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Longitudinal Changes in Strength of Police Officers With Gender Comparisons

Boyce, Robert W1; Jones, Glenn R2; Schendt, Katherine E1; Lloyd, Cameron L1; Boone, Edward L3

Journal of Strength and Conditioning Research: November 2009 - Volume 23 - Issue 8 - p 2411-2418
doi: 10.1519/JSC.0b013e3181bac2ab
Original Research

Boyce, RW, Jones, GR, Schendt, KE, Lloyd, CL, and Boone, EL. Longitudinal changes in strength of police officers with gender comparisons. J Strength Cond Res 23(8): 2411-2418, 2009-Strength is a critical factor in the health and job performance of police officers. Using a retrospective longitudinal design, the purpose of this study was to identify differences in strength scores from initial recruitment to in-service tests and to compare gender differences. Strength changes were also compared in low- to high-strength groups. Strength scores included bench press, bench press/lean weight, and bench press/weight. Body weight, percent body fat, and bench press scores were retrieved for the 1990-1995 recruit classes and were paired to most recent scores on 2006 in-service fitness record. Sample included 327 police officers: 30 females and 297 males. Mean age at initial recruitment was 24.6 years and for in-service was 37.1 years. Average time between tests was 12.5 years. Over this period bench press and bench press/lean weight significantly increased for both gender groups (p ≤ 0.05). Bench press/weight remained relatively consistent. When dividing the men's strength scores into five low- to high-strength groups, there tended to be an averaging effect over time with the strongest group changing least and the weakest group changing most. In conclusion, the results of this study did not follow expected strength trends, which reported annual declines in muscular strength in men and women. Overall, officers increased in strength well into their late 30s. The practical applications of this study include documented benefits gained from maintaining ongoing fitness training and testing, as well as the potentially positive role of weight gain on strength. Also, low- to high-strength groups did not change relative positions over time even with improvements in strength scores, demonstrating the importance of minimum selection criteria for police.

1Health and Applied Human Sciences, University of North Carolina at Wilmington, Wilmington, North Carolina; 2Charlotte Mecklenburg Police Department, Charlotte, North Carolina; and 3Department of Statistical Sciences and Operations Research, Virginia Commonwealth University, Richmond, Virginia

Address correspondence to Dr. Robert W. Boyce,

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Strength is a critical factor in the health and job performance of police officers. Increased muscular strength is positively associated with health (18) and provides protective benefits against disability (7,27) and chronic diseases such as heart disease (33), diabetes (24), and metabolic syndrome (39). Strength supports an officer's ability to safely perform critical emergency functions (14,31) while fulfilling the agency's legal responsibility to deliver adequate protection to the public. Strength remains an important criterion in the selection and hiring of police officers. However, legal ramifications exist as to the potential adverse impact upon the hiring of legally protected groups, such as women (8,29,31).

Strength usually peaks between the ages of 20 to 30 for men and a few years earlier for women and then decreases with age (2,37). Previous studies regarding age-related changes in muscular strength report an annual decline in muscle strength from approximately 0.8% to 5%, depending on gender, the muscle group, and the type of work (3,4,33). Gender differences are reported to be greater in upper body strength than in lower body strength (17,23). Untrained females have been shown to have approximately 50-60% of the upper body strength of men (23,31,32). In addition, some data suggests that the rate of muscular strength decline differs between males and females (19,28,37). A major contributor to the decrease in strength with aging is a loss in muscle cross-sectional area (12).

The literature supports the need for on-going physical fitness training for police officers as the occupation of police officer is primarily sedentary, though mentally challenging, with occasional periods of maximal exertion (34). Furthermore, the physical demands of the occupation are often inadequate to maintain necessary physical fitness to perform the infrequent but possibly life-saving tasks (35).

Understanding longitudinal strength changes over the employment duration of a police officer provides agencies with essential information to address health issues, the proper physical performance of duties, and the potential legal ramifications of hiring and retention criteria (6). It is evident that collecting strength data on police officers from recruitment to in-service status, especially when considering gender and age would identify trends and provide data for designing effective training procedures and requirements.

No longitudinal police research was found that addressed strength changes or changes in upper body strength, nor were any police studies found that examined gender strength changes over time. Also, no studies were found that addressed longitudinal change in upper body strength related to the level of strength at hire. The information found regarding bench press strength in relation to ageing was located in normative tables developed from cross-sectional data (26,38). Other authors also reported a lack of longitudinal strength data (30).

