Background: Flat feet change lower extremity alignment, and it may change the load distribution on Achilles tendon during exercise. The purpose of the present 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 in the present study. Achilles tendon thickness was measured by an ultrasonography device with a linear probe at 3 points: 1 cm (AT-1), 2 cm (AT-2), and 3 cm (AT-3) 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 points measured was thinner in the flat feet group at both pre- and post-exercise conditions compared with that of the control group (p<0.05). Achilles tendon thickness at AT-1, AT-2, and AT-3 decreased after the exercise in both groups (p<0.001). The differences in Achilles tendon thickness at all points measured between pre- and post-exercise conditions were lower in individuals with flat feet than those of the control group (p<0.05).
Conclusion: There was a significant decrease in Achilles tendon thickness after exercise in both groups; however, the tendon thickness markedly diminished in individuals with normal foot posture. The results are thought to result from changes in tendon structure and in load distribution on the Achilles tendon.