Research Paper Assignment

MILITARY MEDICINE, 183, 11/12:e414, 2018

Secular Trends in the Physical Fitness of United States Army Infantry Units and Infantry Soldiers, 1976–2015

Joseph J. Knapik*†; Jan E. Redmond*; Tyson L. Grier‡; Marilyn A. Sharp*

ABSTRACT Introduction: The purpose of this investigation was to examine changes over time in the physical fit- ness of the United States (US) Army infantry. A systematic review was conducted to identify and analyze articles and databases that reported on physical characteristics (height, weight, and body mass index) and/or Army Physical Fitness Test (APFT) scores of male infantrymen or men in infantry units. Materials and Methods: The National Library of Medicine’s PubMed and the Defense Technical Information Center were searched using specific keywords. Reference lists of obtained articles, specific author searches, contact with authors, and secondary analysis of available databases enhanced the search. Studies and databases were selected if they involved infantry soldiers or soldiers in infantry units and provided a quantitative assessment of at least one physical characteristic or APFT measure. Average values for each measure were obtained, plotted by the year of data collection, and fitted to linear regression models. Results: Thirty-one articles and five available databases met the review criteria encompassing years 1976–2015. Regression analysis suggested a small temporal increase in height for infantry soldiers (2%) and soldiers in infantry units (1%). Body weight and body mass index increased over the period (9–15%) in both groups. APFT performance of infantry soldiers was generally higher than that of infantry units. There was little change in APFT performance over the period for infantry soldiers. For soldiers in infantry units, push-up performance changed little over time, whereas sit-up perfor- mance increased (8%) and two-mile run performance decreased (8%). Conclusion: Over the surveyed period, body weight and body mass index increased in US Army infantry soldiers and soldier in infantry units. Infantry soldier per- formance on the APFT appears to have been maintained over the period. For soldiers in infantry units, there has been a small improvement in sit-up performance, but lower two-mile run performance.

INTRODUCTION Physical fitness is widely acknowledged as important for health and optimal physical performance. Lower physical fitness is a risk factor for cardiovascular disease,1–4 certain types of cancers,5,6 type 2 diabetes,7,8 hypertension,9,10 stroke,11

and all-cause mortality.4,12 In the military services, higher levels of physical fitness are important not only for health but also for military task-specific performance and injury prevention. Fitter soldiers exhibit higher levels of perfor- mance on actual or simulated military tasks.13–16 Numerous studies have now shown that the performance on military-

specific tasks such as load carriage, repetitive lifting, obstacle courses, casualty drags, and other tasks are improved by high- er levels of physical fitness.13,15,17–20 Soldiers with lower levels of fitness have also been shown to be at higher risk of injury.21–23

The current Army Physical Fitness Test (APFT) consists of push-ups (PUs), sit-ups (SUs), and a two-mile run. The APFT is administered to all United States (US) Army soldiers twice a year.24,25 Height and weight are usually assessed at the same time the APFT is conducted to assure that soldiers meet the weight for height standards for retention in service.24,26 Until recently, there was no Army-wide database for these data, and it was usually saved at the local level among US Army units, but beginning in 2015, these data could be stored in the Army-wide Digital Training Management System.

Before 2015, a number of studies were published that pre- sented physical characteristic (height, weight, and body mass index [BMI]) and APFT performance of infantry soldiers and soldiers in infantry units. Using these previously published papers and available databases, the purpose of this investigation was to examine changes over time in the physical fitness of US Army infantry soldiers and soldiers in infantry units. This arti- cle will first define physical fitness in relation to the APFT and BMI and then present results of a systematic review.

PHYSICAL FITNESS AND THE APFT Physical fitness can be simply defined as “a set of attributes that allows the performance of physical activity”.27 Table I

*US Army Research Institute of Environmental Medicine, 10 General Greene Ave, Natick, MA 01760.

†Henry M Jackson Foundation, 6720A Rockledge Dr, Bethesda, MD 20817.

‡US Army Public Health Center, 5158 Blackhawk Rd, Aberdeen Proving Ground, MD 21010.

The views, opinions, and/or findings contained in this report are those of the author(s) and should not be construed as an official Department of Army position, policy, or decision, unless so designated by other official documen- tation. Citation of trade names in this report does not constitute an official Department of the Army endorsement or approval of the use of such commer- cial items. In the conduct of research involving human subjects, the investi- gator(s) adhered to the policies regarding the protection of human subjects as prescribed by Department of Defense Instruction 3216.02 (Protection of Human Subjects and Adherence to Ethical Standards in DoD-Supported Research) dated November 8, 2011.

doi: 10.1093/milmed/usx093 Published by Oxford University Press on behalf of the Association of

Military Surgeons of the United States 2018. This work is written by (a) US Government employee(s) and is in the public domain in the US.

