Introduction:
Plasma concentrations of cell-free DNA (cfDNA) serve as markers of overtraining or muscle injury. We have examined, if nuclear (n) and mitochondrial (mt) cfDNA has the potential as a marker of muscle burden or damage.

Material and methods:
Ten healthy, physically active volunteers (6 females, aged 27.1±6.8 years) performed a downhill running test. Samples for cfnDNA and cfmtDNA analysis were collected before, 30 minutes, 1 hour, and 14 days after the downhill run. CfnDNA and cfmtDNA (two markers for both) were analysed using qPCR.

Results:
There was an extreme (~ 40-times) increase in cfnDNA at the 30-min time-point against the baseline (p<0.00001 for both markers), followed by a quick drop to baseline levels after 1 hour after the end of the downhill run for all subjects. In contrast, plasma levels of cfmtDNA did not increase significantly (p=0.27 and 0.12). It reflects the fact, that in 6 subjects, the pattern was similar as for cfnDNA but in 4 subjects rather a decrease of cfmtDNA concentration was observed at the 30-mi time-point. These differences correlate with age, BMI, and sex of the participants. Plasma cfnDNA significantly (p<0.01 for all) correlated with concentrations of muscle damage markers such as AST, ALT, and lactate dehydrogenase, and chemokines MIP-1α and IP-10 (positive). No homogenous correlation between cfmtDNA and biomarkers was detected.

Conclusions:
Our study confirms the extreme release and clearance of cfnDNA in physically active subjects after strenuous exercise. In contrast, the trajectory of cfmtDNA concentrations seems to have much higher inter-individual variability than cfnDNA concentrations.