The grayscale distribution on the ultrasonic images of tendons may be reduced with alignment of collagen fibers, because ultrasound signal intensity changes with alterations in tendon collagen fiber orientation due to acoustic anisotropy in the tendons. The purpose of this study was to investigate changes in the coefficient of variation (CV) of echogenicity in the Achilles tendon during passive dorsiflexion (the angle task) and isometric plantar flexion (the contraction task). Achilles tendon transverse ultrasonic images were collected from 14 healthy individuals every 10° from 20° to -20° (positive values for plantar flexion) in the angle task and every 10% maximum voluntary contraction (MVC) from 0% to 70% MVC in the contraction task. The CV of echogenicity was measured in each image. In addition, relative changes in the measured variables between the former half (20-0° in the angle task, 0% to 30% MVC in the contraction task) and the latter half (0° to -20° in the angle task, 40% to 70% MVC in the contraction task) of each task were compared. The CV of echogenicity decreased with increases in the dorsiflexion angle and intensity of isometric contractions. Furthermore, relative changes in the CV of echogenicity were greater at more dorsiflexed positions in the angle task and at lower torque levels in the contraction task. These results suggested that decreases in the CV of echogenicity were partially related to the alignment of the tendon collagen fibers with tendon stretching.
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