Therefore, the main purpose of this retrospective study was to identify differences, if any, in police officer strength scores from initial recruitment to in-service tests over approximately 12.5 years considering genders and the strength level of the male officers at the beginning of recruit training.

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Experimental Approach to the Problem

Through a retrospective longitudinal design, this study sought to determine whether there were differences in strength scores from the first week of recruit training to police officer tests approximately 12.5 years later and if there were gender change differences. Strength scores were comprised of bench press, bench press/lean weight and bench press/body weight. Bench press/ lean weight and bench press/body weight were included to provide a strength score relative to lean weight and body weight. Bench press/lean weight was used in the police department and bench press/body weight is a commonly used measure of relative strength (38). In addition, we sought to determine if the strength changes were different among the weakest and the strongest male officers based on the officer's recruit strength scores. We planned to provide gender specific strength mean scores with statistical comparisons for each of the testing periods.

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The retrospectively studied sample included 327 police officers (30 women and 297 men) who participated in the recruit classes between 1990 and 1995. Physical fitness test records were obtained from the Charlotte Mecklenburg Police Department, a large metropolitan police department in the southeastern United States of America. Ninety-nine percent of the in-service fitness tests were conducted between the years of 2003 to 2006. Mean values with their standard deviations are provided in the text as mean ± SD. The average number of years between tests was 12.5 ± 2.0 years. The mean age at the initial-recruit test was 24.6 ± 3.4 years and for in-service was 37.1 ± 3.7 years. There were no significant difference between genders for age and time between the two tests. Officers were primarily college educated and their socioeconomic status ranged from lower-middle to upper-middle class.

The University of North Carolina at Wilmington Institutional Review Board approved this study following the Office of Human Research Protection Requirements. Because this research involved existing and unidentifiable data it was exempt from applicable human subject regulation.

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Body weight, percent body fat scores, and bench press strength scores were retrieved for officers in the 1990 to 1995 recruit classes. Officers' recruitment scores were paired to their most recent scores on the 2006 in-service fitness record. Scores for officers in the recruit record that were not in the in-service record were discarded.

Emphasis was placed on encouraging recruits to continue healthy exercise habits following completion of the 16- to 20-week recruit training. The same physical fitness coordinator, certified by the American College of Sports Medicine (ACSM), administered tests over the study period. Sworn officers (in-service) received an annual fitness test. An officer's physical capacity was a consideration at hiring, but the hiring protocol was inconsistent. The physical fitness coordinator suggested nutritional and/or exercise programs for officers following evaluations and when requested.

Included in the physical fitness test battery for recruits and sworn officers was body weight, percent body fat using a skinfold caliper (16), and a 1 repetition maximum (1RM) bench press test (25) using a Smith Bench Press Machine (Atlantis Angled Smith Machine E-155, Quebec, Canada). Tests were administered the first week of recruit training (initial-recruit test). The physical fitness test battery was conducted in the following order for recruits, body fat (16), sit and reach flexibility (38), bench press (25), push-up, and 1.5-mile run (38). The testing protocol for the in-service officers was body fat (16), treadmill (13), bench press (25), muscle endurance (either curl-ups or push-ups), and sit and reach (38). A rest was given between the treadmill test and the bench press test. This investigation's variables of interest, body weight, percent body fat, and the bench press test protocols remained consistent over the time period of the study.

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Statistical Analyses

Paired sample t-tests compared mean differences between initial recruitment to in-service test for age, body weight, lean weight, bench press, bench press/lean weight, and bench press/body weight. Analysis of variance (ANOVA) procedures compared genders in the amount of change from initial-recruit to in-service tests in these same variables. Significance was defined as p ≤ 0.05.

High to low groups within bench press, bench press/lean weight, and bench press/body weight were tracked and changes among groups were compared. The male sample was subdivided into five equal groups or quintiles based on initial-recruit measurement. The cut point for the five male groups corresponded to the 20th, 40th, 60th, and 80th percentiles. Group 1 represented the lowest strength values progressing to group 5, the highest. Gender comparisons were discontinued as further dividing the women (n = 30) into smaller groups would have considerably weakened comparisons. Analyses included paired sample t-tests to test mean differences between initial recruitment and in-service evaluation for each group. ANOVA and Tukey multiple comparison procedures were used to compare changes among groups from the initial-recruit to in-service tests for each strength variable.