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provides the attributes or components of physical fitness and their definitions assembled from a number of sources.28–34

Some investigators have separated the components of physi- cal fitness into those that are health related and those that are performance related, as shown in Table I.28,29,35 Health- related components are those that have been specifically linked to health outcomes, whereas the performance-related components are those related to skilled activity in sports, exercise, or occupation tasks. There is considerable overlap between health- and performance-related fitness components as shown in Table I. The APFT measures two components of physical fitness. The two-mile run is a measure of cardio- respiratory endurance while PUs and SUs assess muscular endurance.36

Body weight and/or BMI (weight/height2) are often con- sidered components of fitness because of their relation with health and their interaction with the other fitness factors. Higher body weight is generally negatively associated with endurance activities37,38 and performance on lower body power tests like the broad jump, vertical jump, and short sprints.39–42 Overweight and obesity are associated with increased risk of cardiovascular disease, type 2 diabetes, musculoskeletal disorders, and certain types of cancers.43

As height and weight are typically obtained during the APFT, it is possible to obtain BMI. BMI is often used an index of obesity and/ or overweight44,45 and there are both advantages and disadvantages with the use of BMI in this regard. BMI corrects body weight for the height of an individ- ual, essentially removing the dependency of weight on height.46

It is an easy to obtain measure, and large, publicly available databases (e.g., Behavioral Risk Factor Surveillance Survey [BRFSS] and National Health and Nutrition Examination Survey47) can be used to describe populations and trends, as many investigations have done.48,49 The correlation between body fat and BMI ranges from about 0.7 to 0.9 in both civilian samples,46,50,51 new Army recruits,52 and infantry soldiers.53

However, correlations ranging from 0.7 to 0.9 indicates that only about 50–80% of the variance in BMI is in common

with body fat. Further, BMI does not reflect the change in body composition (increase in fat and reduction in fat-free mass) that occurs naturally with aging.54 There is also evi- dence that BMI may be associated with different proportions of body fat in different racial groups55 and that leg length and body build can affect the association between fat and BMI.56 An individual can have a high BMI not only because of higher body fat but also because of higher muscle and/or bone mass as the index does not distinguish between the two. Thus, the limitations of BMI should be considered in the interpretation of data derived from this measure.

METHODS For this investigation, the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines were followed.57 The authors followed a review protocol that is described below.

Information Sources and Search To find studies on the physical fitness of infantry soldiers, the National Library of Medicine’s PubMed and the Defense Technical Information Center were searched. Keywords used in the search included {soldier OR military personnel OR infantry AND physical fitness OR push ups OR sit ups OR two-mile run OR body mass index OR weight OR height}. In the Defense Technical Information Center search, only unclassified articles with full-text and unlimited public access were included. Authors known to have conducted research on infantry units or infantry soldiers were queried in the retrieval services and the reference lists of obtained arti- cles were searched for other publications. The files of a senior researcher with experience in studying infantry fitness were also examined. Eight authors were contacted personally to clarify data collection methods, dates, or to obtain original data. No limitations were placed on the dates of the searches and the final search was completed in December 2016. Some original databases containing fitness data on infantry units

TABLE I. Components of Physical Fitness

Category Fitness Component Definition

Health- and performance-related physical fitness components

Cardiorespiratory endurance Ability of a muscle group to sustain external forces for long periods of time Muscular strength Maximal force exerted by a muscle group in a single voluntary contraction Muscular endurance Ability of a muscle group to exert external forces for a short period of time Flexibility Range of motion achieved at a joint Physical characteristics and body composition

Body fat, fat-free mass, and ratio of body weight to height (BMI)

Performance-related physical fitness components

Power Ability to exert force in a short period of time Balance Maintenance of equilibrium while stationary or moving Speed Ability to move the body quickly Agility Ability to change the position of the entire body in space with speed and

accuracy Coordination Ability to use the senses, such as seeing or hearing, together with the body parts

in performing motor tasks smoothly and accurately

Adapted from references.28–30

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were available from the authors. In these cases, information in these databases was used in the analyses.

Study Selection and Data Collection Process Articles were selected for the review if they (1) were written in English, (2) involved infantry soldiers or soldiers in US infantry units, and (3) provided quantitative assessments of height, weight, BMI, or any APFT measure. Publication titles were first examined and abstracts were reviewed if the article appeared to involve military personnel and physical fitness measures. The full text of the article was retrieved if the abstract suggested that US infantry soldiers or infantry units were involved and quanti- tative fitness measures obtained. If authors did not explicitly report on BMI, but height and weight were available in the article, then BMI was estimated as weight/height2 (kg/m2) from the mean data. In cases where original databases were available for secondary analysis, the criteria for selection was essentially the same as for journal articles. That is, the database was selected if it involved infantry soldiers or sol- diers in US infantry units and contained height, weight, BMI, and/or any APFT measure. Data from original databases were footnoted in the tables.

