Background: Responses to exercise training can vary greatly between individuals. For special operation forces, low responses to training can hamper performance. In this study, we objectively measured strength and fitness during special operation forces training, and assessed potential determinants of the training response. Methods: Twenty subjects were enrolled, and measurements were taken before and after a 9-week training program. Muscular strength was measured as one-repetition-maximum on four instruments, and physical fitness by the Cooper-test. Body composition was measured using deuterium dilution, physical activity by accelerometry and diet quality by food records. Level of significance was p<0.05.Results: During the 9-week training period, body strength increased by 0.33±0.24 N/kg (+7%, P<0.001, and physical fitness increased by 3.5±3.4 mL/min/kg (+6%, P=0.001). Gains in strength were inversely associated with strength at baseline, and positively with activity intensity during the training program. We observed no effect of training on body weight, but body composition was significantly different at follow-up as compared to baseline (16.9±2.5% to 14.9±2.5% body fat, P=0.03). Energy intake was 4491±506 kcal/d and energy balance was -243±306 kcal/d (P=0.04). Average physical activity level was 2.6±0.2 and the average duration of moderate-to-vigorous physical activity was 5:53±0:36h. Over time, physical activity did not change significantly. After adjustment for underreporting, intakes of vitamin C and D were insufficient on average and for most participants.Conclusions: Improvements in strength were modulated by strength prior to the intervention, and moderate-to-vigorous physical activity during the training. Thus, compensatory declines in physical activity may hamper the effectiveness of the exercise program.