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Contrary to existing cross-sectional literature, over the 12.5-year period, bench press and bench press/lean weight significantly increased for both gender groups while bench press/body weight remained relatively consistent. When dividing the men's strength scores into five low- to high-strength groups, there tended to be an averaging effect as the officers progressed from initial-recruit to the in-service tests. The strongest group changed the least and the weakest group changed the most, which would be similar to expectations of short-term training studies.

The paired sample t-test comparisons indicated that body weight and lean weight increased significantly from initial-recruit to in-service tests in both gender groups (p ≤ 0.01). Table 1 presents the descriptive characteristics of the subjects by age, body weight, lean weight, and changes by gender over 12.5 years. There were significant differences (p ≤ 0.001) between genders in both body weight and lean weight with males being heavier at both testing periods. Men also increase significantly more (p ≤ 0.001) in body weight and lean weight than women.

Table 1

Table 1

Figure 1 compares the bench press scores by gender from initial-recruit to in-service over 12.5 years. Bench press strength significantly increased (p ≤ 0.001) for both gender groups from initial recruitment to in-service tests. Men were significantly stronger than women at both testing periods (p ≤ 0.001). Males changed significantly more than females from initial-recruit to in-service tests (p ≤ 0.001).

Figure 1

Figure 1

Figure 2 compares bench press/lean weight and bench press/body weight of gender groups of police from initial-recruit to in-service tests over 12.5 years. Both gender groups significantly increased in bench press/lean weight (males p ≤ 0.001, females p ≤ 0.05). There was very little difference in the amount of change in bench press/lean weight and the bench press/body weight between genders. ANOVA comparisons between genders indicated males were significantly stronger than females in bench press/lean weight and bench press/body weight at both testing periods (p ≤ 0.001).

Figure 2

Figure 2

Figure 3 compares low to high bench press strength groups of male police from initial-recruit to in-service over 12.5 years. The cut points of the five strength groups were at 20th, 40th, 60th, and 80th percentile of the initial-recruit bench press values. All the male groups increased significantly in bench press strength (p ≤ 0.01). Male officers in group 1 with the lowest starting bench press scores gained the most by their in-service tests. The strongest officers, group 5, gained significantly less than the other groups from initial-recruit to in-service (p ≤ 0.05). The bench press groups did not change their relative position to each other over time as the strongest continued to be the strongest and the weakest remained the weakest.

Figure 3

Figure 3

Figure 4 compares low to high bench press/lean weight groups of male police from initial-recruit to in-service over 12.5 years. The cut points of the five strength groups were at 20th, 40th, 60th and 80th percentile of the initial-recruit bench press/lean weight values. The strongest officers, group 5, significantly decreased in bench/lean weight (p ≤ 0.05) and the change was significantly different from all other groups (p ≤ 0.001). Groups 1 through 4 significantly gained in bench press/lean weight (p ≤ 0.001) with the weakest, group 1, increasing significantly more than all other groups (p ≤ 0.05). The bench press/lean weight groups did not change their relative position to each other over time.

Figure 4

Figure 4

Figure 5 is a comparison of low to high bench press/weight groups of male police from initial-recruit to in-service over 12.5 years. Groups 1, 3, and 5 changed significantly in bench press/weight strength from initial-recruit to in-service test (p ≤ 0.05) with groups 1 and 3 increasing and the strongest, group 5, decreasing. The negative change in the strongest group, group 5, was significantly different from the lower strength groups (p ≤ 0.001). The positive change in the weakest officers, group 1, was significantly different from groups 2, 4, and 5 (p ≤ 0.05). The bench press/weight groups did not change their relative position to each other over time.

Figure 5

Figure 5

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The results of this study did not follow expected strength trends, as described in earlier studies, which reported annual declines in muscular strength in men and women (33,34,37). Overall, the officers in our study increased in strength well into their late 30s and early 40s. This longitudinal data on police officer strength is unique in the literature, to our knowledge, as the research found previously in regards to bench press strength was cross-sectional (26,38) and none was located regarding bench press strength and police specifically.