Not included in the review were studies that involved (1) military personnel other than infantry soldiers or soldiers in infantry units, (2) studies that did not contain quantitative data, (3) studies of foreign (i.e., other than US) military units, (4) studies on military services other than the Army, or (5) studies of selected populations (e.g., overweight soldiers). There were studies that involved a single data collection period for which the data were used in multiple publications. Authors were contacted to identify these investigations; studies with the largest number of subjects were used in the analyses.

Data Analyses Studies were separated into those that involved infantry soldiers and those that involved soldiers in infantry units. Infantry unit data invariably included infantry soldiers, but it was not clear in any published article what the military occupational special- ties were included in the infantry unit data. Where original data were available and soldiers’ military occupational special- ties were included in the database, infantry soldiers were identified and treated separately. In these databases, infan- try soldiers were defined as any soldier with an 11-series military occupational specialties (infantry specialties).58

Mean data on the measures were placed into a database and the Statistical Package for the Social Sciences (SPSS, Version 21) was used to apply a variety of curve-fitting techniques to the data to examine trends over time (years). Linear, polyno- mial, exponential, logarithmic, and power fits were applied to the mean data, but with few exceptions, a linear fit was able to account for almost as much variance as alternate fits; alter- natives to linear modeling generally added little explanatory information. Linear correlation coefficients (measures vs. years) were calculated and p-values and standard errors of

estimate were determined for these coefficients. These val- ues were shown on graphs plotting the data by year. In the regression analysis, years were indicated by sequential numbers (1, 2, 3, etc.) rather than the actual year so that the intercept was near the estimated value at earliest point at which the data were collected.

RESULTS Figure 1 shows the publications and databases included and excluded at each stage of the literature search. The PubMed search yielded 968 titles and the Defense Technical Information Center search 4,369. After reading titles and selected abstracts, 44 full-text articles were obtained. Thirty-one articles in 18 independent data collection periods fully met the review crite- ria. Original databases encompassing five separate years were included, one of these from an unpublished investigation. Studies and databases collected data between 1976 and 2015, a 39-yr span. Table II shows the studies with each row representing an independent data collection period (19 in total). In some stud- ies, the number of soldiers tested on each measure differed so a range is shown. The average ages of the soldiers in the studies ranged from 20 to 26 yr.

Physical Characteristics Data on physical characteristics (height, weight, and BMI) were available for infantry units from 1976 to 2015 (39 years) and for infantry soldiers from 1979 to 2015 (36 years). In many studies, height and weight data were col- lected from unit records.63,68,75,76,86–88 If these measures were collected by investigators, height was generally mea- sured with a statiometer and weight with a digital or beam balance scale.13,59,62,64,66,69,70,77,79,89 BMI was calculated

Publications identified through database searching

(n = 5,337)

Additional publications and databases identified through

other sources (n = 21)

Publications and databases after duplicates removed (n = 5,316)

Full publications (n = 44) and databases (n = 5) obtained after

review of titles and abstracts

Publications excluded because no original or

relevant data, involved special

populations, or did not involve infantry (n = 13)

Articles included in review (n = 31)

Databases include in review (n = 5)

Data collection periods included in quantitative synthesis

(n = 19)

FIGURE 1. Publications and databases included and excluded at each stage of review.

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directly from these data in some investigations,64,68,69,75,76,86–89 but required calculation from mean data in others.13,59,62,63,66,70,77,79

Figure 2 suggests that body height was higher in later years compared with earlier years (slope of the regression line). This difference was larger in infantry soldiers than among soldiers in

infantry units. For soldiers in infantry units, linear regres- sion suggested an estimated 175.9 cm in 1976 and 177.8 cm in 2015 (1% difference); among infantry soldiers, linear regression suggested an estimated 174.3 cm in 1979 and 177.8 cm in 2015 (2% difference).

TABLE II. Studies Measuring Physical Fitness of Infantry Soldiers and Soldiers in Infantry Units

Study (Reference Number)

Year Data Collected

Location of Data Collection Unit

Type of Soldiers

Infantry Unit

Soldiers (n)

Infantry Soldiers

(n) Measures

Patton and Vogel59; Patton et al60,61

1976 Camp Casey, Korea 2nd Infantry Division Infantry unit 359 NDA Ht, Wt, BMIa

Wright et al62 1979 Fort Hood, TX Infantry Soldiers in Transit

Infantry soldiers NDA 97 Ht, Wt, BMIa

Knapik et al13 1981 Fort Lewis, WA 9th Infantry Division Infantry soldiers NDA 34 Ht, Wt, BMIa, PU, SU, 2MR

Knapik and Drews63

1986 Fort Drum, NY 10th Mountain Division Infantry unit 153–260 NDA Ht, Wt, BMIa, PU, SU, 2MR

Reynolds et al64; Knapik et al65

1988 Fort Hunter Liggett, CA 7th Infantry Division (Fort Ord, CA)

Infantry soldiers NDA 150–159 Ht, Wt, BMI, PU, SU, 2MR

Knapik et al66, 67 1989 Fort Richardson, AK 6th Infantry Division Infantry unit 78–335 NDA Ht, Wt, BMIa, PU, SU, 2MR

