Influence of neglecting the curved path of the Achilles tendon on Achilles tendon length change at various ranges of motion

Achilles tendon length has been measured using a straight-line model. However, this model is associated with a greater measurement error compared with a curved-line model. Therefore, we examined the influence of neglecting the curved path of the Achilles tendon on its length change at various ranges of motion. Ten male subjects participated in this study. First, the location of the Achilles tendon was confirmed by using ultrasonography, and markers were attached on the skin over the Achilles tendon path. Then, the three-dimensional coordinates of each marker at dorsiflexion (DF) 15°, plantarflexion (PF) 0°, PF15°, and PF30° were obtained. Achilles tendon length in the curved-line model was calculated as the sum of the distances among each marker. On the other hand, Achilles tendon length in the straight-line model was calculated as the straight distance between the two most proximal and distal markers projected onto the sagittal plane. The difference of the Achilles tendon length change between curved-line and straight-line models was calculated by subtracting the Achilles tendon length change obtained in curved-line model from that obtained in straight-line model with three different ranges of motion (i.e., PF0°, PF15°, and PF30° from DF15°, respectively). As a result, the difference in Achilles tendon length change between the two models increased significantly as the range of motion increased. In conclusion, neglecting the curved path of the Achilles tendon induces substantial overestimation of its length change when the extent of ankle joint angle change is large.