibre bundles to slide past each other and stretch independently.
In horses, the superficial digital flexor tendon (SDFT), which is used to store energy for propulsion, requires greater stretching of the IFM than in the common digital extensor tendon (CDET), which aids the positioning of the leg.
While the IFM is more elastic, recovering better after loading in the SDFT, it is prone to injury in older age because of the greater strain it is put under, and the stiffening and decreased ability to recover that results.
The SDFT is very similar to the human Achilles tendon and so the results of this study can be directly applied to Achilles injury in people.
The new discovery implies that injuries to different tendons could be treated differently and that new treatments could be developed to prevent injuries in older people.
“We now have a much greater understanding of what happens to tendon structure as people get older and the role that plays in injuries,” said Dr Chavaunne Thorpe, from the School of Engineering and Materials Science at Queen Mary University of London.
“This information could be used to develop measures to reduce the risk of tendon injury or to speed recovery.”
The findings, published in the Journal of the Mechanical Behavior of Biomedical Materials, add to a growing body of evidence on the injury risks associated with tendons.
Tendon injuries are common in horses as well as humans, with an economic impact in the British racing industry alone of more than £3 billion a year. Around 16,000 horses are in training each year and the tendon injury rate is as high as 43 per cent, with few horses returning to racing after injury.
The human cost is even higher. Tendon disorders in people cost the British economy more than £7 billion a year.
Thorpe and her colleagues wrote in their open-access paper that there was now increasing evidence to explain how energy-storing tendons were specialised to meet the mechanical demands placed upon them.
“Previous studies suggest that the greater energy storing capacity is provided by a helical structure at the fascicle level, allowing the fascicle to act as springs.
“However, this does not provide the additional extensibility required by this tendon type.
“The data presented in the current, and previous studies, strongly indicate that the high strain capacity of energy storing tendons is provided by interfascicular sliding, whereby the mechanical properties of the IFM allows the tendon to stretch and store energy when loaded in vivo.
“Indeed, this is the first study to show that, when tested in isolation, the IFM exhibits reversible extension behaviour, and has the ability to recover from a number of loading cycles, maintaining its mechanical properties.”
The IFM in the energy-storing SDFT has a greater ability to recover from loading than its counterpart in the positional CDET, they found.
Their research also indicated that the IFM was less able to resist repetitive loading as it aged, becoming stiffer with increasing age in the SDFT.
A full understanding of the process involved was important for the development of effective preventative measures and treatments for age-related tendon injury, they concluded.
‘The interfascicular matrix enables fascicle sliding and recovery in tendon, and behaves more elastically in energy storing tendons’ by Thorpe, C., Godinho, M., Riley, G., Birch, H., Clegg, P. and Screen, H.
Journal of the Mechanical Behavior of Biomedical Materials
The full study can be read here.