Objective: There is an increasing interest in the application of oscillation-based measurement techniques to evaluate the mechanical stiffness of healthy and diseased tendons. These techniques measure the stiffness of a tendon indirectly by registering the oscillation response of a tendon to an external mechanical impulse. Although these measurement techniques seem to be comparatively easy and time-saving, their applicability is implicitly limited by their indirect measurement principle. Approach: In this study, we aim to find evidence that the oscillation response of a tendon to an external mechanical impulse is not only affected by the stiffness of a tendon but also by the tendons’ cross-sectional area (CSA), length, and tension. Therefore, we reviewed the current literature on oscillation-based techniques that measure in vivo tendon properties. Further, a phantom material was used to mimic the nature of tendons and to test the impact of four factors on oscillation-based measurements. Main results: Our results indicate that the mechanical properties, geometrical dimensions (length and CSA), and tensional state affect oscillation-based measures. Surprisingly, most studies on tendon behavior often exclusively associate their oscillation-based measurements with the mechanical stiffness of a tendon. Significance: While this narrow perspective bears the risk of misinterpretation or false implications, a broader understanding of oscillation-based measurements has the potential to shed new light on the interaction of muscles and tendons in vivo.

Achilles tendon adaptation is a key aspect of exercise performance and injury risk prevention. However, much debate exists about the adaptation of the Achilles tendon in response to exercise activities. Most published research is currently limited to elite athletes and selected exercise activities. Also, existing studies on tendon adaptation do not control for genetic variation. Our explorative cross-sectional study investigated the effects of regular recreational exercise activities on Achilles tendon mechanical properties in 40 identical twin pairs. Using a handheld oscillation device to determine Achilles tendon mechanical properties, we found that the Achilles tendon appears to adapt to regular recreational exercise at comparatively low intensities by increasing its stiffness. Active twins showed a 28% greater Achilles tendon stiffness than their inactive twin (p < 0.05). Further, our research extends existing ideas on sport-specific adaptation by showing that tendon stiffness seemed to respond more to exercise activities that included an aerial phase such as running and jumping. Interestingly, the comparison of twin pairs revealed a high variation of Achilles tendon stiffness (305.4–889.8 N/m), and tendon adaptation was only revealed when we controlled for genetic variance. Those results offer new insights into the impact of genetic variation on individual Achilles tendon stiffness, which should be addressed more closely in future studies.

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