**5. Conclusions**

In conclusion, a brief-duration, supplementary, circuit-type strength and power training program was effective in increasing single- and double-leg vertical jump, jumping agility, and sport-specific skill performance in adolescent female gymnasts. Improvement of performance by devoting only 7–9 min per session may be an attractive model of supplementary training in female adolescent athletes, which may also reduce injury risk. The fact that gymnastics skills training alone was not adequate to improve important fitness and sport-specific parameters, may indicate that a supplementary strength and power training program is necessary for an effective and safe athlete development in this age group. Practitioners should consider incorporating a short-duration, supplementary strength–power program in "Gymnastics for All" training, as it was shown that substantial gains in neuromuscular performance can be obtained with minimum time investment.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2075-4663/8/8/104/s1, Table S1: Plyometric and strength exercises used by the athletes of the training group.

**Author Contributions:** Conceptualization, G.C.B. and K.K.; methodology, G.C.B., O.D., C.K., and K.K.; investigation, K.K.; data analysis, G.C.B. and K.K.; writing—original draft preparation, O.D. and K.K.; writing—review and editing, G.C.B., O.D., C.K., and K.K.; supervision, G.C.B. and O.D. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors would like to thank the young athletes and their parents.

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