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.
Highlights
In humans, the gastrocnemius muscle inserts together with the soleus muscle via a well-developed Achilles tendon onto the calcaneus (Swindler and Wood, 1973; Standring et al, 2016)
The present study was primarily designed to determine the effect of regular recreational exercise activity on Achilles tendon stiffness
We identified a large variability in tendon stiffness among twin pairs, which seems at least partially genetically predetermined
Summary
The gastrocnemius muscle inserts together with the soleus muscle via a well-developed Achilles tendon onto the calcaneus (Swindler and Wood, 1973; Standring et al, 2016). The chimpanzees (Pan troglodytes) and gorillas (Gorilla gorilla), the gastrocnemius muscle inserts almost immediately onto the calcaneus, so that the Achilles tendon is barely visible (Swindler and Wood, 1973; Myatt et al, 2011). Several studies consider the well-developed Achilles tendon as an adaptation that fosters energy-efficient locomotion (Alexander, 1984, 1991), during bipedal running (Bramble and Lieberman, 2004). Achilles Tendon Adaptation in Twins element, which is stretched and loaded with strain energy during the initial phase of stance and recoils during the late phase of stance to support the foot push-off (Hof et al, 2002; Arampatzis et al, 2006; Lichtwark et al, 2007). While the evolutionary advantage of this adaptation seems generally accepted (Bramble and Lieberman, 2004), debate continues about structural adaptations of the Achilles tendon in response to exercise-related mechanical loading
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