Musculoskeletal disorders, stress perception and physical activity in police officers

| The purpose of this study is to analyze association between stress perception and physical activity with the prevalence of symptoms of musculoskeletal disorders (SMSD) in police officers. The police officers (n=142) answered the Perceived Stress Scale-10 (PSS-10) and the Nordic Musculoskeletal Questionnaire. Pearson’s linear correlation coefficient was used to measure strength and direction of relationship between PSS-10 and prevalence of SMSD and showed a positive correlation between variables in the last 7 days (r=0.34, p=0.00001) and in the last 12 months (r=0.40, p=0.00001). The mean prevalence of SMSD in the last 12 months was lower among police officers practicing physical activity (W=1888.5, p=0.04). Multiple logistic regression analysis showed that the practice of physical activities as well as efforts to reduce stress perception both decrease the odds of musculoskeletal disorders. This study indicated that stress level and physical activity practice influenced in prevalence of SMSD. Multidisciplinary interventions with actions to reduce stress level and encourage physical activity practice are needed to control SMSD prevalence.


INTRODUCTION
Police officers often perform activities associated with stress-causing factors, such as ostensible surveillance work and preservation of public order [1][2][3] . Excessive exposure to risks and violence, corporative efficiency charges and poor working conditions all contribute to make police officers most susceptible to stress 4,5 . Work stress costs are high and can be reflected both in mental health (frustration, anxiety and depression) and in physical health including the prevalence of symptoms of musculoskeletal disorders (SMSD) 6 .
Stress may be associated with an increase in SMSD 7 . Studies have shown that police officers have a higher prevalence of SMSD complaints in the following areas of the body: lower back, back, neck and knee [6][7][8] . Prevalence of these symptoms is directly related to these professionals' work specifics, such as running, jumping, and shooting, which consequently may cause physical trauma, the main reason for orthopedic problems 3 .
There is evidence for effectiveness of physical activity in significantly reducing SMSD prevalence among office workers 9 . The association analysis between stress level, physical activity practice and SMSD can provide important information in developing public strategies for policework. Thus, the aim of this study was to investigate association of SMSD prevalence with stress perception and physical activity among police officers.
METHODOLOGY This is a cross-sectional study conducted with the Brazilian Military Police. The subjects to this study were 142 male police officers. Through a brief initial questionnaire, sociodemographic variables were collected, including age, height, weight, body mass index (BMI), marital status and number of children, as well as occupational variables such as working time in the corporation, working time in current function, working hours, if they had been away from work in the last 12 months and if they performed some type of physical exercise more than three times a week. BMI-the ratio of mass (kg) and height (m²)-was calculated from values reported by police officers. BMI values were classified as <18.5kg/m² (underweight); 18.5kg to 24.9kg/m² (normal); 25kg to 29.9kg/m² (overweight); and >30kg/m² (obese) 10 . Police officers developing their functions in Radio Patrol Standard with a shift of 12×24 (12 hours working, followed by 24 hours of rest) and 12×48 (12 hours working, followed by 48 hours of rest).
The inclusion criteria were police officers with more than a year of activity in the military police operational sector, that accepted answering the questionnaires and that filled out the informed consent form. Exclusion criteria consisted of police officers away from their activities and who did not attend during the day of the data collection.
To analyze perceived stress, the Perceived Stress Scale-10 (PSS-10) was used, proposed by Cohen, Kamarck and Mermelstein 11 and validated in Brazil by Reis, Hino and Añez 12 . This instrument, based on officer self-reporting, intends to achieve global measure of stress in order to assess what situation in the individual's life is perceived as stressful. The results of stress level can range from 0 to 40, correlating a higher score with a higher perceived stress level 11 .
To evaluate SMSD prevalence, the general content of the Nordic Musculoskeletal Questionnaire developed by Kuorinka et al. 13 was used, adapted and validated to Portuguese by Barros and Alexandre 14 . The Nordic questionnaire objectively standardizes analysis to SMSD prevalence and allows data comparison from different studies.
An exploratory analysis of the variables was performed, using graphics, equations, percentages, means and standard deviations. Pearson's linear correlation coefficient was used to measure the strength and direction of the relationship between PSS-10 and the SMSD prevalence. The Shapiro-Wilks test was used to check data for normal distribution. Student's t-test was used to compare the means of two groups under the assumption that both samples were random, independent, and originated from normally distributed population, while the Mann-Whitney w-test was used to test the null hypothesis that two independent samples came from the same population with non-normal distribution. We used a multiple logistic regression model to calculate probabilities and odds ratios for SMSD prevalence. The statistical methods used in this work are described by Peat and Barton 15 . Data were analyzed using R software R Core team 16 .

