In the present study, we report the improvements in fitness and strength observed during a 9-week special operation forces training in soldiers of the Dutch Army. This is the first study to objectively measure physical activity and activity intensity using accelerometry during the training period. Thereby, this study allows to assess whether improvements in fitness and strength are affected by the intensity of physical activity during the training period. In addition, we measured body composition and collected dietary records to determine nutritional adequacy during this high-intensity training program.
The present study demonstrates that the 9-week special operation forces training achieved the expected improvements in physical fitness and whole-body strength (+ 6–7%). Adjusting for individual changes in fat-free mass, largely composed of muscle mass, the increase in fitness and strength were small (+ 3%), but remained significant. Thus, strength and fitness improved likely as a result of increased muscle mass and quality and respiratory capacity. The inter-individual range in response to the training program was 25%. For strength, the increase negatively associated with strength prior to training, demonstrating that strong individuals benefit less from the training program. For fitness, no such association was observed and thus, the prescribed training achieves improvements in fitness even in fit individuals.
The training regimen in this observational study imposed an average physical activity level of 2.6, which is high. Inter-individual differences in physical activity during the training program, in moderate, vigorous, and very-vigorous intensities, associated positively with changes in strength. Thus, this study suggests a dose-response relationship with intense activities, but not with low-intensity activities. The training-effect on fitness was not determined by the variability in physical activity levels during the 9-week training. Of note, maintaining a physical activity level at this level for > 60 days approximates to 67% of the maximal human physical capacity, which has been estimated using studies of long-term endurance capacity, e.g. artic trekking, ultra-marathons and mountaineering . Importantly, these estimates are derived only from week-days, thus training days, and physical activity during the weekends was significantly less (-8 hours).
At the observed physical activity level, we estimated energy intake requirements during the training to be 4773 kcal/d, or ~ 60 kcal/kg body weight per day. These estimates confirm energy requirement studies using gold-standard methodology, that is doubly labeled water [7, 16, 17]. Importantly, accelerometry measures body movements and energy requirements are estimated as a function of body movements and body weight. During many activities, soldiers were required to carry additional weights, e.g. 20 kg-backpacks, and thus physical activity levels, energy requirements and energy intakes were likely higher than reported.
While the imposed training program did not affect body weight, soldiers lost 1.8 kg of fat mass, while gaining 1.2 kg of fat-free mass. The gains in fat-free mass are comparable to other exercise training studies [18, 19]. Based on the changes in body composition, we estimated that soldiers consumed 330 kcal/d less than they expended. Energy balance was calculated based on changes in body composition and is therefore not affected by confounding by carrying additional weights. Despite being in a small energy deficit, dietary records demonstrate sufficient intake of proteins, the primary dietary driver of changes in muscle mass and strength during exercise training. In contrast to other studies , gains in muscle mass during an energy deficit were not associated with protein intake, which is likely because protein intake was sufficient in all individuals. The association between changes in strength and energy deficit is explained by changes in fat mass, because it is a factor in calculating energy balance and a denominator in calculating strength. If strength is adjusted for fat-free mass only, changes in strength do not associate with energy balance.
We did not observe any associations between changes in fitness or strength with diet quality, i.e. adequacy of nutrient intake. Nevertheless, after adjustment for underreporting of energy intake (~ 30%), we observed significant deficiencies in vitamin C and D intake. Although these deficiencies did not cause any adverse effects on outcomes of this study, others have shown detrimental effects of vitamin deficiencies on physical function, metabolic health and bone quality [21–23].
This study is the first to use objective measurements of physical activity during high-intensity exercise training for special forces operations. In light of the intense training, burden to the participants of this study had to be minimized, and therefore, collection of this data, and in addition of diet and body composition data, is exceptional. Due to the specialized nature and target population, it is limited in its subject size, which is small, and the burden of the study in addition to the demanding training may explain the missing data. However, the results of this study, despite preliminary in nature, are valuable to inform optimization of training strategies.