Author(s): NOBUE, A., ODA, T., SANO, K., ISHIKAWA, M., Institution: MORINOMIYA UNIVERSITY OF MEDICAL SCIENCES, Country: JAPAN, Abstract-ID: 2108

INTRODUCTION:
Kendo, a traditional Japanese martial art, is characterized by asymmetrical footwork in which the driving left leg maintains a pre-tensioned "floating heel" position for explosive propulsion, while the leading right leg performs a sharp stomping motion upon striking. Clinically, approximately 90% of Achilles tendon (AT) ruptures in kendo athletes occur in the left leg. Bilateral differences in the mechanical properties of the AT are thought to contribute to this predisposition. However, it remains unclear whether these differences are associated with morphological variations at the AT sub-tendon level.
The AT consists of three sub-tendons originating from the medial gastrocnemius (MG), lateral gastrocnemius (LG), and soleus (SOL), which form a twisted architecture. The purpose of this study was to evaluate bilateral differences in the cross-sectional proportion and torsional morphology of AT sub-tendons in kendo athletes to clarify the structural basis underlying their asymmetrical mechanical properties.
METHODS:
Experienced kendo athletes (n=12; experience >10 years) participated in this study. High-resolution ultrasonography was used to perform a longitudinal sweep scan of the AT from the proximal musculotendinous junction to the distal calcaneal insertion, with participants in a prone position. Utilizing a spectral clustering-based tracking workflow, the MG-AT, LG-AT, and SOL-AT were identified in vivo. The total structural twist angle (the cumulative rotation of each sub-tendon along the scanned length) was calculated and compared between the leading right and the driving left legs.
RESULTS:
The MG-AT exhibited the highest twist degree among the three sub-tendons in both legs. Significant bilateral differences were observed in the LG-AT and SOL-AT twist angles. The LG-AT twist angle was significantly greater in the left leg than in the right leg (p<0.05). The SOL-AT twist angle was greater in the right leg than in the left leg (p<0.05). The cross-sectional area (CSA) of the MG-AT was significantly smaller in the right leg than in the left leg (p<0.05).
CONCLUSION:
The high degree of MG-AT twist observed in kendo athletes exceeds values reported for the general population, suggesting a sport-specific adaptation. This likely reflects structural remodeling in response to their characteristic rapid, barefoot movements. Furthermore, the greater LG-AT twist in the left leg likely reflects the torsional demands of the explosive "kick-off" motion, while the smaller MG-AT CSA and greater SOL-AT twist in the right leg may result from the repeated stomping motion upon striking. These findings highlight that long-term specialized training can induce sub-tendon-level morphological plasticity.