Detailed understanding of tendon mechanical properties – and change thereof if injured – is essential for ensuring maximal performance, especially about key tendons such as the patellar and Achilles tendon. Dr Hans-Peter Wiesinger and his research team from the University of Salzburg, Austria, look at the effects of tendinopathy on the mechanical properties of injured tendon tissues to improve treatment methods. They also find that long-term training influences tendon viscoelastic properties as the healthy tendon adapts to different stress scenarios to preserve an ultimate strength and to better resist fatigue. This is particularly important for sport-specific contexts.
Tendons are crucial tissues of the body for movement. They are the connection between the muscle and the bone and withstand considerable tension. Despite the main function of the tendon to transmit force from the muscle to the bone, there are other functions which make the body movements flawless. For instance, they stretch, thus store and release elastic energy to aid in muscle power output amplification or shock absorbance for muscle protection from damage. Tendons also provide rapid mechanical feedback for stability.
Tendons are fibrous connective tissues consisting of collagen; healthy tissues have no sensory nerves deep inside them, but only in the periphery. Tendon injuries, known as tendinopathy, happen very often, at a rate of almost one in every two sports- or occupation-related injuries. Nerve ingrowth is frequently observed in this condition which potentially triggers a pain response. People who are exposed to a high level of load on their muscles and tendons from repeated movements on a daily basis in their occupations, such as professional athletes like gymnasts or basketball players, are at an increased risk of tendon injury.
Injuries usually happen from overusing or ageing of the tendons. The precise mechanisms of tendinopathy occurrence, however, are still not fully understood. Healing of the tendons varies a lot between the different types of tendons as well as across different individuals. People might be prescribed anything from oral medication and local injections, through abstaining from activity, to surgeries or shockwave therapies.
Unpredictably, tendons in some people are able to heal and in others are not. A lot of studies have addressed this inconsistency, but there is still no agreed explanation as to why this is happening. Moreover, scientists still do not fully understand how chronic tendinopathy affects tissue function and integrity and what consequences tendinopathy has on different sports and activities.
Dr Hans-Peter Wiesinger from the University of Salzburg, Austria, and his team of researchers have taken up the difficult task to investigate the underlying mechanism of tendinopathy and suggest a novel method for treating the condition – the whole body vibration training. They are working with world-class sports players in vivo, such as volleyball and water polo players, ski jumpers, and runners, to study the short- and long-term changes in the mechanical, morphological, and material properties of the tendons used the most in these sports. The research team focus predominantly on the patellar and the Achilles tendons as models, taking into account the whole muscle-tendon unit when they study the loading patterns and the properties of the tendons.
The patellar and Achilles tendons are two of the highest loaded tendons in the human body and can be relatively easily studied using ultrasonography. The patellar tendon attaches at the bottom of the kneecap to the top of the shinbone and enables the front thigh muscles to extend the knee for movements like kicking, running, or jumping. The Achilles tendon is the largest and strongest tendon in the body, and it helps when the calf muscle flexes to pull the heel for movements such as walking, running and jumping.
Tendon properties include morphological, mechanical and material properties. Hence, Dr Wiesinger and his team measured the cross-sectional area (CSA) of the patellar and Achilles tendon (the transverse section through the tendon) as well as the stiffness (how much the tissue elongates under a given load), hysteresis (how much elastic energy can be used) and Young’s modulus (which characterises material properties as a measure of stiffness when tendon dimensions are taken into account). Additionally, they related the tendon properties to the corresponding muscle architecture to characterise whole muscle-tendon units.
Tendon loading and injuries
Constant tendon overloading is one of the main explanations scientists have for injuries in tendons. The group from Salzburg believe that injuries occur when a load is applied on a tendon that exceeds its capacity to synthesis relevant proteins to repair degradation processes after exercise. Tendon loading of high injury risk is usually associated with any activity that requires the tendon to store and release energy like a spring. It includes activities such as landing from stop jumping, running, or hopping. Other activity types such as heavy lifting, leg presses, or calf raises do not stress the tendons as much, as they apply different types of pressure onto the muscle-tendon unit.
There has been evidence, however, that specific types of loading could have beneficial long-term effects on tendons and might could be crucial for injury treatment. In one of their reviews, Dr Wiesinger and colleagues compared adaptations of some tendon properties to increased mechanical loading. They concluded that healthy tendon tissue adapts systematically to training. After a heavy workout, for example, the synthesis of collagen increases. The increased load during training acts as stress over the tendon and changes its morphology, increasing the stiffness of the tendon. These changes strengthen the tendon to maintain an efficient but safe movement in stressful situations.
Moreover, Dr Wiesinger and his team came to the conclusion that chronic patellar tendinopathy did not have a significant effect on knee extension strength. Patellar tendinopathy also did not affect the strain, hysteresis, and energy storage of the tendon. However, the condition decreased the tendon stiffness, stress and Young’s modulus.
Effects of whole-body vibration
The researchers looked at an alternative strategy for tendon recovery – the whole-body vibration training. Dr Wiesinger investigated what effects the whole-body vibration would have on the patellar tendon cross-sectional area, stiffness and Young’s modulus of the healthy tissue. They found that a single bout of vibration does not produce any changes in these properties. However, they showed that 8 weeks of whole-body vibration training remodels the tendon based on an anabolic response, which increases the tendon cross-sectional area along with muscle strength (Rieder et al., 2016b). The researchers, together with Dr Florian Rieder from the Paracelsus Medical University Salzburg, are currently investigating a course of whole-body vibration training in patients with chronic patellar tendinopathy to explore any property, functional and pain-related changes as it might be the answer to effective treatment.