Reynolds et al68, 69 1990 Fort Drum, NY 10th Mountain Division Infantry soldiers NDA 154–181 Ht, Wt, BMI, PU, SU, 2MR

Moore et al70; Friedl et al71,72; Nindl et al73,74

1991 Fort Benning, GA Ranger Training Brigade Infantry soldiers (ranger trainees)

NDA 190 Wt, Ht, BMIa

Schneider et al75 1995 Fort Bragg, NC 82nd Airborne Division Infantry unit 799–1018 NDA Ht, Wt, BMI, PU, SU, 2MR

Reynolds et al68 2001 Fort Carson, CO 10th Special Forces Group

Infantry soldiers (special forces)

NDA 162 Ht, Wt, BMI, PU, SU, 2MR

Knapik (unpublished)76,b

2004 Fort Campbell, KY 101st Airborne Division Infantry soldiers and infantry unit

1,349–1,441 837–926 Ht, Wt, BMI, PU, SU, 2MR

Knapik et al87,b 2005 Fort Polk, LA 10th Mountain Division Infantry unit 595–656 NDA Ht, Wt, BMI, PU, SU, 2MR

Sharp et al77, 78 2006 Fort Polk, LA 10th Mountain Division Infantry unit 103–110 NDA Ht, Wt, BMIa

Allison et al79; Sell et al80; Abt et al81

2007–2012 Fort Campbell, KY 101st Airborne Division Infantry unit 347 NDA Ht, Wt, BMIa

Grier et al88,b 2010 Schofield Barracks, HI 25th Infantry Division Infantry soldiers and infantry unit

380–765 64–291 Ht, Wt, BMI, PU, SU, 2MR

Martin et al82;Grier et al83,84; Anderson et al85, b

2011 Fort Carson, CO 4th Infantry Division Infantry soldiers and infantry unit

2,758–4,968 388–727 Ht, Wt, BMI, PU, SU, 2MR

Teyhen et al86 2011 Joint Base Lewis– McCord, WA

Infantry soldiers (rangers) and infantry unit

211 636 Ht, Wt, BMI, PU, SU, 2MR

Redmond et al89,b 2014 Fort Stewart, GA; Joint Base Lewis–McCord, WA; Fort Carson, CO

3rd Infantry Division, 7th Infantry Division, 4th Infantry Division

Infantry soldiers and infantry units

113–121 58–67 Ht, Wt, BMI, PU, SU, 2MR

Redmond et al89,b 2015 Fort Stewart ,GA, Fort Carson, CO

3rd Infantry Division, 4th Infantry Division

Infantry soldiers and infantry units

177–190 85–91 Ht, Wt, BMI, PU, SU, 2MR

aEstimated from reported height and weight. bData obtained from original dataset. NDA, no data available on this group; Ht, height; Wt, weight; 2MR, two-mile run; AK, Alaska; CA, California; CO, Colorado; GA, Georgia; HI, Hawaii; KY, Kentucky; LA, Louisiana; NC, North Carolina; NY, New York; TX, Texas; WA, Washington.

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Figure 3 suggests that body weight was greater in later years, with a similar rate of increase among infantry units and infantry soldiers. For soldiers in infantry units, linear regres- sion suggested that weight was 72.8 kg in 1976 and 83.8 kg in 2015 (15% difference); for infantry soldiers, these values were 72.4 in 1979 and 82.2 in 2015 (14% difference).

Figure 4 suggests that BMI was greater in later years, with a greater change among soldiers in infantry units com- pared with infantry soldiers. For soldiers in infantry units, lin- ear regression indicated that BMI was 23.6 kg/m2 in 1976 and 26.6 kg/m2 in 2015 (13% difference); for infantry soldiers, these values were 23.8 kg/m2 in 1979 and 25.9 kg/m2 in 2015 (9% difference).

APFT Measures APFT measures were available from 1976 to 2015 for sol- diers in infantry units (39 years) and 1979 to 2015 for infan- try soldiers (36 years). In most studies, APFT values were obtained from records of military units.63,64,66,68,69,75,76,86,87

In one case, the APFT was administered by the investigators13

and in another case, scores were self-reported by soldiers.89

Testing order for the APFT was PUs, SUs, and the two- mile run. For PUs, the soldier was required to lower his body in a generally straight line to a point where his upper arms were parallel to the ground and then return to the start- ing point with elbows fully extended. For the SUs, the sol- dier’s knees were bent at a 90° angle, fingers interlocked behind the head, and a second person held the participant’s ankles to keep his heels firmly on the ground. The soldier raised his upper body to a vertical position so that the base of the neck was anterior to the base of the spine and then returned to the starting position. Test administrators recorded the number of PUs and SUs successfully completed in two separate 2-min periods. Following these two assessments, test administrators had soldiers lined up at a starting line, began the two-mile run and recorded the time it took for each soldier to complete the distance. At least 10 minutes of rest was allowed between the three events.

