## 4 Discussion
### Heterogeneity of Acute Alterations of Viscoelastic Properties  
The heterogeneity of these changes, particularly in AT stiffness, is unexpected. For instance, [PERSON] et al. [23] reported that a 30-min constant-intensity run did not significantly alter [PERSON] tendon stiffness. Relative strain and elongation remained constant during the run, and the tendon length while standing was unchanged post-exercise. In contrast, our study found a significant increase in left AT stiffness and a decrease in right AT stiffness, which may be partially attributable to the progressive increase in intensity until exhaustion. An increase in exercise intensity can impose greater stress on the tendon, potentially altering its post-exercise viscoelastic properties [24]. The decrease in right AT stiffness and its reduced oscillation frequency may signal a decline in the tendon's viscoelastic properties [25]. Although the literature on tendon viscoelasticity is limited, fluid flow and structural interactions within the extracellular matrix during loading are likely to be key factors influencing this behavior [18; 19]. Additionally, tendinopathy--characterized by reduced stiffness and elasticity--has been linked to diminished explosive performance in athletes [26]. The observed differences in tendon response between legs might also reflect varying viscoelastic conditions, possibly due to disturbed movement patterns or unilateral stress during treadmill running, though movement patterns were not assessed in the current study. Supporting these observations, several studies have documented heterogeneous responses in viscoelastic properties across muscle groups. [PERSON] et al. [23] underscored that the tendon's response is highly sensitive to both exercise intensity and duration, while [PERSON] et al. [24] demonstrated that the dynamic viscoelastic behavior of lower extremity tendons is markedly influenced by the specific loading conditions during simulated running. These findings may help explain the asymmetrical changes observed between the dominant and non-dominant limbs in our investigation. The role of muscle fiber composition and neuromuscular recruitment patterns in mediating fatigue-induced changes in tissue stiffness is well documented. [PERSON] [27] detailed cellular mechanisms of muscle fatigue, indicating that muscles with a higher proportion of fast-twitch fibers might exhibit a different response to exhaustive exercise compared to those dominated by slow-twitch fibers. Similarly, [PERSON] and [PERSON] [28] discussed how neuromuscular factors contribute to fatigue, potentially underpinning the differential stiffness alterations observed among various muscle groups. Clinically, the implications of altered tendon stiffness are significant. [PERSON] et al. [26] found that decreased tendon stiffness is associated with tendinopathic conditions, which can compromise explosive performance in athletes. These insights emphasize the need for precise post-exercise monitoring of tissue viscoelastic properties, particularly in elite athletes. Moreover, [PERSON] et al. [10] validated the use of stiffness measurements as objective markers of muscle fatigue, reinforcing the relevance of our methodological approach utilizing the MyotonPRO device. In summary, the inhomogeneous changes observed in tissue stiffness across muscle groups suggest that factors such as exercise intensity, muscle fiber composition, and neuromuscular recruitment patterns may differentially influence the viscoelastic properties of muscles and tendons. Viscoelastic Properties of the Rectus Femoris as an Indicator of Readiness for Exhaustive Exercise Testing  
The present study revealed notable correlations between baseline MyotonPRO measurements and VO2 max, suggesting that non-invasive assessments of muscle viscoelastic properties may serve as potential indicator of maximal endurance performance. Notably, these significant correlations were observed exclusively in the rectus femoris.