Introduction
A soldier must engage in many physically demanding activities, both near and on the battlefield, such as carrying heavy backpacks over long distances and rough terrain, and shorter, more intense activities, such as sprinting across the battlefield and traversing obstacles in rural and urban terrain. The speed at which these activities can be performed can affect a soldier's fighting effectiveness and survivability. Thus, it is important to seek the best training programs, within Army time constraints, to prepare soldiers for battle.
Few studies have been performed to investigate the improvement in militarily relevant physical performance attributable to physical training programs, and they have been mainly limited to observing the effects of training on the speed of medium- to long-distance individual load carriage, rather than other common battlefield activities, such as sprinting for cover or negotiating obstacles. Kraemer et al. (18,20) reported that 12 weeks of upper- or lower-body weight-based training by men, combined with high-intensity endurance training, improved 3.2-km foot travel time with a 44.7-kg backpack load by 14% and 11%, respectively. However, weight-based training or high-intensity endurance training alone produced no improvement. Furthermore, Kraemer at al. (19) observed that women who participated in 6 months of total-body or only upper-body weight-based training along with aerobic training improved significantly in the speed at which they could carry 34 kg for 3.2 km, whereas women engaged solely in aerobic training did not improve their speed.
Knapik et al. (14) found that, over 9 weeks, soldiers who marched 2 or 4 times a month in addition to performing endurance training, weight-based training, interval training, and calisthenics could carry loads significantly faster for 20 km than soldiers who did not march or marched only once a month in addition to the other training. Knapik and Gerber (15) observed that a 14-week program combining resistance and aerobic training reduced 5-km, 19-kg load carriage time by 4% among young women. Harman et al. (11) reported that the 3.2-km, 34-kg load carriage time of young women was shortened by 32.5% as a result of 24 weeks of weight-based training, running, backpack hiking, and drills. Williams and Rayson (26) found that 10 weeks of British Army recruit training reduced 3.2-km, 15-kg load carriage time of 20 men and 14 women by 6.7% and reduced 3.2-km, 25-kg load carriage time of 50 men by 16%. In addition to improving performance, physical fitness has been shown to reduce injury risk. A lower rate of injury during basic training was found among men and women who were at higher aerobic fitness levels (16). Collectively, these studies show the effectiveness of weight-based training for improving medium- to long-distance load carriage speed.
Because of the lack of research into the effects of physical training on military physical performance other than the speed at which loads could be carried over medium to long distances, the current study was designed to examine the effects of training on the performance of simulated battlefield activities. Two physical training programs were examined. A weight-based training program was chosen because of its widespread use in sport training. The Army's new Standardized Physical Training program was also examined because it recently became standard training for Army basic trainees and those in advanced individual training. Because previous Army basic training was found to improve the muscular strength of recruits by only approximately 10% (13), it was hypothesized that a weight-based training program could achieve more improvement in militarily relevant physical performance. The training programs examined in this study were meant for training new Army recruits and reservists called up for deployment, rather than for training soldiers assigned to their regular duty stations. The 8-week training period was selected based on the typical time allocated to such training.
It is noteworthy that the weight-based training program was designed to improve overall soldier physical performance, rather than to improve strength maximally. Battlefield physical performance requires speed, agility, and anaerobic endurance, in addition to strength. Thus, a program designed to improve battlefield performance must balance the types of training so as to achieve the most overall improvement within the limited amount of time the Army allocates for physical training. By consequence, compromises had to be made by which no individual physical ability, including strength, could be improved to the greatest degree possible. Therefore, strength increases brought about by the weight-based training program were not expected to be as great as those that could be engendered by a pure strength training program.
Methods
Experimental Approach to the Problem
The weight-based training program and the Army's new Standardized Physical Training program were compared as to improvement on 5 different simulated Army battlefield activities to determine how well each program physically prepares soldiers for the challenges they would face during military conflict. In addition, other physical abilities were tested to characterize the effects of the training programs. The simulated battlefield activities cover much of what a soldier must do to survive and fight effectively in combat, including carrying military loads for short and medium distances, repeatedly sprinting between protected locations, negotiating obstacles, and rescuing fallen comrades.
