## 4 Discussion
Moreover, the strength of the leg muscles has been found to be a determinant parameter for vertical jump height in some [14; 36] but not all studies [37]. In addition, a further study reported that strength of the knee extensors is only related to vertical jump height when it is relative to body mass [26; 38]. When quadriceps strength is related to muscle stiffness of the rectus femoris, no association has been reported [17]. However, these authors reported that rectus femoris muscle stiffness was significantly negatively related to power [17]. This would indicate that a more compliant muscle can lead to higher power outcomes. Besides strength and power, a further possible predictor for CMJ jump performance has been found in flexibility of the quadriceps muscle, although this relationship was only to a moderate extent [26]. This study has some limitations. We did not control for the knee angle position at the start of the concentric phase of the CMJ. A smaller squad depth might change the performance of the SSC due to a change of the muscle-tendon unit [13]. Hence, a deeper position might result in in bigger jump height due to an increase in, e.g., storage and release of elastic energy. However, in the work of [PERSON] et al. [39] it was shown that the self-selected knee flexion angle resulted in the same jumping heights compared to a bigger knee flexion angle (>90\({}^{\circ}\)), and outperformed a knee flexion angle (<90\({}^{\circ}\)) smaller than the preferred position. A reason might be that acute changes in the squad position, prior to the concentric phase, might affect the intersegmental coordination and, therefore, beneficial effects might be diminished [39]. Taken together, the authors assume that a self-selected initial start position did not influence the results of the study. Another limitation is the generalizability of the results. Our subjects reflected a recreationally trained cohort; therefore, results cannot be transferred to elite or highly trained athletes. Previous studies have reported that elite or highly trained athletes have another muscle-tendon structure (e.g., stiffness [8]) as well as function (e.g., strength [40]) compared with the recreational trained or non-athletes. In summary, the data obtained in the current study showed no significant association between muscle and tendon stiffness of the respective muscles and DJ or CMJ performance variables. Therefore, it is likely that other variables, such as jump technique, body weight, or strength, play a more important role in DJ and CMJ performance. Hence, future studies should investigate such potential relationships. This would help to draw a clearer picture and to give training recommendations to improve jump performance. Conceptualization, A.K. and F.K.P. ; methodology, A.K. and F.K.P. ; validation, A.K. and F.K.P. ; formal analysis, A.K. and F.K.P. ; investigation, A.K. ; writing--original draft preparation, A.K. ; writing--review and editing, A.K. and F.K.P. ; funding acquisition, A.K. All authors have read and agreed to the published version of the manuscript. This research was funded by the Austrian Science Fund (FWF), grant number J 4484. Open Access Funding by the Austrian Science Fund (FWF). The study was conducted according to the guidelines of the Declaration of Helsinki, and was approved by the Ethics Commission of the Technical University of Munich (protocol code 762/20 S-KH 14 December 2020). Informed Consent Statement Informed consent was obtained from all subjects involved in the study. Data Availability Statement: All data supporting the conclusions are provided at the following link: [[https://doi.org/10.6084/m9.figshare.18297692.v1](https://doi.org/10.6084/m9.figshare.18297692.v1)]([https://doi.org/10.6084/m9.figshare.18297692.v1](https://doi.org/10.6084/m9.figshare.18297692.v1)). This study was supported by a grant (Project J 4484) from the Austrian Science Fund (FWF). The authors declare no conflict of interest.