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Objectives: Generally, operational military duties are associated with a variety of stressors, such as prolonged physical activity (PA). However, limited information is available on the occupational workload or changes in PA during international military operations. Thus, the aim of the study was to investigate the changes in body composition, stress biomarkers, PA, and heart rate (HR) responses of 79 male soldiers during a 6-month international crisis management operation.Material and Methods: Measurements were conducted 3 times in South-Lebanon during the operation. Body composition was assessed by the bioelectrical impedance method. Blood samples were analyzed for serum testosterone, sex-hormone binding globulin (SHBG), cortisol and insulin-like growth factor. Saliva sampling was used for analyzing stress biomarkers, cortisol and α-amylase. Heart rate and physical activity were monitored by a recordable belt and tri-axial accelerometer, respectively.Results: Increases in muscle mass (39.2±4.1 vs. 39.5±4.2 kg, p < 0.05) and testosterone (15.9±4.6 vs. 17.2±4 nmol/l, p < 0.01), and reductions in PA variables (e.g., daily step count 9472±2547 vs. 8321±2720, p < 0.05) were observed during the first half (i.e., PRE-MID) of the study. The increase in muscle mass remained significant during the latter half (PRE-POST, 39.2±4.1 vs. 39.6±4.4 kg, p < 0.05), but also fat mass increased (MID-POST, 10.6±4.6 vs. 11.0±4.7 kg, p < 0.05) while SHBG (MID-POST, 31.8±12.1 vs. 26.6±13.2 nmol/l, p < 0.01) and cortisol (MID-POST, 445±116 vs. 400±123 nmol/l, p < 0.05) decreased. With the exception of increased concentration of salivary α-amylase (PRE-POST, 36.5±33.7 vs. 55.1±39.7 U/ml), the acute stress biomarkers and HR responses remained unchanged. Furthermore, the low quantity of PA, low HR values and subjective ratings of exertion refer to rather light physical workload.Conclusions: Due to the operatively calm nature of the working environment, the present soldiers did not express any significant signs of physical overload during the study period. Int J Occup Med Environ Health 2018;31(2):185-197. [ABSTRACT FROM AUTHOR]