Subjects
The subjects (Table 1) were civilian men between the ages of 18 and 35 years, who responded to recruitment flyers, Internet postings, or advertisements in local newspapers. No women were recruited for the study because only men are accepted into ground combat positions in the U.S. Army, and the study was intended to relate to physical performance on the battlefield. Because of institutional standards for safe carriage of heavy loads, only volunteers weighing at least 61 kg were accepted. Furthermore, to simulate a recruit population, only volunteers who were not currently or recently engaged in an intense combination of aerobic and weight-based training were accepted, although those engaged in aerobic only, resistance only, or moderate recreational exercise were allowed. The volunteers came from varied backgrounds and included students, white-collar workers, and blue-collar workers. Their physical activity backgrounds ranged from sedentary to fairly active. Some had been doing some weight-based training, and some had been engaged in aerobic training. This is similar to a typical Army recruit population. After written informed consent was obtained from the volunteers, they underwent physical examinations to ensure they had no physical problems that would increase their injury risk or impede maximal efforts during testing or training. All volunteers met U.S. Army height and weight induction standards. The investigators adhered to the policies for the protection of human subjects as prescribed in Army Regulation 70-25, and the research was conducted in adherence with the provisions of 45 CFR part 46. The study was approved by the Human Use Review Committee of the U.S. Army Research Institute of Environmental Medicine and the Human Subjects Research Review Board of the U.S. Army Medical Research and Materiel Command.
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Table 1: The subjects in each training group.
Procedures
The Training Groups
The following are descriptions of the weight-based training (WBT) group and the Army Standardized Physical Training (SPT) group, both of which trained for 1 to 1.5 hours a day, 5 days a week, including warm-up and stretching. The ratio of volunteers to trainers was never more than 4:1.
Weight-based Training Group
This group trained with weight-based training exercises, medium-distance runs, sprint intervals, agility training, and load carriage hikes. Table 2 provides a training schedule for members of this group. Table 3 lists the weight-based training exercises designed to work all major body movements (10). For the 16 weight-based training sessions (2 sessions a week for 8 weeks), there was no waiting time between exercises beyond what it took to walk between stations and set the correct weight. The target pace was 90 seconds per set, including between-exercise time. The first day was focused on instruction and finding the correct starting weight for each trainee in each exercise. After the first day of weight-based training, the volunteers performed 2 sets of each exercise until they could perform 3 sets of each in the allotted time, a goal they all reached within 2 weeks (i.e., 4 sessions). The 8-km backpack hikes were performed at a prescribed speed of 6.4 km per hour and lasted 75 minutes. The first day's hike was without a load. Thereafter, weight was added week to week in a backpack or weighted vest, up to a maximum of 33 kg, based on the ease with which the volunteer completed the previous march. The agility training included drills, such as running around rubber cones set in different patterns, crawling on the ground, jumping over obstacles, running up hills, and shuffling laterally. The interval training started with fewer (2 or 3), longer (600-800 m) intervals and progressed to more (8-10), shorter (100-200 m) intervals.
Table 2: A training schedule for the weight-based training group.
Table 3: The weightlifting exercises performed by the weight-based training group.
Army Standardized Physical Training Group
This group performed exercises in accordance with the U.S. Army's Standardized Physical Training program, which became mandatory for basic training approximately when the study began in 2004. The program had been carefully developed in consideration of the physical demands placed on a soldier on the battlefield and incorporated many calisthenic exercises in addition to the push-up and sit-up variations typical of previous military training. The authors used an interim training manual, a version of which has since been published by the Army's Training and Doctrine Command as the IET Standardized Physical Training Guide (7), and were trained in administering the program by experienced Army personnel. Contrary to previous Army physical training, which gave a lot of flexibility to individual fitness trainers to select exercises from the Army's Physical Fitness Training: Field Manual No. 21-20 (5), the Standardized Physical Training program is a highly structured and standardized program with exercise sequence, sets, and repetitions prescribed for each exercise session during the full course of basic training. All exercises were performed with body weight as resistance, and the only equipment used was an array of 4 pull-up bars affixed to the ground. Table 4 shows the components of the Standardized Physical Training program, and Table 5 shows the Standardized Physical Training program workouts during week 6, as an example. As prescribed in the manual, the distance and time for each session of running were specified for each of 4 speed groups, in which individuals were placed depending on their time on a 1.6-km (1-mile) run test administered early in the program. For example, the manual specifies that the fastest group runs for 20 to 30 minutes in training, progressing during the course of the program from a pace of 7:30 per mile to 7:00 per mile, whereas the third fastest group runs for 20 to 25 minutes, progressing during the course of training from a pace of 9:30 per mile to 8:00 per mile. Detailed descriptions of the exercises and the complete program can be found in the manual (7).