Our study found that female police officers had approximately 44% of the upper body strength of the male officers. This trend was slightly lower than the 50-60% reported by others (23,31,32). Although the literature reports a difference in muscular decline between males and females (19,28,37), our study showed strength increases in both genders. The female strength increase was approximately 42% of that of the males over the 12.5-year period.

It has been reported in short-term fitness training studies that untrained individuals who are low in strength can have greater potential for strength gain than those who are already closer to their maximum strength potential (9,10). Our 12.5-year longitudinal findings followed these same trends as the weakest group demonstrated the largest strength gains while the strongest group had the smallest gains in strength. This led to an averaging effect among the groups. However, it is interesting to note that the strength groups maintained their relative positions over time meaning that even with their greater strength gains; the weakest individuals did not overtake the higher groups. This has implications in the selection of police officers and continues to support recommendations for minimum physical standards at hire.

Another study, using this same police officer data but beginning at post-recruit training, demonstrated similar strength gains after recruit training (36). This indicated that the reported gains in our study were not simply the result of recruit training but had been influenced by other factors. Some factors may have included the department's ongoing fitness program as well as the officer's own maturing process and increases in lean weight.

The literature suggests that increases in body weight correspond with increases in lean weight by as much as 44% (11). The officers in this study gained a significant amount of body weight and correspondingly, a significant amount of lean weight. Lean weight is associated with increases in strength (11,20,21). Therefore, we would expect to see an increase in absolute bench press strength related to lean weight gain alone. However, the strength gains were negated when dividing the body weight of the officer into their bench press scores. Even though the bench press/body weight measure did not increase over the 12.5 years, it also did not decline as indicated in other cross-sectional research (38).

Consistent data collection as part of a real-life ongoing program, in place for over 25 years, has both strengths and weakness in regards to the stability of this data. One primary weakness is that the data does not provide test-retest outcomes for either testing period. Also, inherent errors exist in skinfold measurements, such as when subjects are extremely obese or lean (15,38) or have large amounts of visceral fat (5).

However, the stability of the data is substantially reinforced by other factors. The test-retest for the 1RM bench press and for lean weight assessments with skinfold measures have been reported by other studies to be highly reliable with intra-class correlation coefficients (r = 0.99-0.93) (1,22). A unique aspect of this study is in the fact that over this extended time period the same highly trained tester gave all the tests, essentially eliminating interclass testing errors. The test administrator is certified by the ACSM and holds a master's degree with an emphasis in exercise science. The tests were well rehearsed as the in-service tests were administrated for at least 48 weeks out of each year for more than 20 years and the recruit tests were given multiple times each year. Furthermore, the test scores used in this data represent multiple recruit classes over 5 years. The department takes the tests seriously as the department is legally bound to follow testing protocols which are strictly controlled in order to provide for the safety of the officers and the public and the test results are part of the employment profile of the officers.

Further longitudinal study is needed regarding the actual physical performance of police officer job functions as opposed to documenting a single strength measure. As this study examined the first half of a police officer's career, officers need to be studied for the second half of their career in order to evaluate the protective benefits of strength against disability and chronic disease and the ability of the officer to safely perform critical emergency functions.

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Practical Applications

One of the most important implications of this report is that it demonstrated how police departments, with the implementation of a properly designed physical fitness program, can expect to see increases in the strength of their personnel over the first half of their careers. Clearly there are positive benefits to be gained from maintaining ongoing fitness training and testing, even though there is no on-duty physical training for the in-service police officers. It is also important to emphasize the role of lean weight as a component in body weight gain. Lean weight can contribute to improved functional ability in terms of strength. Therefore, weight gain can have positive implications in relationship to the overall strength of a police officer. However, the substantial weight gain by the officers in this study over the 12.5 years also revealed a need for programs that address the potential health implications of body weight gain over time.

The importance of having minimum selection criteria for police officers was demonstrated when examining the males in low- to high-strength groups. The strength groups did not change relative positions over time even when they showed improvements in their strength scores. Overall, this study illustrated the long-term benefits of maintaining fitness training and a fitness testing program, as directed and overseen by an exercise professional. This emphasizes the critical importance of a structured physical fitness program in a police environment for the purpose of encouraging and promoting improvements in physical strength over time.

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bench press; police; longitudinal study; body weight; sex differences

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