Dynamic changes in the moment arm of the Achilles tendon during weight bearing conditions measured by dual fluoroscopy imaging system.

BACKGROUND

There are various methods used to estimate the length of the Achilles tendon moment arm, but these methods often provide substantially different results. Using radiographic images can help with better localization of the bones, but it is often used during static conditions. Using a dual fluoroscopy imaging system enables us to obtain in vivo data during dynamic movement, which could provide a more detailed description of the ankle joint position-dependent length changes of the Achilles tendon moment arm angle.

METHODS

Ten healthy male university students volunteered to participate in this study. Fluoroscopic images of the ankle during a task mimicking the stance phase of walking were captured under barefoot conditions. Achilles tendon moment arm length was estimated from sagittal-plane dual fluoroscopic images by identifying the line of action of the tendon and computing the center of rotation as the center of a best-fit circle applied to the talar dome. The moment arm was normalized to calcaneus length.

RESULTS

Achilles tendon moment arm increased its length from 4.4 ± 0.3 cm at -20° dorsiflexion to 5.1 ± 0.4 cm at 0° ankle joint position and further increased its length to 5.8 ± 0.5 cm at 40° plantarflexion ankle joint position. The regression model showed a moderate polynomial association between moment arm length and ankle joint position (R2=0.55).

CONCLUSION

The observed parabolic relationship between Achilles tendon moment arm and angle under dynamic conditions is in line with previous reports using static and unloaded conditions. However, the high inter-individual variation in normalized moment arm length prevents establishing a general model that could estimate the joint position dependent length changes of the moment arm.