Table 4: The components of the Army's Standardized Physical Training program.
Table 5: Week 6 workout for the Standardized Physical Training program group.
Before and After Testing
The following tests were performed over 15 days by all volunteers before and after the 8-week training period. Unpublished preliminary evaluation of these tests has shown them to be reasonably reliable, with intracorrelation coefficients in the range of 0.80 (M. Sharp, personal communication, January 2007). Volunteer familiarization and practice with the test activities were provided before testing to ensure stable scores (24).
Militarily Relevant Tests
Timed 400-m Run With an 18-kg Load
This test simulated a sprint across the battlefield. The load consisted of Army battle uniform, boots, helmet, dummy rifle of authentic weight and shape, body armor with protective ceramic plates, and fighting vest with dummy ammunition. The total weight varied to a small degree depending on helmet and body armor sizes. This is considered a standard infantry fighting load (6).
Timed 3.2-km Run or Walk With a 32-kg Load
This test simulated a rapid approach on foot to the battlefield. The load consisted of the fighting load described earlier, with an additional 14-kg backpack. This is considered a standard battlefield approach load (6).
Timed Obstacle Course With the Fighting Load
This test simulated running across an obstacle-strewn battlefield. The course involved leaping over 4 61-cm high hurdles; running zigzag around 9 rubber cones arranged in a rectangle 27 m long and 1.5 m wide; crawling through a rectangular tunnel 0.6 m high, 0.9 m wide, and 3.7 m long; shimmying along a horizontal pipe 3.7 m long; climbing over a 1.4-m high sheer wooden wall; sprinting 29 m; climbing up on a 1.55-m high sheer-faced platform; and climbing 2 flights of stairs. Before the timed runs, the volunteers were familiarized with the course and given 2 practice runs, the first without and the second with the fighting load. For this event, a hockey helmet replaced the Army helmet for safety.
Timed 30-m Rushes With the Fighting Load
This test simulated a soldier's forward movement on the battlefield consisting of short rushes between points of cover. The volunteer lay prone on a mat, facing another mat 30 m away. On the command “go,” the volunteer rose, sprinted 30 m, and lay prone on the other mat as quickly as possible. After 5 seconds, during which the volunteer pivoted in the prone position to face the first mat, the command “go” signaled the volunteer to run back to the first mat and lie prone. This process continued, with the volunteer running back and forth until 5 30-m rushes were completed.
Simulated Casualty Rescue With the Fighting Load
This test simulated the rescue of a wounded fellow soldier on the battlefield. On a verbal signal, the volunteer ran to an 80-kg Survivor manikin (Dummies Unlimited, Pomona, CA) 50 m away, gripped a web handle on the manikin's military vest, and dragged the manikin 50 m back to the starting line.
Other Physical Tests Performed in T-shirts, Shorts, and Running Shoes
Maximal Oxygen Uptake
This value was measured on a Trackmaster TMX425C treadmill (Fullvision, Inc., Newton, KS) with a TrueMax 2400 metabolic measurement system (ParvoMedics, Salt Lake City, UT) by using a continuous, uphill, stepwise protocol.
U.S. Army Physical Fitness Test
The U.S. Army Physical Fitness Test is a standardized test intended to evaluate soldier physical fitness and is required for all physically able soldiers twice a year. Points awarded for push-ups, sit-ups, and 3.2-km (2-mile) run are based on table values for gender and age. A score of 60 of a possible 100 points is passing for each test. The test was administered to the volunteers according to the protocol specified in the Army physical fitness manual (5). The volunteers first performed as many push-ups as they could in 2 minutes. Then they rested 10 to 20 minutes before doing as many bent-leg sit-ups (i.e., hands behind the neck and feet held down) as they could in 2 minutes. After another 10 to 20 minutes of rest, they ran 3.2 km (2 miles) as fast as possible.
Standing Vertical Jump
The difference between each volunteer's standing upward reach height and maximal jump reach height was recorded to the nearest 1.3 cm (0.5 inch) with a Vertec jump meter (Sports Imports, Columbus, OH), on which the volunteer displaced horizontal plastic fins indicating the height reached. Volunteers were allowed arm swing and countermovement, but no steps before the jump. The best of 3 attempts was taken as the volunteer's score.
