## 4 Discussion  
The purpose of this study was to relate tissue stiffness (i.e., muscle stiffness or tendon stiffness) with DJ and CMJ performance. We hypothesized that DJ performance would be related to Achilles tendon stiffness, gastrocnemius medialis stiffness, and gastrocnemius lateralis stiffness. Moreover, we assumed a relationship between CMJ performance to patellar tendon stiffness, rectus femoris stiffness, vastus medialis stiffness, and vastus lateralis stiffness. Against our hypotheses, the correlation analysis showed no significant association between DJ performance parameters (contact time, jump height, RSI) and Achilles tendon stiffness or muscle stiffness of both gastrocnemius. Similarly, there was no significant association between CMJ performance parameters and patellar tendon stiffness, rectus femoris stiffness, vastus medialis stiffness, or vastus lateralis stiffness. A previous study [12] reported a positive moderate relationship between gastrocnemius medialis stiffness and some (jump height and RSI) but not all (contact time) DJ parameters. However, no such correlation was seen in the soleus muscle. The authors \begin{table}
\begin{tabular}{c c c c c} \hline  & **rP** & **CI 95\%** & \(p\) & **ES** \\ \hline GM—DI\_CT & **0.24** & (\(-\)0.29–0.66) & 0.38 & 0.24 \\ GM—DI\_RSI & **0.02** & (\(-\)0.48–0.51) & 0.94 & 0.02 \\ GM—DI\_JH & **0.13** & (\(-\)0.39–0.59) & 0.62 & 0.13 \\ GL—DI\_CT & **0.16** & (\(-\)0.37–0.61) & 0.56 & 0.16 \\ GL—DI\_RSI & **0.02** & (\(-\)0.48–0.51) & 0.93 & 0.02 \\ GL—DI\_JH & **0.08** & (\(-\)0.43–0.56) & 0.76 & 0.08 \\ AT—DI\_CT & \(\mathbf{-0.07}\) & (\(-\)0.54–0.44) & 0.81 & \(-\)0.07 \\ AT—DI\_RSI & **0.11** & (\(-\)0.41–0.57) & 0.70 & 0.11 \\ AT—DI\_JH & **0.09** & (\(-\)0.42–0.56) & 0.73 & 0.09 \\ \hline \end{tabular}
\end{table} 
Table 2: [PERSON]’s correlation(=rP) between drop jump (DJ) performance values (CT—ground contact: RSI—reactive strength index; JH—jump height) and the stiffness of the gastrocnemius medialis (GM), gastrocnemius lateralis (GL), and Achilles tendon (AT). The correlation is in bold; CI—confidence interval; ES—effect size. \begin{table}
\begin{tabular}{c c c c c} \hline  & **rP** & **CI 95\%** & \(p\) & **ES** \\ \hline PT—CMJ\_JH & \(\mathbf{-0.03}\) & (\(-\)0.52–0.48) & 0.92 & \(-\)0.03 \\ PT—CMJ\_SD & \(\mathbf{0.36}\) & (\(-\)0.17–0.72) & 0.18 & 0.36 \\ PT—CMJ\_RSImod & \(\mathbf{-0.04}\) & (\(-\)0.52–0.47) & 0.90 & \(-\)0.04 \\ VL—CMJ\_JH & \(\mathbf{-0.02}\) & (\(-\)0.51–0.48) & 0.95 & \(-\)0.02 \\ VL—CMJ\_SD & \(\mathbf{-0.19}\) & (\(-\)0.63–0.34) & 0.48 & \(-\)0.19 \\ VL—CMJ\_RSImod & \(\mathbf{-0.13}\) & (\(-\)0.59–0.39) & 0.64 & \(-\)0.13 \\ VM—CMJ\_JH & \(\mathbf{0.15}\) & (\(-\)0.37–0.60) & 0.57 & 0.15 \\ VM—CMJ\_SD & \(\mathbf{0.05}\) & (\(-\)0.46–0.53) & 0.85 & 0.05 \\ VM—CMJ\_RSImod & \(\mathbf{0.15}\) & (\(-\)0.37–0.60) & 0.58 & 0.15 \\ RF—CMJ\_JH & \(\mathbf{0.22}\) & (\(-\)0.31–0.65) & 0.41 & 0.22 \\ RF—CMJ\_SD & \(\mathbf{-0.01}\) & (\(-\)0.50–0.49) & 0.98 & \(-\)0.01 \\ RF—CMJ\_RSImod & \(\mathbf{-0.02}\) & (\(-\)0.51–0.48) & 0.94 & \(-\)0.02 \\ \hline \end{tabular}
\end{table} 
Table 3: [PERSON]’s correlation (=rP) between countermovement jump (CMJ) performance values (JH—jump height SD—squat depth; RSImod—reactive strength index modified) and the stiffness of the patellar tendon (FT), vastus lateralis (VL), vastus medialis (VM), and rectus femoris (RF). The correlation is in bold; CI—confidence interval; ES—effect size. assumed that this discrepancy can be likely explained by the different slack angles between the gastrocnemius medialis and soleus muscles [12; 30]. Furthermore, another study reported a moderate positive relationship between tendon stiffness and ground contact time during a DJ [11]. Bringing the findings of [PERSON] et al. [11] and [PERSON] et al. [12] together, there seems to be evidence that, overall, a stiffer MTU might be favorable for DJ performance. However, the current study did not support these findings.