Figure 5 suggests that for PUs, there was a very small improvement in performance over time, but the regression was not significant for either infantry soldiers or infantry units.

Figure 6 suggests that SU performance in later years was higher than in earlier years. The regression was not significant for the infantry soldiers. Infantry units showed about half the variability (standard errors of estimate) of the infantry soldiers and the regression fit was significant. For infantry units, linear regression indicated that SU performance was 63 repetitions in 1986 and 68 in 2015 (8% difference). Despite the variability and non-significant regression fit, the difference in SU perfor- mance among infantry soldiers was similar to infantry units, 67 repetitions in 1981 and 72 repetitions in 2015 (7% difference).

Figure 7 suggests that for the two-mile run, there was lit- tle difference in performance over time for infantry soldiers. On the other hand, infantry units tended to show lower per- formance in later years (i.e., longer run times) compared

y = 0.05x + 175.82 (Infantry Units) R2 = 0.32, SEE = 0.86, p = 0.04

y = 0.10x + 174.23 (Infantry Soldiers) R2 = 0.56, SEE = 1.22, p = 0.01

160

165

170

175

180

185

190

1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 2020

H e ig

h t

(c m

)

Infantry Units (Dotted Line)

Infantry Soldiers (Solid Line)

FIGURE 2. Temporal trends in height of US Army infantry soldiers and infantry units. Solid lines are regression line for infantry soldiers and dashed lines for infantry units.

y = 0.29x + 72.50 (Infantry Units) R2 = 0.92, SEE = 1.10, p<0.01

y = 0.28x + 72.08 (Infantry Soldiers) R2 = 0.82, SEE = 1.79, p<0.01

60

65

70

75

80

85

90

1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 2020

W e ig

h t

(k g

)

Infantry Units (Dotted Line)

Infantry Soldiers (Solid Line)

FIGURE 3. Temporal trends in body weight of US Army infantry soldiers and infantry units. Solid lines are regression line for infantry soldiers and dashed lines for units.

y = 0.08x + 23.48 (Infantry Units) R2 = 0.82, SEE = 0.46, p<0.01

y = 0.06x + 23.76 (Infantry Soldiers) R = 0.81, SEE = 0.41, p<0.01

18

20

22

24

26

28

30

1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 2020

B o

d y

M a

s s

I n

d e

x (

k g

/m 2 )

Infantry Units (Dotted Line)

Infantry Soldiers (Solid Line)

FIGURE 4. Temporal trends in BMI of US Army infantry soldiers and infantry units. Solid lines are regression line for infantry soldiers and dashed lines for infantry units.

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with earlier years. For infantry units, linear regression indi- cated an estimated two-mile run time of 13.3 min in 1986 and 14.4 min in 2015 (8% difference).

DISCUSSION This study examined temporal trends in US Army infantry physical fitness by reviewing and analyzing quantitative fit- ness measures available in the literature or accessible data- bases. Linear regression analysis suggested a small temporal increase in the height of both infantry soldiers and infantry units (1–2%). Data strongly suggested an increase in weight and BMI over the survey period ranging from 9% to 15% for both infantry soldiers and soldiers in infantry units. APFT performance of infantry soldiers was generally higher than that of infantry unit soldiers. Among infantry soldiers, there was little change over the survey period on any APFT measure. For soldiers in infantry units, PU performance changed little over time, whereas SU performance increased (8%) and two-mile run performance decreased (8%).

Physical Characteristics There is historical evidence that BMIs have been increasing since at least the late nineteenth century in the US.90,91 Data col- lected from large national US samples (i.e., NHANES) did not begin until about 1960 and suggested little change in BMI from about 1960 to 1980, but after this BMI increased over years.92–94

The average increase in BMI from about 1980 to the early 2000s has been estimated at 11% for men (25.5–28.3 kg/m2).95

The most recent NHANES data suggested a leveling off in the prevalence of obesity after 2000s.48,96 Similar sec- ular increases in weight and/or BMI have been reported in other countries.97–108 Soldier BMI data reported here involved the period from 1976 through 2015 and, in consonance with the civilian data, showed a trend of increasing BMIs with similar rates among infantry soldiers and soldiers in infantry units. The absolute BMI values among these soldiers were lower than in the general US population.95,109