Standing Horizontal Jump
Volunteers stood on a rubber jump mat imprinted with measuring lines and, beginning with their toes at the starting line, jumped horizontally as far as possible. The score was the distance between the starting line and the rearmost portion of the body landing on the mat. Volunteers were allowed prejump arm swings and countermovement, but no steps before the jump. The best of 3 attempts was taken as the volunteer's score.
Maximal Bench Press
After warming up with 3 sets of increasing submaximal loads, the volunteer performed 3 to 6 lifting attempts to determine the 1 repetition maximum (1RM) lift. The weight for each attempt was estimated by the volunteer and experimenter as difficult but achievable. Spotters assisted any subject unable to complete a lift.
Maximal Squat
The squat 1RM was determined in a similar manner to that of the bench press.
Statistical Analyses
The method of Cohen (4) was used to estimate the sample size. Entering Cohen's tables with the SDs of the tests, the desired effect size of 1 SD, a power of 0.80, and a 2-tailed α of 0.05, it was determined that the needed sample size was 15 per group. Twenty-one volunteers per group were initially recruited to account for attrition. A 2-by-2 analysis of variance was used to test the effects of before vs. after and training group.
Results
Forty-two volunteers began the study, and 32 volunteers completed it. Of the 10 volunteers who did not finish the study, one injured his finger during obstacle course testing; another had migraine headaches; and the rest had reasons unrelated to their health, such as academic pressures and travel difficulties. None of the volunteers withdrew because of injuries sustained during the physical training programs.
The pretraining and post-training results of the various tests are shown in Figures 1-5. In both training groups, there were statistically significant (P ≤ 0.05) improvements in all of the tests.
Figure 1: Point-to-point military load carriage time trials. *Significant (P < 0.05) pre-post difference. #Significant (P < 0.05) inter-group difference in improvement.
Figure 2: Simulated battlefield time trials. *Significant (P < 0.05) pre-post difference. #Significant (P < 0.05) inter-group difference in improvement.
Figure 3: Army Physical Fitness Test. *Significant (P < 0.05) pre-post difference. #Significant (P < 0.05) inter-group difference in improvement.
Figure 4: Maximal jumps. *Significant (P < 0.05) pre-post difference. #Significant (P < 0.05) inter-group difference in improvement.
Figure 5: Treadmill
o
2max. *Significant (
P < 0.05) pre-post difference. #Significant (
P < 0.05) inter-group difference in improvement.
Figures 1 and 2 show the results of the militarily relevant tests. In the 3.2-km run or walk with the 32-kg approach load (Figure 1), the SPT group reduced its mean ± SD time by 14%, from 24.5 ± 3.2 minutes to 21.0 ± 2.8 minutes, whereas the WBT group reduced its time by 15%, from 24.9 ± 2.8 minutes to 21.1 ± 2.2 minutes, with no significant difference in improvement between the 2 groups. In the 400-m run with the 18-kg fighting load (Figure 1), the SPT group reduced its time by 11%, from 94.5 ± 14.2 seconds to 84.4 ± 11.9 seconds, whereas the WBT group reduced its time by 16%, from 100.1 ± 16.1 seconds to 84.0 ± 8.4 seconds, with no significant difference in improvement between the 2 groups. The post-test times of both groups were very similar. In the obstacle course (Figure 2), the SPT group reduced its time by 16%, from 73.3 ± 10.1 seconds to 61.6 ± 7.7 seconds, whereas the WBT group reduced its time by 10%, from 66.8 ± 10.0 seconds to 60.1 ± 8.7 seconds. The improvement in the SPT group was significantly (P < 0.05) greater than in the WBT group. However, the post-test times of both groups were similar. In the 30-m rushes (Figure 2), the SPT group reduced its time by 6%, from 63.5 ± 4.8 seconds to 59.8 ± 4.1 seconds, whereas the WBT group reduced its time by 2%, from 60.4 ± 4.2 seconds to 58.9 ± 2.7 seconds, with no significant difference in improvement between the 2 groups. In the 80-kg casualty rescue (Figure 2), the SPT group reduced its time by 36%, from 65.8 ± 40.0 seconds to 42.1 ± 9.9 seconds, whereas the WBT group reduced its time by 23%, from 57.6 ± 22.0 seconds to 44.2 ± 8.8 seconds, with no significant difference in improvement between the 2 groups. The post-training test scores of the 2 groups were similar.
