Author(s): YAM, T., CHAN, L., FUNG, J., NGAI, E., LEUNG, V. , Institution: HONG KONG METROPOLITAN UNIVERSITY, Country: HONG KONG, Abstract-ID: 1287

INTRODUCTION:
The Achilles tendon (AT) demonstrates exceptional elasticity and resilience during high-intensity loading. However, its acute mechanical response to fatigue remains unclear. Shear wave elastography (SWE) allows non-invasive, quantitative assessment of tendon stiffness and may provide new insights into tendon behavior under fatigue. This study aimed to investigate fatigue-induced changes in AT and medial gastrocnemius (MG) stiffness and examine their relationships with plantarflexor performance.
METHODS:
Twenty-eight physically active adults (21 males, 7 females; mean age = 23.5 ± 4.9 years) participated. AT and MG stiffness were evaluated using SWE with the ankle fixed at 5° dorsiflexion. Functional performance was assessed using maximum voluntary isometric contraction (MVIC), rate of force development (RFD), relative strength index (RSI), and jump height (JH). Participants completed a single-leg heel raise fatigue protocol at 60 beats per minute with the knee fully extended. Testing ceased after two consecutive failed raises or loss of rhythm. Measurements were taken pre-fatigue (PRE), immediately post-fatigue (POST), and after 20 minutes of rest (REC).
RESULTS:
AT stiffness increased significantly immediately after fatigue (PRE: 3.74 ± 1.17 m/s; POST: 4.12 ± 1.47 m/s; p = 0.002), accompanied by higher RFD (p < 0.001) and reduced RSI (p = 0.031). After 20 minutes of recovery, AT stiffness and RSI remained stable (p > 0.1), while RFD decreased (p = 0.025). MG stiffness and JH showed no significant changes across timepoints (p > 0.1). Although AT stiffness did not correlate significantly with RSI or RFD (p > 0.1), a positive correlation with JH was observed at REC (r = 0.40, p < 0.05).
CONCLUSION:
SWE revealed acute increases in AT stiffness following a fatigue protocol, indicating short-term tendon adaptation to repetitive loading. Despite altered mechanical properties, jump performance remained stable, suggesting compensatory mechanisms within the muscle–tendon unit. These findings support SWE as a rapid and sensitive tool for evaluating tendon responses to fatigue. Future research should explore its application in athlete monitoring, rehabilitation, and return-to-play decision-making.