Effects of static stretching on mechanical properties and collagen fiber orientation of the Achilles tendon in vivo

Background: The study was designed to examine changes in tendon properties measured during ramp and ballistic contractions after static stretching and to elucidate the relationship between stretching-induced changes in tendon properties (particularly hysteresis) and collagen fiber orientation. Methods: Thirteen males performed static stretching, in which the ankle was passively flexed to 36 degrees dorsiflexion and remained stationary for 10 min. Before and after stretching, the stiffness and hysteresis of tendon structures in the medial gastrocnemius muscle were measured using ultrasonography during ramp and ballistic contractions. Tendon collagen fiber orientation was also estimated from the coefficient of variation (CV) of echogenicity on transverse ultrasonic images of the Achilles tendon. Findings. The hysteresis of tendon structures significantly decreased by 15.5% (p = 0.005) during ramp contractions and by 15.3% (p = 0.003) during ballistic contractions after stretching, whereas stiffness did not. The mean echogenicity of the Achilles tendon significantly increased by 6.0% (p = 0.002) after stretching, whereas the CV of echogenicity did not (p = 0.148). Furthermore, the relative change in mean echogenicity, which reflected interstitial fluid movement within tendons, tended to be correlated to that in hysteresis measured during ballistic contractions (r = 0.439, p = 0.133). Interpretation: These results suggest that the hysteresis, but not stiffness, of tendon structures measured during ramp and ballistic contractions significantly decreased after stretching. Furthermore, a decline in the hysteresis of tendon structures after static stretching was associated with interstitial fluid movement within tendons, but not to changes in collagen fiber orientation.