Figure 3 shows the effects of the 2 types of training on the Army Physical Fitness Test results. In the push-up test, the SPT group increased the number of repetitions by 31%, from 32.2 ± 13.5 to 42.3 ± 10.4, whereas the WBT group increased the number of repetitions by 32%, from 36.3 ± 8.5 to 47.8 ± 10.9, with no significant difference in improvement between the 2 groups. In the sit-up test, the SPT group increased the number of repetitions by 50%, from 36.9 ± 10.9 to 55.4 ± 10.2, whereas the WBT group increased the number of repetitions by 28%, from 39.3 ± 14.6 to 50.3 ± 13.1. The SPT group improvement was significantly greater than the WBT group improvement. In the 3.2-km unloaded run test, the SPT group reduced its run time by 13%, from 16.8 ± 2.5 minutes to 14.6 ± 1.4 minutes, while the WBT group reduced its run time by 12%, from 17.2 ± 3.2 minutes to 15.1 ± 2.2 minutes, with no significant difference in improvement between the 2 groups.
Figure 4 shows the results for jump performance. In the standing vertical jump test, the SPT group increased its vertical jump distance by 1%, from 47.8 ± 8.1 cm to 48.3 ± 7.1 cm, whereas the WBT group increased its vertical jump distance by 5%, from 49.5 ± 7.4 cm to 52.1 ± 7.5 cm, with no significant difference in improvement between the 2 groups. In the standing horizontal jump test, the SPT group increased its horizontal jump distance by 3%, from 214 ± 26 cm to 221 ± 22 cm, and the WBT group also increased its horizontal jump distance by 3%, from 212 ± 31 cm to 219 ± 30 cm, with no significant difference in improvement between the 2 groups.
Figure 5 shows the results of the treadmill maximal oxygen uptake test. The SPT group improved by 10%, from 47.2 ± 5.9 mL·kg−1·min−1 to 51.9 ± 5.4 mL·kg−1·min−1, whereas the WBT group improved by 13%, from 48.1 ± 5.6 mL·kg−1·min−1 to 54.5 ± 5.7 mL·kg−1·min−1, with no significant difference in improvement between the 2 groups.
Figure 6 shows the results of the weight-based training tests. In the bench press, the SPT group improved by 11%, from 77.2 ± 10 kg to 85.5 ± 10 kg, whereas the WBT group improved by 12%, from 72.7 ± 10 kg to 81.2 ± 10 kg. In the barbell squat, the SPT group improved by 10%, from 94.0 ± 14 kg to 103.0 ± 17 kg, whereas the WBT group improved by 12%, from 92.3 ± 6 kg to 103.0 ± 7 kg. In neither weight-based training test was there a significant difference in improvement between the 2 groups.
Figure 6: Weight lifting 1-repetition maxima. *Significant (P < 0.05) pre-post difference. #Significant (P < 0.05) inter-group difference in improvement.
Discussion
Although the volunteers in both groups improved significantly in all tests, there were no practical differences in training effect between the 2 programs. The similarity in outcome of the 2 programs may have been related to the comprehensive nature and similarity between both forms of training. Both training programs were designed to cover all major body movements and improve speed, agility, and endurance; started at a moderate level and built up to relatively high intensity; included sprint interval training, requiring the muscles of the lower body to exert high forces at high speeds; and had some sort of agility training that required rapid changes in direction. The main difference between the 2 programs was that one used weight-based training exercises and the other used calisthenics.
The strength improvements by the WBT group in this study, a mean 12% in the bench press and squat, were comparable to those in a study of similar duration and initial volunteer fitness status (23), but less than those in another such study (2). There is little doubt that if the focus of training had been strictly on strength development, changes in strength test scores would have been greater. The lack of dramatic strength gains in the WBT group may be attributed in part to a degree of incompatibility between strength and endurance training. Work by Hickson (12) was the first to elucidate the inhibition of strength gains when strength and endurance training are conducted concurrently. Recent work has attributed the inhibition to interference of intracellular signaling mechanisms. AMPK, activated by endurance exercise, inhibits the activity of mTOR and its targets, thereby blocking signaling to the protein synthesis mechanisms (22). Also, protein kinase B/Akt and AMP-activated protein kinase, respectively, are responsible for strength and endurance adaptations and inhibit each other's downstream signaling (3).