Tendon properties in different sports
In order to gain practicality on their results, Dr Wiesinger and his team worked with professional athletes to determine how particular tendon properties are developed in diverse sports. What Dr Wiesinger and colleagues found was that ski jumpers had a 21% bigger CSA of the patellar tendon, 13% bigger Achilles tendon CSA, and 11% stiffer patellar and 27% stiffer Achilles tendon compared to sedentary individuals. Runners had a 26% increase in the cross-sectional area of the patellar tendon in comparison with the sedentary individuals. These sports were classified as high-intensity excercises and the larger CSA seen in sportsmen doing these activities is consistent with findings of other studies.
On the other hand, water polo players showed completely opposite results: their patellar and Achilles tendon cross-sectional areas were smaller than the controls with 24-28%. Despite the high number of loading cycles on their bodies, the researchers speculated that water polo players still show thinner tendons because the impact on tendons and the muscle contractions are reduced in water environment.
These results point to the fact that tendons are adaptable and change their properties according to load and function. Thinner tendons are thought to operate at higher stress levels having more capacity to store and release elastic energy, whereas thicker tendons usually serve for safety. Consequently, could specialised training affect the tendon’s capacity to store and release energy? In other words, can the tendons be trained? Dr Wiesinger’s research group investigated this question as well, addressing the energy storage, hysteresis, and strain energy recovery of the knee extensor and plantar flexor muscle-tendon units.
The scientists found that the CSA ratio of muscle-to-tendon was 31-33% lower in the knee extensors of ski jumpers and runners than in water polo players. There was no difference in the energy storage between the groups. However, the hysteresis was about 30% lower in the ski jumpers and runners than in controls. This meant that some activities can change the mechanical properties of tendons, such as lowering the hysteresis of the tendons, resulting into the tendon to retain and utilise more elastic energy and adapt to the different conditions.
Overall, tendinopathy is a very common pathology on the tendons that needs to be addressed early, even though only about 4% of the people diagnosed with it actually develop extreme injury, such as a rupture. It causes persistent daily pain, functional limitations, or in some cases an ending of a sporting career. Dr Wiesinger and his team of researchers are only a few of the people who have dedicated their time to seek solutions for the condition, paying attention to sports-specific particularities.
The Austrian research team are calling scientists all over the globe to combine efforts and focus on finding how differences in the tendon properties can be used to predict tendinopathy occurrence across disciplines. Investigating the impact of lower patellar stiffness on how the muscles function is also another key point in the further research that needs to be done.
- Chan, J.J., Chen, K.K., Sarker, S., Hasija, R., Huang, H.H., Guzman, J.Z., Vulcano, E. (2020). Epidemiology of Achilles tendon injuries in collegiate level athletes in the United States. International Orthopaedics, 44(3), 585-594. Available at: https://doi.org/10.1007/s00264-019-04471-2
- Wiesinger, H.P., Seynnes, O.R., Kösters, A., Müller, E., Rieder, F. (2020). Mechanical and material tendon properties in patients with proximal patellar tendinopathy. Frontiers in Physiology, 11, 704. Available at:
- Wiesinger, H.P. et al. (2017). Sport-specific capacity to use elastic energy in the patellar and Achilles tendons of elite athletes. Frontiers in Physiology, 8, 132. Available at: https://doi.org/10.3389/fphys.2017.00132
- Rieder, F., Wiesinger, H.P., Kösters, A., Müller, E., Seynnes, O.R. (2016a). Immediate effects of whole body vibration on patellar tendon properties and knee extension torque. European Journal of Applied Physiology, 116, 553-561. Available at: https://doi.org/10.1007/s00421-015-3316-4
- Rieder, F. et al. (2016b). Whole-body vibration training induces hypertrophy of the human patellar tendon. Scandinavian Journal of Medicine & Science in Sports, 26(8), 902-10. Available at: https://doi.org/10.1111/sms.12522
- Wiesinger, H.P., Rieder, F., Kösters, A., Müller, E., Seynnes, O.R. (2016). Are sports-specific profiles of tendon stiffness and cross-sectional area determined by structural or functional integrity? PLoS ONE, 11(6), e0158441. Available at: https://doi.org/10.1371/journal.pone.0158441
- Wiesinger, H.P., Kösters, A., Müller, E., Seynnes, O.R. (2015). Effects of increased loading on in vitro tendon properties: a systematic review. Medicine & Science in Sports & Exercise. Available at: https://doi.org/10.1249/MSS.0000000000000603
- Jung, H.J., Fisher, M.B., Woo, S.L.Y. (2009). Role of biomechanics in the understanding of normal, injured, and healing ligaments and tendons. Sports Medicine, Arthroscopy, Rehabilitation, Therapy & Technology, 1(1), 9. Available at: https://doi.org/10.1186/1758-2555-1-9
Prof Wiesinger examines the underlying mechanism of tendinopathy and suggests a promising new recovery strategy.
The Austrian Science Fund (FWF; KLI585B30)
- Olivier Seynnes – Department of Physical Performance, Norwegian School of Sport Sciences, Oslo, Norway
- Florian Rieder – Institute of Physical Medicine and Rehabilitation, Paracelsus Medical University, Salzburg, Austria
- Yusuke Sato – Department of Orthopedic Surgery, Chiba University Hospital, Chiba, Japan
Dr Hans-Peter Wiesinger graduated in sports science, law, and economics at the University of Salzburg, where he teaches in various courses, including Sports Science and Physiotherapy or Journalism. He was responsible for EU projects and received an Austrian Science Fund. With this grant,
Dr Wiesinger continued researching the patellar and Achilles tendon.
MMMag Dr Hans-Peter Wiesinger
University Assistant (PostDoc)
Department of Sport and Exercise Science
University of Salzburg