RESULTS
Most of the workers were married (65%), practiced regular physical activity (62%) with time in company of 149.85 months (±78.35) ( Table 1). Regarding SMSD prevalence in the last twelve months, police officers reported feeling more pain in the lower back (47%) and dorsal region (33%), as shown in Table 2. The linear correlation coefficient between stress level with SMSD in the last seven days was statistically significant (r=0.34, p=0.00001), and the correlation coefficient between stress level and SMSD in last 12 months was, also, statistically significant (r=0.40, p=0.00001). This, therefore, indicates a moderate positive linear correlation between two variables at the two situations. Mean stress level tended to be lower among police officers who practiced physical activity more than three times/week (mean=24.01) than among police officers who did not (mean=25.33). However, there were no statistically significant differences (t=−1.69, p=0.09). Similarly, the mean prevalence of SMSD in last 12 months is lower among police officers who practiced physical activity more than three times/week than among police officers who did not (mean=2.40 vs 3.09, w=1888.5, p=0.04) as shown in Figure 2. The log odds of the prevalence of SMSD in the last 12 months were estimated by the logit model ln(odds)=ln(p/(1-p))=−2.67+0.19×stress_leve -0.35×physical_activity. It shows that practicing physical activities in the last 12 months reduces the odds of getting musculoskeletal symptoms by 30%, while a unit increase in stress perception increases the odds by 21%.
The model also shows that a police officer with a maximum stress level (PSS-10=40) who did not practice physical activities in the last 12 months presents a probability of 0.82 of developing some type of musculoskeletal disorder. On the other hand, a police officer without stress (PSS-10=0) who has practiced physical activities in the last 12 months has a very low probability (equal to 0.04) to develop some type of musculoskeletal symptom.

DISCUSSION
This study has shown that stress is directly associated with SMSD prevalence in the last seven days and SMSD prevalence in the last 12 months. Police officers are among the professionals most vulnerable to accidents and deaths in exercise of their work and most exposed to danger and aggression, often involved in everyday conflict situations generating tension, SMSD prevalence and occupational stress [17][18][19] .
Occupational stress is an imbalance perceived between work demands and the worker's ability to cope, with differences between each person's ideal working routine and the reality of their working conditions being the only reason to report stress perceived at work 20,21 . Occupational stress is considered a risk factor in triggering metabolism changes, and stress can appear through minor psychological trauma that police officers might face 22,23 .
Studies worldwide claim that the effects of occupational stress may result in anxiety, depression, alcoholism, insomnia, increased aggression, suicidal attempts or ideation, obesity, gastrointestinal diseases, hypertension, coronary heart disease, as well as other conditions 24,25 . In this context, strategies such as the practice of physical activity are important to be considered, as they deal with stress risks and improve both quality of work and personal life, avoiding consequences to occupational stress for these professionals. It is important to highlight that stress is the cause of several health problems, such as SMSD prevalence, and that the most common and unavoidable stressors are those connected with work 26 .
Some studies shown SMSD prevalence related to occupational stress [26][27][28] . The SMSD risk factors are known to be generated by work activities, such as carrying of work equipment (ballistic vest, gun, ammunition, among others), repetitive tasks and incorrect posture, generating SMSD prevalence in specific body parts such as the lumbar spine, which corroborates this study's findings 29 . Economic losses due to SMSD prevalence affect not only the individual but also the corporation and society, since SMSD can lead to inability and absenteeism 30 , contributing to the relevancy of this study.
In this study, the police officers that practiced physical activity reported less occupational stress than participants that did not. Physical activity can decrease stress because people who practice it display lower cortisol levels, better heart rate response, more calmness and less frequent anxiety states than people who do not practice it, suggesting that physical activity may produce a protective effect against diseases related to occupational stress [31][32][33] .
Police officers may be required to use force in search of a suspect, showing physical fitness is essential to this population 34 . This study demonstrated police officers who exercise more than three times a week had fewer SMSD prevalence in the last seven days and in the last 12 months. One possible explanation for these findings may be due to the protective factor that physical exercise causes in the body, bringing better joint stability, flexibility and muscular endurance 35,36 . In this sense, regular physical exercise practice is an important factor for police officers to avoid stress, SMSD prevalence and reduce absenteeism 37,38 .
In conclusion, there was significant association between stress and SMSD prevalence. The prevalence of SMSD is lower among police officers who practice physical activity. This study indicated that stress level and physical activity influenced positive or negative in SMSD prevalence. Therefore, multidisciplinary intervention which includes actions to reduce stress level and encourage physical activity practice are needed to control SMSD prevalence in police officers.