Posited as one of the main adaptations that allowed early humans to adopt a terrestrial lifestyle, the Achilles tendon has played a significant role in our evolution. `It is one of the special characteristics in the human body that facilitate our endurance running and it may have assisted in the pursuit of prey`, says Jussi Peltonen and colleagues from the University of Jyväskylä, Finland. However, this critical tendon has an unfortunate tendency to rupture, leading many scientists to query why it is so vulnerable to damage when it has played such an essential role in our survival. Explaining that Achilles tendons rupture when they have lost stiffness, the Finnish team wondered whether long-distance endurance exercise might weaken the tendon, reducing its stiffness to make it increasingly vulnerable to damage (p. 3665). Measuring the stiffness of the Achilles tendons of runners before and shortly after the Finlandia Marathon (42 km) and Half Marathon (21 km), the team found that the tendon's stiffness hadn't changed at all. Despite repeatedly stretching and releasing the enormous tendon over the 20,000 strides that it took most of the competitors to complete the longer course, their Achilles tendon showed no sign of fatigue, maintaining a stiffness of around 200 N mm-1. However, after completing the race, the athletes did show signs of physical fatigue, running less efficiently and consuming more oxygen during a brief run an hour after completing the marathon. In addition, many of the runners had adjusted their running style, with some switching from running on their toes to landing on their heels, while others switched to running with a more flat-footed gait. Overuse-induced injuries have been proposed as a predisposing factor for Achilles tendon (AT) ruptures. If tendons can be overloaded, their mechanical properties should change during exercise. Because there data are lacking on the effects of a single bout of long-lasting exercise on AT mechanical properties, the present study measured AT stiffness before and after a marathon. AT stiffness was determined as the slope of the force-elongation curve between 10 and 80% of maximum voluntary force. AT force-elongation characteristics were measured in an ankle dynamometer using simultaneous motion-capture-assisted ultrasonography. Oxygen consumption and ankle kinematics were also measured on a treadmill at the marathon pace. All measurements were performed before and after the marathon. AT stiffness did not change significantly from the pre-race value of 197±62 N mm-1 (mean ± s.d.) to the post-race value of 206±59 N mm-1 (N=12, P=0.312). Oxygen consumption increased after the race by 7±10% (P<0.05) and ankle kinematic data revealed that in nine out of 12 subjects, the marathon induced a change in their foot strike technique. The AT of the physically active individuals seems to be able to resist mechanical changes under physiological stress. We therefore suggest that natural loading, like in running, may not overstress the AT or predispose it to injury. In addition, decreased running economy, as well as altered foot strike technique, was probably attributable to muscle fatigue. `The Achilles tendon of physically active individuals seems to be able to resist mechanical changes under physiological stress. We therefore suggest that natural loading, like in running, may not overstress the Achilles tendon or predispose it to injury`, says the team, adding that poor-quality footwear and incorrect technique probably pose a more significant threat.
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