In civilian investigations, the increase in body weight and BMI since 1980 has been ascribed to both a reduction in physi- cal activity and an increase in food intake.94,110–112 With regard to physical activity, early civilian surveys and other data dating back to 1965 suggest a small increase in the prevalence of adult leisure-time physical activity from 1965 to 1982,113 but occupa- tional activity has decreased.114 One study compared adult data from three national survey systems including the BRFSS, NHANES, and National Health Interview Survey (NHIS) using standard definitions for “active” and “inactive.” The BRFSS indicated a slight increase in the prevalence of active indivi- duals and a slight decline in inactive individuals from 2001 to 2007. NHANES indicated a similar trend from 1999 to 2006. The NHIS indicated no significant trend from 1998 to 2007. Military personnel are considerably more active than civilian personnel115–118 likely because of their younger age, occupa- tional requirements, and requirement to take and pass the

y = 0.18x + 63.22 (Infantry Units) R2 = 0.40, SEE = 2.42, p = 0.05

y = 0.15x + 66.75 (Infantry Soldiers) R2 = 0.13, SEE = 4.91, p = 0.31

40

50

60

70

80

90

1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 2020

S it

-U p

s (

n )

Infantry Units (Dotted Line)

Infantry Soldiers (Solid Line)

FIGURE 6. Temporal trends in SU performance of US Army infantry sol- diers and infantry units. Solid lines are regression line for infantry soldiers and dashed lines for infantry units.

y = 0.20x + 58.67 (Infantry Units) R2 = 0.24, SEE = 3.87, p = 0.15

y = 0.12x + 63.92 (Infantry Soldiers) R2 = 0.11, SEE = 4.41, p = 0.34

40

50

60

70

80

90

1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 2020

P u

s h

-U p

s (

n )

Infantry Units (Dotted Line)

Infantry Soldiers (Solid Line)

FIGURE 5. Temporal trends in PU performance of US Army infantry sol- diers and infantry units. Solid lines are regression line for infantry soldiers and dashed lines for infantry units.

y = 0.04x + 13.24 (Infantry Units) R2 = 0.45, SEE = 0.48, p = 0.03

y = 0.01x + 13.92 (Infantry Soldiers) R2 = 0.10, SEE = 0.50, p = 0.37

10

12

14

16

18

1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 2020

T w

o -M

il e R

u n

( m

in )

Infantry Units (Dotted Line)

Infantry Soldiers (Solid Line)

FIGURE 7. Temporal trends in two-mile run times of US Army infantry soldiers and infantry units. Solid lines are regression line for infantry sol- diers and dashed lines for infantry units.

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APFT twice a year. Available data suggest that physical activ- ity has not decreased in military personnel and some data actu- ally indicates an increase during the survey period.116,119

possibly associated with the requirement for twice yearly APFT for all soldiers that was initiated in 1982.25 Thus, it is unlikely that a reduction in physical activity accounted for the increase in BMI seen here.

In contrast to the physical activity data, there is firmer evidence that energy intake has increased in civilian popula- tions. Although complicated by differences in data collection methods over time, national data from NHANES indicate that energy intake increased in the four decades from the early 1970s into the 2000s. The increase in energy intake came primarily from carbohydrate sources and was found in most BMI groups (normal, overweight, and obese).95,120,121

More recent NHANES data extending to 2010 suggested energy intake peaked in the 2003–2004 period then decreased slightly in subsequent years accounted for by small decreases in carbohydrates and fat intake.122 No studies could be found on temporal trends in the energy intake of soldiers.

To be retained in service, soldiers must not exceed pub- lished weight for height standards. These have changed somewhat over time and could have affected the secular trends. A sample of the male weight for height standards con- verted to BMIs is shown in Table III. Before 1983, there were only minimum and maximal standards for all soldiers regard- less of age. Beginning in 1983, maximal standards became dependent on age. Table III shows that the minimal allowable BMIs were generally higher beginning in 1983. The maximal allowable BMIs beginning in 1983 tended to be lower in the younger age groups (≤39 years) compared with the standards before 1983. Despite the more rigorous post-1983 standards, body weight and BMI rose over time as shown in Figures 3 and 4. It should also be noted that beginning in 1983, if a sol- dier exceeded the weight for height standards, their body fat could be estimated using circumferential techniques. Maximal allowable body fat values for men were 20% for 17- to 20-yr olds, 22% for 21- to 27-yr olds, 24% for 28- to 39-yr olds, and 26% for those ≥40 years.

Another factor to consider is that the increase in BMI may reflect not only an increase in fat but also an increase in lean mass. This was the case when secular trends were examined

in new recruits entering the US Army.127 Olds128 reviewed 154 studies conducted from 1951 through 2004 that had provided skinfold data to estimate body fat. He found that the average increase in body fat was 0.8 kg/decade, whereas the increase in fat-free mass was 0.6 kg/decade. Thus, the possibility that the increase in BMI also reflects an increase in lean body mass must be considered.

