An Identical Twin Study on Human Achilles Tendon Adaptation: Regular Recreational Exercise at Comparatively Low Intensities Can Increase Tendon Stiffness

Abstract

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.

The Myometric Assessment of Achilles Tendon and Soleus Muscle Stiffness before and after a Standardized Exercise Test in Elite Female Volleyball and Handball Athletes—A Quasi-Experimental Study

“…Older sedentary controls exhibited higher values of dynamic stiffness (N/m) compared to active controls [ 17 ]. In an identical twin study, myometry detected significantly higher Achilles tendon stiffness in the recreationally active twin compared to the inactive one [ 18 ]. It was also shown that AT stiffness adaptation could be specific to the sport type [ 18 ] due to different locomotion patterns.…”

In Achilles Tendinopathy the Symptomatic Tendon Differs from the Asymptomatic Tendon While Exercise Therapy Has Little Effect on Asymmetries—An Ancillary Analysis of Data from a Controlled Clinical Trial

“…It is well known that in general, exercise activity even at recreational level induces structural (i.e., material, and morphological) adaptations of the tendon which affect the mechanical properties of the structure and cause e.g., increases of tendon stiffness [29,30]. Accordingly, exercise also plays a key role in rehabilitation and therapy of tendinopathic tendons [31][32][33][34].…”