**5. Conclusions**

Athletes showed significant changes in NADH fluorescence in skin cells after a 7-week training period. We found that they achieved higher post-training values in basal NADH fluorescence (Bmean) (pre-exercise 41% increase and post-exercise 49% increase). Additionally, the maximal increase in fluorescence during occlusion (FImax) and the maximal drop in fluorescence after reperfusion (FRmin) were higher at rest and post-exercise after training (FImax 42% at rest, and 47% post-exercise, FRmin (39% at rest, and 47% post-exercise). In conclusion, physical training results in an increase in the skin NADH fluorescence levels at rest and after exercise in highly trained athletes. We sugges<sup>t</sup> that the measurements can reflect the training-induced changes in the metabolic status of the skin mitochondria.

**Author Contributions:** Conceptualization, O.B. and J.Z.; Methodology, J.Z., K.K., and O.B.; Software, J.Z.; Validation, J.Z. and O.B.; Formal analysis, J.Z., D.W., and O.B.; Investigation, O.B., J.Z., D.W., K.K., A.K., and P.K.; Resources, K.K. and J.Z.; Data curation, J.Z. and O.B.; Writing—original draft preparation, O.B.; Writing—review and editing, K.K., J.Z., P.K., and A.K.; Visualization, O.B., J.Z., and A.K.; Supervision, J.Z. and K.K.; Project administration, J.Z.; Funding acquisition, J.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by ANG/ZK/2/2016 project, being a part of the project funded by the European Union from the resources of the European Regional Development Fund under the Smart Growth Operational Program, gran<sup>t</sup> number POIR.01.01.01-00-0540/15.

**Acknowledgments:** We thank all the participants for their full cooperation.

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