Introduction

The mental health of military personnel has been studied in different circumstances, such as war situations [1, 2]. The emotional reactions of people affected by war situations can have harmful psychological consequences, and majority of the studies focuses on post-traumatic stress disorder among active personnel and veterans [3,4,5] or on mental health intervention [6]. On the other hand, few observational studies have investigated mental health among the military in peacetime [7,8,9].

Higher levels of physical fitness are associated with lower cardiovascular risk [10], all-cause mortality [11] and other harmful conditions [12, 13]. Investigations also indicate that physical fitness is associated with mental health in different populations [14,15,16]. Willis et al. [17], using a retrospective cohort design, showed that midlife physical fitness is associated with a lower risk of later-life depression. Job strain is also related to mental health [18]. Harvey et al. [18], using a British cohort study found that high job stress increases the risk of common mental disorders in midlife after 5 years of follow-up. Among the existing theoretical models to assess psychosocial stress in the workplace, the demand–control model proposed by Robert Karasek in 1979 has been the most widely used [19, 20]. The bases of the model are: (a) adverse health reactions from simultaneous exposure to heavy psychological demands and limited control over the work process (the negative scenario); (b) there is a “positive effect” from stress in the face of elevated psychological demand and control (the positive scenario).

Although few studies [21, 22] have been made in military populations, the association between physical fitness and mental health is particularly relevant because of the nature of the army environment, which is perhaps more physically and mentally demanding than any other occupation.

Psychological distress, defined as emotional suffering, is characterized by symptoms including depression and anxiety, and sometimes by somatic symptoms [23]. Physical fitness has been consistently associated with psychological distress [16, 24]. However, few studies [21, 25] have investigated the association between physical fitness and psychological distress among military personnel. A recent study [21] found an inverse association between physical fitness levels and depressive symptoms among American soldiers at a specific moment in time (training). Therefore, the present study may offer novel information regarding physical activity domains and mental health outcome in the work routine. Physical fitness involves a set of attributes that are skill-related and can be measured by different physical tests. In the military environment, it is quite common to walk long distances carrying all necessary equipment, highlighting the demand of cardiorespiratory and muscular systems. In this sense, it is plausible to suppose that these are the most important physical fitness domains.

The Brazilian Army is responsible for the protection of borders. Additionally, the Army has become more involved in international military peace operations and health care programs in hard-to-reach areas. Brazilian soldiers are classified into two categories: operational and non-operational. The operational category comprises soldiers actively engaged in different operations. Soldiers in the non-operational group, by contrast, have administrative functions. Considering the different demands of work, we hypothesized that low levels of physical fitness may be more relevant among operational personnel.

Therefore, the objective of the present study was to evaluate the association between objectively measured physical fitness and psychological distress among Brazilian soldiers, and to investigate differences by operational status.

Method

Participants

This cross-sectional study of male Brazilian soldiers ranking sergeant or higher was conducted during the year 2008 (from March to December). Twenty-two strata (11 × 2) were formed to contemplate the eleven military regions in the Brazilian Army’s administrative structure and the soldiers’ operating status (operational or non-operational). To select a representative sample of each of the 22 strata, the sampling plan was carried out for each stratum, their sum forming the whole study sample. Each sample size was calculated regarding a relative precision of 0.1, a significance level of 0.05 and assuming a true proportion of 0.8. The study sample comprised 1396 individuals, 137 of whom refused to participate. The final sample was thus 1259 participants (40 ± 8.5 years). Written informed consent was obtained from all participants, and the research protocols were approved by the Rural Federal University of Rio de Janeiro Ethics Committee.

Data collection

Data were gathered using the self-administered questionnaire filled out individually in the workplace or physical tests that were conducted in the physical education center where the participant is situated were attached. The tests were made with a maximum of five soldiers per time. An average of 8 min was needed to fill in the questionnaire during free time provided especially for the procedure by the participant’s immediate boss under an institutional agreement.

Outcome

Psychological distress was measured by the 12-item General Health Questionnaire (GHQ-12). The Brazilian version of GHQ-12 has been validated against a structured psychiatric interview and identified a sensitivity of 85% and a specificity of 79% [26]. A previous study, using a confirmatory factor analysis, supports the psychometric properties of the instrument (Cronbach’s alpha = 0.89) [27]. Each item of the GHQ-12 has four possible response options. For instance, the first one is: have you recently been able to concentrate on what you are doing? Scores for individual items were coded as absent (first or second option) or present (third or fourth option), and then added. Total scores of four or more were classified as having psychological distress.