Additional factors could well have contributed to the lack of difference in strength gains between the 2 training groups. The training was restricted to 1.5 hours a day, 5 days a week, to keep within normal Army time strictures. Because workout time had to be allocated to the development of speed, agility, and load carriage ability, weight-based training was limited to twice a week. In addition, the energy devoted to running, interval training, agility drills, and load carriage may have somewhat muted the strength gains. However, there was no evidence of overreaching or overtraining, and the volunteers never stagnated or regressed in their lifts. The intensity of the training was increased gradually, and the volunteers were not deprived of sleep or adequate nutritional intake. The volunteers were civilians who led their normal lives except for their training sessions 1.5 hours a day, 5 days a week.
Many soldiers, particularly those in elite units, engage in weight-based training in the gym facilities that are provided in virtually all military bases. Some unit leaders even direct weight-based training sessions. The authors hypothesized an advantage to WBT over SPT primarily because the resistance provided by barbells, dumbbells, and weight stack machines could reach higher levels than the resistance provided by calisthenics and could be adjusted with more precision to allow exercises to be kept within the range of 1 to 12 repetitions, which produces strength increases (8). Because calisthenics use body weight for resistance, it is more difficult to adjust resistive force than with weights. Thus, for exercises such as push-ups or sit-ups, fit people can do considerably more than 20 repetitions, a range that improves muscular endurance more than muscular strength. However, the predicted advantages of weight-based training did not materialize. The Standardized Physical Training program somewhat compensated for the lesser adjustability of resistance of calisthenics than of weight-based training. For example, most young men can do fewer than 5 pull-ups, and many cannot do any at all. To allow pull-ups to be used for a training exercise, even for those who cannot do any, the Standardized Physical Training program manual specified that a partner could grasp the trainee's legs and provide enough vertical force to enable several repetitions to be performed (7). This technique had to be used with most volunteers, and some progressed so that they could do at least 5 repetitions unassisted.
The improvements in both training groups were not only statistically significant for all of the tests, but also of substantial magnitude in most of the tests. Because the volunteers had not recently engaged in intense, comprehensive exercise programs before the study, they clearly had potential for improvement. Relatively untrained subjects typically respond dramatically to training. Thus, the weightlifting exercises and the calisthenics provided sufficient training stimulus to improve the strength- and power-based physical performance of the volunteers.
Based on their aerobic capability, the volunteers were not an unfit group. Their initial mean o2max of approximately 48 mL·kg−1·min−1 placed them in approximately the 70th percentile for men their age, and their post-training o2max of 52 to 55 mL·kg−1·min−1 placed them in approximately the 85th percentile for men their age (1). It was not unexpected that the volunteers would be of an above-average physical fitness level. Individuals with the desire and confidence to participate in an intense physical training program are likely to be more athletic than average. Second, the medical screening process eliminated individuals above the Army induction weight for height limits and those with high blood pressure or other physical problems. However, the relatively high initial fitness level of the volunteers did not make them less representative of an Army recruit population because similar self-selection and medical-selection factors come into play for those enlisting in the military. Indeed, a study of more than 1,500 U.S. military male recruits showed that they were also relatively high in aerobic fitness compared to civilian men of their age (25).
The 10% to 13% improvement in o2max of the volunteers was considerable, especially because they started out somewhat above average in aerobic fitness and neither group averaged more than 12 km per week of running, including the distance runs and interval training. The degree of improvement one can expect in o2max depends on the pretraining level, and fit volunteers generally show relatively small percentage changes. A reasonable comparison can be made to a group of moderately active college students, who trained by running 10 to 40 minutes at 80% to 90% of their maximal heart rate 3 days a week for 9 weeks and improved 9.3% in o2max (21).
In the timed 3.2-km, 32-kg load carriage trial, some of the volunteers were able to jog the whole distance, whereas others had to walk occasionally. Overall, the volunteers reduced the time taken to cover the distance by approximately 15% in 8 weeks, an improvement that compares favorably with those in other studies. In the studies by Kraemer et al. (18, 20), the 12-week program of weight-based training and high-intensity endurance training produced 11% to 14% improvements in 3.2-km, 44.7-kg load carriage time of men. Among 20 male and 14 female recruits who underwent 10 weeks of British Army recruit training, Williams and Rayson (26) observed a 6.7% improvement in 3.2-km, 15-kg load carriage time, and among 50 male recruits, an improvement of 16% in 3.2-km, 25-kg load carriage time. In the authors' previous 24-week training study, 3 times as long as the current one, women improved by 32.5% in 3.2-km, 34-kg load carriage time (11).
