*Article* **Measurement of Lipid Peroxidation Products and Creatine Kinase in Blood Plasma and Saliva of Athletes at Rest and following Exercise**

**Aleksandr N. Ovchinnikov 1,2,\*, Antonio Paoli 2,3, Vladislav V. Seleznev <sup>4</sup> and Anna V. Deryugina 2,5**

	- antonio.paoli@unipd.it (A.P.); derugina69@yandex.ru (A.V.D.)
	- <sup>3</sup> Department of Biomedical Sciences, University of Padova, 35122 Padova, Italy
	- <sup>4</sup> Department of Theory and Methodology of Sport Training, Lobachevsky University, 603022 Nizhny Novgorod, Russia; vseleznev92@mail.ru
	- <sup>5</sup> Department of Physiology and Anatomy, Lobachevsky University, 603022 Nizhny Novgorod, Russia
	- **\*** Correspondence: alexander\_ovchinnikov91@mail.ru

**Abstract:** This study aimed to assess the agreement between quantitative measurements of plasmatic and salivary biomarkers capable of identifying oxidative stress and muscle damage in athletes at rest and following exercise. Thirty-nine high-level athletes participating in track and field (running), swimming or rowing were recruited and assigned to one of three groups depending on the sport. Each athlete group underwent its specific exercise. Blood and saliva samples were collected before and immediately after the exercise. Diene conjugates (DC), triene conjugates (TC), Schiff bases (SB), and creatine kinase (CK) were measured. Comparisons were made using Wilcoxon signed-rank test. Correlation analysis and Bland–Altman method were applied. DC levels were elevated in plasma (*p* < 0.01) and saliva (*p* < 0.01) in response to exercise in all three groups, as were the plasmatic (*p* < 0.01) and salivary (*p* < 0.01) TC and SB concentrations. CK activity was also significantly higher at postexercise compared to pre-exercise in both plasma (*p* < 0.01) and saliva (*p* < 0.01) in all groups. Strong positive correlation between salivary and plasmatic DC (*p* < 0.001), TC (*p* < 0.001), SB (*p* < 0.01), and CK (*p* < 0.001) was observed at rest and following exercise in each athlete group. The bias calculated for DC, TC, SB, and CK using the Bland–Altman statistics was not significant at both pre-exercise and postexercise in all three groups. The line of equality was within the confidence interval of the mean difference. All of the data points lay within the respective agreement limits. Salivary concentrations of DC, TC, SB, and CK are able to reliably reflect their plasma levels.

**Keywords:** saliva; blood plasma; oxidative stress; lipid peroxidation products; muscle damage; creatine kinase; exercise; athletes
