Aging and the effects of a half marathon on Achilles tendon force-elongation relationship

Purpose: We aimed to determine whether there are different changes in Achilles tendon (AT) mechanical properties in middle-aged, compared to younger runners that might indicate that tendon fatigue, induced by long-distance running, is age-dependent. Methods: 27 middle-aged (50–67 years) and 22 younger (21–29 years) participants ran a 21 km route at their own pace (mean and SD: old: 3.1 ± 0.3 m s ⁻¹; young: 3.6 ± 0.5 m s ⁻¹). We tested for changes in the AT force–elongation relationship using dynamometry and ultrasonography during isometric voluntary ankle plantarflexion ramp contractions, conducted 20–28 h pre-run, immediately pre-run, immediately post-run and 20–28 h post-run. Stride frequency and number were examined to estimate cyclic tensile loading characteristics of the tendon during running. Results: Muscle strength decreased significantly (P < 0.05) in both groups immediately post-run (old: 17 %; young: 11 %) and recovered to baseline within 20–28 h post-run. AT stiffness did not change for the younger adults, whereas the middle-aged adults showed a significant (P < 0.05) decrease in AT stiffness (22 %). However, tendon stiffness recovered to baseline 20–28 h post-run. Middle-aged, compared to young adults, demonstrated significantly (P < 0.05) greater stride frequency and number, but no correlations with tendon fatigue changes were determined (R ² ≤ 0.038). Conclusions: The results suggest that the plasticity of the AT in response to short-term mechanical loading may be age dependent and that the AT length–tension properties of middle-aged runners may be more vulnerable to change following running compared to younger athletes. However, the observed AT changes in the middle-aged runners dissipated within 20–28 h post-run, suggesting that a tendon viscoelastic recovery mechanism may occur in vivo.