Immediate Effect of Cumulative Transverse Strain via Exercise on the Achilles Tendon in Individuals with and Without Flat Feet

Background: Flat feet change lower-extremity alignment, and they may change the load distribution on the Achilles tendon during exercise. The purpose of this study was to investigate the immediate effect of cumulative transverse strain via resistive ankle plantarflexion exercise on the Achilles tendon in individuals with flat feet. Methods: Fourteen individuals with flat feet and 14 age-matched individuals with normal foot posture were enrolled. Achilles tendon thickness was measured by an ultrasonography device with a linear probe at three points: 1, 2, and 3 cm proximal to the superior aspect of the calcaneus. Ultrasonography measurements were performed before and after participants completed 90 repetitions of double-leg calf raise exercises, which included moving the foot from full ankle dorsiflexion to full ankle plantarflexion. Results: Achilles tendon thickness at all three points measured was thinner in the flat feet group at preexercise and postexercise conditions compared with that of the control group ( P < .05). Achilles tendon thickness at all three points decreased after the exercise in both groups ( P < .001). The differences in Achilles tendon thickness at all three points measured between preexercise and postexercise conditions were lower in individuals with flat feet than in those of the control group ( P < .05). Conclusions: There was a significant decrease in Achilles tendon thickness after exercise in both groups; however, tendon thickness markedly diminished in individuals with normal foot posture. The findings are thought to result from changes in tendon structure and load distribution on the Achilles tendon.