Exposure

Cardiorespiratory fitness

The Cooper 12-min Run Test was performed to measure cardiorespiratory fitness. It consisted in running for 12 min on a 400 m athletics track [28]. The test presents a high level of reliability in adults (coefficient = 0.96) [29]. Individuals were familiar with the test, which is part of their routine. The total number of laps was counted and the finishing point was marked. Total distance (in meters) covered in 12 min was calculated by multiplying the number of complete laps by 400, and adding the distance covered (in meters) in the final incomplete lap. Instead of using the standard formula for the estimation of maximal oxygen consumption, those who covered more than 2800 m were allocated to the High-level group. This cut-off point is the same as that used by the army to evaluate the soldiers [28].

Muscle endurance

Muscle endurance was assessed with push-ups test [30]. The participants were familiar with the test procedure and started in an arm-extended (up) position with forearms and wrists pronated, feet at biacromial (shoulder) width, and fingers flexed. In the down position, forearm and wrists were kept pronated, whereas the elbow was flexed 90° and the shoulder abducted 45°. Individuals were asked to perform as many repetitions as possible in 1 min. For analytic purposes, those who performed more than 22 repetitions were allocated to the High-level group. This cut-off point is the same as that applied by the army.

Covariates

The covariates were (a) age, (b) educational level, (c) marital status, (d) operational status, and (e) rank. Education was measured using three categories according to years of education: (a) “elementary”—until 9 years, (b) “high school”—10–12 years, and (c) “college”—at least 13 years. Marital status was measured using two categories: (a) “married/cohabiting” and (b) “unmarried”. Operational status was reported dichotomously by respondents: (a) “operational” or (b) “non-operational”. Rank was categorized as: (a) “Superior officers and Captains” (captain, major, lieutenant colonel and colonel), (b) “Lieutenants”, (c) “Sergeant Majors and (d) Sergeants”. All these variables were evaluated as possible confounders in the associations between physical fitness and psychological distress because they are associated with physical fitness [31] and/or influence psychological distress [32].

Statistical analysis

We calculated descriptive statistics in the form of means and standard deviations for continuous variables and frequencies for nominal variables, by operational status. Logistic regression models were fitted for the dichotomous outcome. Odds ratios (OR) and confidence intervals (95% CI) were estimated before and after adjusting for confounders. The modeling process was performed in two steps: step 1 involved the overall sample and step 2 fitted the models separately for operational status. The adjusted models included the following independent variables: physical fitness variables, age, education, marital status, operational status, and rank. The analysis was performed using R, version 3.2.2 software.

Results

Participants’ average age was 40 years (standard deviation 8.5); 84% were in the highest category of education; 47.6% were classified as operational; 75.1% were classified as ‘low cardiorespiratory fitness’; and 56.5%, as “low muscle endurance”. The average physical fitness test performance was 2800 ± 331 (1400–3600) meters and 24 ± 9 (1–44) push-up repetitions, respectively, for cardiovascular test and muscle endurance. Overall prevalence of psychological distress was 16%. Detailed characteristics can be seen in Table 1.

Table 1 Characteristics of the sample according to operational status
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Analysis based on the overall sample showed that both muscle endurance and combined cardiorespiratory fitness and muscle endurance were significantly (p < 0.05) associated with psychological distress. In this connection, the fully adjusted model showed that participants classified as ‘low muscle endurance’ (OR 1.65; 95% CI 1.2–2.3) and with low performance in both tests (OR 1.91; 95% CI 1.2–3.0) were associated with greater psychological distress (Table 2).

Table 2 Association between physical fitness and psychological distress
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Analysis restricted to the operational group showed that individuals with low physical fitness had higher odds of psychological distress in the fully adjusted models. For instance, low muscle endurance (OR 1.81; 95% CI 1.2–2.8) was associated with greater psychological distress. In addition, statistically significant (p < 0.05) and positive association was also detected among those who were classified as ‘low fit’ in both physical fitness domains (OR 2.70; 95% CI 1.2–5.1). By contrast, no significant (p < 0.05) association was detected in non-operational group (Table 3).

Table 3 Association between physical fitness and psychological distress according to operational status
Full size table

Discussion

This study sought to investigate the association between physical fitness and psychological distress in military personnel, and investigated differences by operational status. Our results largely confirmed that scores of physical tests were associated with psychological distress. These findings may reflect how stressful it is to be a soldier with poor physical fitness, even in peacetime. It is important to consider how relevant is, in a military environment, to be in good physical condition, which is seen as an inherent part of the profession. In this regard, unfit individuals may experience feelings of not belonging and/or uselessness, which may lead to psychological distress. Therefore, it is relevant to consider the peculiarities of military environment: rigid hierarchy, heightened competition and the possibility of changes occurring against the individual’s wishes [8]. These peculiarities could influence psychological distress [8].