Muscular Endurance PU and SU performance generally increased over time. However, for PUs, the trend was not statistically significant and there was considerable variability in performance over the years. For SUs, the rate of performance improvement (slope of the regression) was similar among infantry units and infantry sol- diers but variability (standard errors of estimate) was more than twice as great among infantry soldiers so that the performance increase was not significant. There were no civilian studies found on secular trends in PU performance, but there were investigations on SU performance, although samples involve adolescents and methods (performance time and body position) differed. In SU performance of young US men collected yearly between 1980 and 1989, the SU performance (methods not specified) increased,129 in consonance with the SU data among soldiers in infantry units. In other countries, results have been mixed. In Lithuanian 18-yr-old males, abdominal curls in 30 sec increased from 1992 to 2012,98 and in Finnish 13- to16-yr-old males, 30-sec SU performance increased substantially (14%) when comparing males in 1976 with those tested in 2001.103

On the other hand, in 16-yr-old Swedish males, SU perfor- mance (posture similar to the APFT but SUs were in cadence at 25 repetitions and to exhaustion) was lower in 1995 com- pared with 1974.100

Cardiorespiratory Endurance The two-mile run is considered a measure of cardiorespiratory endurance because it has a high correlation with VO2max, which is considered the criterion measure of this component of physical fitness.36 Although infantry soldiers appeared to have largely maintained their two-mile run performance over the sur- vey period, performance of infantry units declined. There are a few studies of secular trends in the cardiorespiratory fitness in

TABLE III. Sample of Minimum and Maximal BMI (Calculated from Height and Weight) for Retention in US Army at Various Periodsa

Height (in.)

1976–1983 (kg/m2) 1983–2015 (kg/m2)

Minimum Regardless of Age

Maximum Regardless of Age

Minimum Regardless of Age

Maximum for Age

17–20 yr 21–27 yr 28–39 yr ≥40 yr

60 19.5 27.6 18.9 25.8 26.6 27.2 27.6 65 17.6 27.5 19.0 25.8 26.5 27.1 27.5 70 17.7 27.6 18.9 25.8 26.5 27.1 27.6 75 17.9 27.5 19.0 25.8 26.5 27.1 27.5 80 18.2 27.4 19.0 25.7 26.4 27.1 27.5

aFrom references.26,84,123–126

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the US,130–132 but these involve unique populations. Only one investigation has included a nationally representative sample and this investigation did not examine secular trends.133 Data on estimated VO2max have been collected at the Cooper Clinic in Dallas, Texas, for some time, but these data only include individuals who could afford an exclusive preventive medicine examination. In an analysis of temporal changes using these data, it was found that men 20- to 34-yr old who were tested between 1980 and 1989 had a higher estimated VO2max (treadmill time and Balke protocol) than those tested between 1970 and 1979 (45 vs. 41 mL/kg/min), but those attending the clinic in later decades (up to 2009) had only modestly lower VO2max values compared with the 1980–1989 group (43 vs. 45 mL/kg/min).130 Another cross-sectional study examined students at a Seventh-Day Adventist College (average age 22 years) in each year from 1996 to 2008. They found a secular decline in estimated VO2max (bicycle ergometer, Astrand protocol) that amounted to yearly linear declines of 0.8 and 0.4 mL/kg/min for men and women, respectively.132 In a nationally representative sample of Canadians, there was a decline in estimated VO2max (step test) from 1981 and 2007–2009 (27-year period).134 Several civilian studies have suggested a secular decline in the performance of endurance-running tasks like aerobic shuttle runs and long- distance runs for time (e.g., 1 and 1.5 miles) among both

US males and females of high school age129,135–138. This has been reported not only in the US but also more recently in other countries,97,98,101,104,139–141 over periods ranging from 7 to 40 years.

Higher body mass will generally decrease running per- formance as weight added to the body increases energy cost.37,142,143 Thus, the increase in body weight seen in this study (Fig. 3) may at least partly account for the decline in running performance among soldiers in infantry units. However, infantry soldiers were able to apparently maintain running performance despite the increase in their body weight. This may suggest that at least part of the body weight increase in infantry soldiers may have involved an increase in lean body weight as an increase in muscle mass (comprising about 50% of lean mass27) will increase whole-body oxidative capacity. However, the increase in lean mass is only specula- tion as the present data do not consider body composition. Further, studies that have looked at body composition in infantry soldiers or units13,59,62–64,70,77,80 have used a variety of methods to estimate fat and fat-free mass (skinfolds using different sites and equations, circumference, dual-X-ray absorptiometry, and densitometry) making comparisons dif- ficult. One civilian study that matched younger individuals for age, gender, BMI, and triceps skinfold found overall performance declines in 1.6-km runs and 20-m shuttle run

TABLE IV. Army Physical Readiness/Fitness Test Events with Maximal and Passing Scores Over Time

Years Age Group (yr)

Maximal Score (100 Points) Passing Score (60 Points)

PU (n) SU (n) Two-Mile Run (min) PU (n) SU (n) Two-Mile Run (min)

