## 4 Discussion  
This study explored the complex relationships between the morphological parameters of the quadriceps femoris MTU and ME during various SSC contractions considering that the mechanical properties of tendons are related to the structural arrangement of collagen fibers [32]. We found a significant correlation between the patella tendon CSA and ME during SSC contractions at a lower stretch load, which highlights the importance of tendon morphology in ME. This observation aligns with previous research that suggests that a larger CSA can effectively reduce tendon stress under a given force, potentially mitigating the risk of strain injuries [33]. However, the lack of association between the AmAt ratio and ME suggests that the mechanical properties of the MTU could be more determinant of ME. The stretch load and pretension levels significantly influenced ME, with the highest ME observed at a 50% pretension level combined with a 20 J stretch load, suggesting an  
Figure 4: The results of the correlation analysis. The first row shows the results between the mechanical efficiency (ME) of the four stretch–shortening cycle contractions and the patella tendon (PT) mean cross-sectional area (CSA); the second row shows the results between the ME and PT length; and the third row shows the results between ME and the muscle to tendon cross-sectional area ratio (AmAt). The solid red line indicates the magnitude and direction of the correlation, and the red dashed line represents the corresponding 95% confidence intervals. Blue dots show the individual data points. optimal condition for efficient joint work. Increasing the stretch load generally resulted in lower ME, likely due to lower tendon stress in these conditions. Under a low stretch load, the knee joint flexion remained lower compared with high stretch loads at corresponding pretension levels; hence, the total MTU elongation was lower, assuming a lower strain in the patella tendon as well. In this case, when tendon stress is lower with smaller tendon strain, the tendon stiffening of the patella tendon also decreases [34]. Consequently, the tendon could be more compliant in this condition, which seems to be beneficial for achieving higher ME because reduced tendon stiffness is associated with greater elastic energy storage for a given force applied during SSC contractions [35]. When the stretch load was set to 20 J, less mechanical work was required to stop the servomotor's lever arm which resulted in smaller degree of knee flexion; however, participants with a higher MVC torque could stop the lever arm quicker, indicating that muscle force generation capacity plays an important role, despite the lack of association with muscle volume or PCSA. In low stretch load SSC contractions, the amount of positive work during knee extension was similar, resulting in higher ME at smaller stretch loads. This suggests that the stretch load could influence ME more than the pretension level, even though the pretension level affects the initial muscle length and tension, which should impact SSC performance [20]. Given the smaller knee flexion and increased muscle tension after SSC initiation, it is plausible that the MTU stretch occurred mainly in the tendon, which contributed to higher ME at lower stretch loads [18; 19; 21]. For example, at a 20 J stretch load, where MTU lengthening was less, it is likely that most elongation occurs in the patella tendon while the muscle operates quasi-isometrically. Since isometric contractions consume less metabolic energy while generating more force than shortening contractions [36], this could enhance ME. Contraction near the optimal muscle length (for myofilament overlap) can increase the force output for a given activation level [37], supporting our observation of higher ME under low stretch loads. On the other hand, higher stretch loads require greater mechanical work to stop knee flexion, especially with low knee flexion velocity (i.e., 30\({}^{\circ}\cdot\)s\({}^{-1}\)), which results in a higher knee flexion angle.