**5. Conclusions**

This study investigated the change in the optimum angle of knee extension force production due to growth. The results showed that the optimal angle is temporarily changed to the loose position of the quadriceps femoris muscle at approximately the same time as the increase in femur length in a previous study, and returned to the original within a few years. This suggests that the force–angle relationship temporarily changes with muscle elongation due to skeletal growth.

**Author Contributions:** Conceptualization, J.S. and T.M.; methodology, J.S.; formal analysis, J.S.; data curation, J.S. and A.S.; writing—original draft preparation, J.S.; writing—review and editing, S.T.; supervision, S.T.; project administration, S.I., H.S. and S.T.; funding acquisition, J.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by a Grant-in-Aid from the Japan Society for the Promotion of Science Fellows (18J01881).

**Institutional Review Board Statement:** The study was conducted according to the guidelines of the Declaration of Helsinki, and approved by the Ethics Review Committee on Human Research of Waseda University (2013-167).

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** We appreciate Shinoda for helping in recruitment.

**Conflicts of Interest:** The authors declare no conflict of interest.