Pre-1986 17–25 68 69 13.1 40 40 17.9 26–30 66 67 13.7 38 38 18.5 31–35 61 65 14.3 33 36 19.2 36–39 60 63 15.1 32 34 19.6 40–45 a 20 25 20.0 46–50 20 25 21.0 51–55 15 20 22.0 56–60 15 20 23.0

1986–1998 17–21 82 92 11.9 42 52 15.9 22–26 80 87 12.6 40 47 16.6 27–31 78 82 13.3 38 42 17.3 32–36 73 78 14.0 33 38 18.0 37–41 72 73 14.7 32 33 18.7 42–46 66 69 15.1 26 29 19.1 47–51 62 67 15.6 22 27 19.6 ≥52 56 66 16.0 16 26 20.0

1998-Present 17–21 71 76 13.0 42 53 15.9 22–26 75 80 13.0 40 50 16.6 27–31 77 82 13.3 39 45 17.0 32–36 75 76 13.3 36 42 17.7 37–41 73 76 13.6 34 38 18.3 42–46 66 72 14.1 30 32 18.7 47–51 59 66 14.4 25 30 19.5 52–56 56 66 14.7 20 28 19.8 57–61 53 64 15.3 18 27 19.9 62+ 50 63 15.7 16 26 20.0

References:24,153–160 aOnly passing standards required for those ≥40 years.

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tests amounting to 29–61%, suggesting that while markers of fatness might account for some of the decline, other fac- tors are also involved.144

One potential confounder for APFT events is the fact that the point standards for the test have changed over time25 and these changes may have affected the secular trends. Various performance levels on each APFT events are assigned point values ranging from 100 to 0. The performance level at 100 points is the maximal recorded score and 60 points the mini- mum “passing” score. Scores on the three events are summed to a maximum of 300 points. Soldiers are generally focused on the point standards because this could influence their eval- uation reports and chances for promotion. Table IV shows the maximal and passing levels during the time period in this study. Note that the age groups in the pre-1986 period dif- fered from that of subsequent periods. Table IV shows that the performance level to achieve 100 points on each of the three events was higher in 1986–1998 than periods before or after this. Minimal passing scores (60 points) on the PUs changed little, but performance levels required to pass the SUs and the two-mile run were higher during and after 1986. APFT point values are established on a normative basis and maximal and minimal scores involve approximately the 90th

and 10th percentile, respectively (Dr Louis Tomasi, personal communication). Examination of Figures 5–7 does not sug- gest any systematic changes in the three time periods. On the whole, it appears that these changes had had little influence on the average performance measures analyzed here.

Limitations Besides the considerations with regard to the BMI and the APFT measures mentioned above, there are several other limita- tions to the analyses reported here. Some studies on which this review is based were cross-sectional, not specifically collected to examine longitudinal trends, and generally involved conve- nience samples. For some fitness measures, the number of sub- jects was low, with the possibility that the sample was not representative of the year indicated. Although the average age range was relatively narrow, some units could have had older soldiers that were included in the data and older indivi- duals generally have lower APFT scores and higher BMI.145,146

In one case, APFT measures were self-reported,89 but studies have demonstrated that self-reported APFT scores are valid measures of actual APFT scores.82,147

CONCLUSIONS Acknowledging the limitations above, the present analysis suggested that despite an increase in body weight and BMI, PU, SU, and two-mile run performance of infantry soldiers have changed little between 1979 and 2015, indicating little change in aerobic fitness and muscular endurance over the period. Soldiers in infantry units demonstrated almost identi- cal increase in weight and BMI to those of infantry soldiers

but demonstrated trends suggesting improvements in SU per- formance and declines in two-mile run performance.

As noted above, physical characteristics (height and weight) and APFT performance are routinely collected twice a year in US Army units.24 Data can now be entered into the Digital Training Management System148 and an execute order (EXORD #081-17149) was issued in January 2017 for many units to begin using the Digital Training Management System. However, units need to be instructed on use of the sys- tem and it is not clear how many units or soldiers are entering information at this point.150,151 Also, there are efforts in prog- ress to change the APFT25 and this could make comparisons with past data difficult if the test items change. In the civil- ian sector, self-reported height and weight are available in publicly available databases such as the BRFSS (https:// apps.nccd.gov/brfss/trends) and the NHANES where peri- odic measurements of height and weight over time have been captured.48,93,110,112,152 However, no measures of physical performance are collected among adults in the US. A compre- hensive fitness surveillance system that provides overlapping fitness measures from civilian and military sources would allow systematic tracking of fitness trends, periodic assessments, and comparisons between populations.

ACKNOWLEDGMENTS We would like to thank the following individuals who provided clarification of their methods or data: Dr Katelyn Allison, Dr Ludmila Cosio-Lima, Dr Takashi Nagai, Dr Bradley Nindl, Dr Katy Reynolds, Dr Dan Rhon, Dr Timothy Sell, and Dr Deydre Teyhen.

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