Previous studies [1, 2] of military personnel have detected several psychiatric disorders, including major depression, anxiety disorders and post-traumatic stress disorder after different kinds of missions, showing the importance of being physically active. It is well-known that psychiatric disorders affect occupational effectiveness, which is strongly related to retention and productivity of military personnel [33]. Thus, factors related to mental health problems should be one of the concerns of military work.

A theory proposed by Hellerstedt and Jeffery [34] is that ‘high demand’ work may reduce workers’ capacity to exercise regularly. In a Finnish study that corroborated this hypothesis, an inverse association between job stress and leisure time physical activity was observed in 46,573 subjects [33]. In a military context, physical exercise is part of the work routine (not leisure time). In this sense, it is possible to speculate that physical exercise does not have a positive impact on mental health. A recent meta-analysis [35] showed that only leisure time and transport-related physical activities were associated with mental health.

Distress can be considered a contributing factor to sedentary lifestyles, because of the psychological and physical symptoms. For example, individuals with depressive symptoms, which are associated with distress, display lifestyle changes, such as social isolation, fatigue, low motivation and physical inactivity [36, 37]. A review showed that patients with severe psychiatric disorders have lower levels of physical activity as compared to the general population [38]. Another study, focused on symptoms of depression and distress, found that at high levels of distress, both sexes tended to be more inactive [39].

Considering the whole sample, our findings demonstrated that psychological distress was more strongly associated with low levels of muscle endurance than with cardiorespiratory fitness. We postulate two explanations for this: (a) low performance in the muscle endurance test may be related to stress because it is associated with physically demanding military activity. In this situation, military personnel may feel useless, which, in turn, could be attributed to psychological distress; (b) alternatively, job stress may play a role in this finding. Taking into account the demand–control model [40], we hypothesize that participants with low muscle resistance may have low job control and high job demand (e.g., mission, training). In this direction, previous studies [8, 41] have observed an association between job stress and psychological distress.

The models stratified by operational status showed an association between physical fitness and psychological distress only in the operational group. Two explanations could be postulated: First, operational military personnel (those in mission-priority functions) face higher potential demand than non-operational personnel and low control. In fact, in some jobs, the employees have no control or low level of control in what they have to do. For instance, soldiers in operational group could be engaged in an unexpected mission. Consequently, they will be away from home for at least 2 weeks. Higher demands and lower control is the worst-case scenario according to the demand–control model [42]. Alternatively, unfitted operational soldiers may feel uncomfortable in a military environment. Probably, strong feelings of uselessness could influence psychological distress.

A study conducted in a large American military cohort showed complementary findings to the present approach [9]. In this line, the authors found that older participants and those with longer lengths of service had more favorable mental health but less favorable physical health. They also observed that participants with low levels of educational attainment had less favorable mental and physical health. Although this is not the focus of the present approach, these findings emphasize the idea of investigate both physical and mental health among military population.

Our findings detected a relatively high percentage of military personnel with low physical fitness in the operational group, exactly those who were intended to be more involved in physical training activities. Two explanations could be postulated. First, officers may be less required in physical training since they are usually in charge of them. Second, some operational functions in the army, such as communication occupations do not require higher levels of physical fitness, but rather specific skills.

Some limitations of the study should be mentioned. First, the study population did not include women, corporals or privates. Accordingly, our findings cannot be extrapolated, but they do indicate a clear association between psychological distress and specific indicators of physical fitness in operational military personnel. Second, only two physical tests were used, although several physical tests are applied in the military routine to access different components of physical fitness. Nevertheless, the tests chosen were the most commonly used, and evaluated two important components of physical fitness. In this sense, this approach increases the external validity of the study. However, the muscle resistance test used evaluates only superior muscle group. Third, as the study design was based on data at one point in time, it was not possible to evaluate possible changes in physical fitness related to psychological distress or, consequently, to detect a causal relationship.

Conclusion

Several studies [13, 36, 43] have investigated the association between physical activity and psychological distress, but little is known about this relationship in military personnel. This research used a representative sample of Brazilian soldiers and found evidence that physical fitness was associated with psychological distress with higher effect for soldiers in the operational group. These findings could have implications for soldier’s health in peacetime. In this line, improved fitness is related to low psychological distress, which is useful to increase the levels of physical fitness. Future studies should investigate the directionality of the association. It is plausible to wonder if physical fitness influences psychological distress or vice versa. Furthermore, it would be useful to investigate the mechanisms underlying the relationship between physical fitness and psychological distress among armed force personnel.