The 8-week training period was used because of the Army's interest in relatively short-term training for recruits and for reservists called up for deployment. However, the relative shortness of the training period may well have contributed to the minimal differences in performance improvement between the 2 training programs. One would expect that weight-based training would allow strength to increase over a longer period than would calisthenics, because weight-based resistance can be increased in precise increments and to an unlimited extent. However, such differences may take somewhat longer to become evident. Indeed, in a 12-week study by Kraemer et al., a weight-trained group continued to show significant improvements in tests of strength and power over a longer period of time than did a group that trained without weights. Thus, it would be valuable to conduct a comparison of training interventions that would extend over several months to determine whether calisthenic training would produce an earlier plateau in simulated battlefield performance than would weight-resisted training.
As with most sports, the primary power for the militarily relevant physical activities tested in this study comes more from the lower body than from the upper body. The muscles in the hips, calves, and thighs propel the body upward and forward with each step in the 3.2-km load carriage test and the 400-m sprint with fighting load. In the obstacle course, these muscles are heavily involved in running, jumping over hurdles, zigzagging, climbing the wall and platform, and running up the stairs. To a lesser extent they are also involved in crawling. The obstacles that bring the muscles of the torso and arms more into play are the crawl and horizontal pipe shimmy. The simulated casualty rescue relies somewhat on grip strength, but the primary movers are also the muscles of the lower body. This helps to explain the similar training effects of the 2 programs. Although the weight-based training program used the barbell squat, barbell step-up, and box jump to strengthen the lower body, the calisthenics program used bodyweight squats, lunges, and jumps. That both programs included exercises that trained the leg and hip muscles to accelerate against gravitational and inertial resistance helps to explain why performance improvements on all the militarily relevant tests, which depend mainly on lower-body power, were similar for the 2 programs.
The U.S. Army for decades has shown a preference for physical training programs and physical fitness tests that do not require the use of equipment. The obvious motivation for this is to control costs and avoid transport, storage, securing, and accounting for equipment. This is why Army physical training has been based in calisthenics and the Army Physical Fitness Test consists of push-ups, sit-ups, and unloaded run, although these tests have not correlated strongly with military performance, such as load carriage speed (9). The Army's Standardized Physical Training program represents a departure from the no-equipment policy, in that it incorporates exercises on a pull-up bar. The purpose for adding such exercises was to improve the soldier's ability to perform climbing maneuvers, such as entering houses through windows and getting over walls, maneuvers that challenge soldiers, especially in the urban environments commonly encountered during current military operations and in possible future conflicts. The Standardized Physical Training manual (7) actually includes construction plans for a 4-bar pull-up station. That the pull-up bars are outdoors and fixed to the ground obviates the need for secure storage and property accountability.
The results of this study support the use of the Army's Standardized Physical Training program for the basic training of recruits. A previous study (17) showed the program's effectiveness in producing fewer recruits who failed the Army Physical Fitness Test than did the Army fitness training that had been the norm until the introduction of the Standardized Physical Training program (1.7% vs. 3.3%) (P = 0.03), while producing 38% fewer injuries. The current results, showing major improvements in simulated battlefield tasks, provide further support for implementation of the standardized program. Currently, the Standardized Physical Training program is mandatory for Initial Entry Training and Advanced Individual Training, which the soldier undergoes before being placed into a regular unit. Afterward, individual Army units have the option of continuing to follow the Standardized Physical Training program or creating their own programs from the exercises listed in Army Field Manual No. 21-20 (5).
Practical Applications
Military recruits and called-up military reservists can considerably improve their ability to perform physically demanding combat-related physical activities in 8 weeks of the Army's new Standardized Physical Training program or a weight-based training program. That the standardized Army program does not require moveable equipment that must be stored and accounted for and could be lost or stolen makes it attractive to the Army. The program is well suited to training large groups and could be adapted by schools and other organizations that wish to train large groups for general fitness with minimal equipment. Eight weeks is a relatively short physical training period, and the overload principle makes it likely that individuals trained with weight-based training would continue to improve in strength and, concomitantly, military performance over a longer period than those trained with calisthenics. It is thus important to emphasize that this study in no way negates the potential benefits of weight-based training for longer-term improvement of militarily relevant physical performance. The availability of weight-based training facilities and their use by military personnel would likely enhance the combat readiness of fighting forces.
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