## 3 Results  
The morphological parameters of the quadriceps femoris muscle-tendon unit are presented in Table 1.  
The mean MVC was 95.6 \(\pm\) 17.7 N\(\cdot\)m in 30\({}^{\circ}\) knee flexion. The SSC condition significantly impacted ME (F = 31.03, \(p\) = 0.0003, \(\eta^{2}\) = 0.37), with the highest ME recorded at a 50% pretension level with a 20 J stretch load SSC (54.14 \(\pm\) 2.24%), while the lowest ME was observed at a 100% pretension level with a 100 J stretch load SSC (39.71 \(\pm\) 6.58%) (Table 2). Peak torque also reached its maximum value (207.58 \(\pm\) 27.02 N\(\cdot\)m) at a 50% pretension level with a 100 J stretch load SSC. Interestingly, greater ME was observed with a lower degree of knee joint flexion (Table 2). At 100 J stretch load, the knee flexion was nearly three times greater than that observed with a 20 J stretch load, irrespective of pretension level. The ANOVA results confirm a significant effect of stretch load on knee flexion (F = 214.2, \(p\) < 0.001, \(\eta^{2}\) = 0.40), indicating that stretch load influences knee flexion more strongly than pretension level. This effect appears consistent, as similar knee flexion angles were observed across different pretension levels for the same stretch load. Additionally, mechanical work during the concentric phase and peak torque remained comparable when applying the same stretch load, regardless of the pretension level (Table 1). The peak tendon \begin{table}
\begin{tabular}{c c} \hline
**Morphological Parameter** & **Value** \\ \hline Quadriceps femoris volume (cm\({}^{3}\)) & 1584 \(\pm\) 142 \\ Quadriceps femoris muscle length (cm) & 40.22 \(\pm\) 1.92 \\ Quadriceps femoris PCSA (cm\({}^{2}\)) & 228 \(\pm\) 38 \\ Patella tendon length (cm) & 5.77 \(\pm\) 0.81 \\ Patella tendon mean CSA (cm\({}^{2}\)) & 0.67 \(\pm\) 0.07 \\ Muscle-to-tendon cross-sectional area ratio & 349 \(\pm\) 93 \\ \hline \end{tabular} Abbreviations: PCSA = physiological cross-sectional area, CSA = cross-sectional area.  
\end{table} 
Table 1: The descriptive results (mean and SD) of the calculated mechanical variables for each contractile condition. force was affected by SSC conditions as well (F = 42.3, \(p\) = 0.0001, \(\eta^{2}\) = 0.43) (Table 2). The estimated tendon stress was similar between matched stretch conditions and was almost twofold higher in high stretch load SSC contraction. In line with these results, an increase in both the stretch load and pretension level corresponded with a decrease in ME and an increase in EMG activity during both the eccentric and concentric phases of contraction. Namely, the mean EMG activity was the lowest at a 50% pretension level with a 20 J stretching load across all muscles during the concentric phase, although significant differences were only observed in the biceps femoris and vastus medialis muscles (Figure 3). The EMG activity during the concentric phase gradually increased, reaching its peak with a 100% pretension level and a 100 J stretching load. Additionally, the EMG activity was consistently higher during the eccentric phase compared to the concentric phase, suggesting that the stretch reflex may have been activated within the muscles. For the majority of the muscles, the EMG activity was greater with a 100 J load than with a 20 J load in both phases; however, this was not the case for the vastus medialis muscle. Specifically, during the eccentric phase with a 100% pretension level, the mean EMG activity of the vastus medialis muscle was higher with a 20 J load than with a