Author(s): YANG, W., ZHAO, Z.1, ZHANG, X.1, LIAO, J.2, CUI, Y.2, Institution: BEIJING SPORT UNIVERSITY, Country: CHINA, Abstract-ID: 2271

INTRODUCTION:
The mechanical properties of the Achilles tendon (AT) are fundamental to elastic energy storage and athletic performance. However, conventional assessment methods are often limited to static, single-point measurements or require cumbersome laboratory setups, failing to capture the tendon's behavior across its functional range of motion. This study aimed to establish a standardized "Functional Stiffness Spectrum" of the AT in elite male athletes using a portable force-ultrasound fusion device and to evaluate its reliability and sensitivity in detecting sport-specific adaptations.
METHODS:
Sixty elite male athletes (ranking top 8 in national-level, e.g., basketball, volleyball, football, sprint, long distance running and tennis) participated. AT shear modulus (G) was quantified using a portable force-ultrasound fusion system (M5, Xijian Technology, Beijing, China) employing a novel biomechanical inverse analysis algorithm to ensure measurement precision. A custom-designed protocol was employed to assess the tendon under six distinct conditions: a relaxed baseline and five fixed ankle angles (0°, 20° plantarflexion (PF), 40° PF, 20° dorsiflexion (DF), 40°DF), stabilized by an adjustable rigid boot. Intra-trial reliability was assessed using the coefficient of variation (CV). Linear mixed models (LMM) were used to analyze the effects of ankle angle, sport type and limb dominance on tendon stiffness.
RESULTS:
The protocol successfully mapped a non-linear increase in stiffness from PF (slack) to DF (tension) (p < 0.001). Crucially, a significant Angle × Sport interaction (p = 0.049) was observed, indicating that stiffness adaptations are angle-specific rather than uniform. Post-hoc analyses revealed that differences were most pronounced in the mid-range of the spectrum: At 20° PF: Basketball players (ES=0.58) and Long-distance runners (ES=0.62 ) exhibited significantly higher tendon stiffness compared to Tennis players (p < 0.05). At Neutral Position (0°): Basketball players continued to display significantly higher stiffness than Tennis players (ES= 0.66). At 40° Plantarflexion (Slack): no significant differences were found among sports, suggesting a convergence of mechanical properties under minimal load. Intra-trial reliability was high (CV < 0.25) within these functional ranges.
CONCLUSION:
This study demonstrates that AT stiffness is not a singular constant but a dynamic spectrum that adapts specifically to sport demands. Basketball players and Long-distance runners possess stiffer tendons at functional plantarflexion angles (20°PF and 0°), likely an adaptation to enhance elastic energy return during repetitive sagittal-plane activities (jumping and running). In contrast, Tennis players exhibit greater compliance in this range, potentially to accommodate multi-directional court movements. Consequently, athletic screening must assess stiffness across the functional spectrum, as single-point measurements may fail to capture these critical, sport-specific mechanical signatures.