University of Jyväskylä

Dissertation: 19.12 Achilles tendon viscoelastic properties and mechanical responses to a single bout of exercise (Peltonen)

Start date: Nov 19, 2014 12:00 AM

End date: Dec 19, 2014 03:00 PM

Location: Mattilanniemi, Agora, Auditorio 2

Jussi Peltonen. Kuvaaja: Katja PylkkänenM.Sc. Jussi Peltonen defends his doctoral dissertation in Biomechanics. Opponent Professor S. Peter Magnusson (University of Copenhagen) and custos Professor Janne Avela (University of Jyväskylä). Doctoral dissertation in the English language.


When excised tendons are cyclically loaded, their maximum tensile strength is reduced. This is known as tendon fatigue and is accompanied by a reduction in stiffness, the gradient of the force-length curve. As tendon fatigue has been linked to tendon damage, it may constitute a mechanism for tendon overuse injuries. The current thesis investigated Achilles tendon (AT) fatigue by measuring its stiffness after two different exercise bouts that were characterized by high-intensity and long-duration. In addition, tendon viscoelastic properties were investigated. As viscous properties arise from internal friction, they are responsible for production of heat and increased temperature of the tissue. This may also damage the tendon. Thus the relation between AT hysteresis and increase of temperature was investigated. AT stiffness and hysteresis were derived from the force-length curves obtained in an ankle dynamometer during voluntary contractions up to 80 % of maximum force. AT force was calculated from measured plantar flexion force and ankle lever arm lengths, and AT length was determined using motion capture assisted ultrasonography. Increase of AT skin temperature was assessed with infrared thermography imaging. The results showed that AT stiffness was unchanged after high-intensity hopping exercise and after a marathon run. Previously, tendon fatigue has been demonstrated in isolated AT’s and in vivo after isometric loading in the patella tendon. In hopping and running, AT is unloaded once every movement cycle, and is therefore not exposed to creep stress. This may explain why tendon fatigue was not induced in the current thesis, and why it may be difficult to impose during natural movements. The results of viscoelastic studies showed that AT did not demonstrate strain rate sensitivity within a range of 2–10% s-1. However, AT did demonstrate hysteresis (mean: 9%), and larger hysteresis was related to a larger increase of AT skin temperature during 15 stretch-shortening cycles. Thus, the high variation in hysteresis that is typically observed in vivo could be of physiological origin rather than a measurement artefact. The current results provide support for the idea that some tendons are at risk of elevated temperature, which may damage the tendon. The current thesis increases understanding of the mechanisms of physical activity-related tendon injuries.

Keywords: stiffness, hysteresis, ultrasonography, tendon fatigue, fatigue damage, hyperthermia

The dissertation will be published in the series Jyväskylä Studies in Sport, Physical Education and Health, 215. ISSN 0356-1070; 215; ISBN 978-951-39-5968-5. It is available at the University Library’s Publications Unit, tel. +358 40 805 3825,

  • Further information:

Jussi Peltonen, +358 40 726 5247,

Communications intern Birgitta Kemppainen, +358 40 805 4483,