Military operations often require soldiers to traverse ground while carrying very heavy loads (5,19). This is especially true in areas where motor vehicles cannot be used because of the roughness of the terrain or because the vehicle sounds might result in unwelcome detection by hostile forces. In some cases, mission success and battlefield survival can be dependent on the speed at which the soldier can cover the ground while carrying the equipment needed for the operation.
Although soldiers have progressively become larger and stronger than their earlier counterparts, loads that soldiers carry have also progressively increased since the British Crimean War at the start of the Industrial Revolution (19,22). The increase in soldier loads is presumably because of new technologies that have increased the lethality of the individual soldier while at the same time increasing the soldier's survivability. For example, infantry soldiers can now carry weapon systems that can disable and destroy aircraft and armored vehicles, whereas developments in body armor have provided enhanced individual protection from hostile fires (5,9,31).
In 1987, the Army Employment and Development Agency recommended 5 ways of reducing or compensating for the loads that soldiers carried. One of these recommendations was the development of special physical training programs to enhance soldiers' physical capability for load carriage (1). Since this recommendation was made, several scientific efforts have been undertaken to examine the effects of physical training on load carriage performance. The purpose of this article is to systematically review these investigations to determine effects of physical training on load carriage performance and to describe the modes of physical training that can most optimally enhance load carriage performance.
For this review, several literature databases were explored to find original, English-language studies that examined the effects of physical training on load carriage performance. The databases included PubMed (MEDLINE), the Cumulative Index to Nursing and Allied Health Literature, Academic Search Premier, Biomedical Reference Collection (Comprehensive), Ovid, and the Defense Technical Information Center. Keywords for the search included load, load carriage, physical training, physical fitness, backpacking, hiking, walking, military, and military personnel. To find additional studies, the reference lists of the articles obtained were searched, and the files of a senior investigator were examined. Six investigators with experience in physical training and load carriage issues were also contacted to assure that all the studies were located. Personal contacts were made with 3 authors to clarify methods and other parts of their investigations.
Studies were selected for review if they were original investigations that examined the effects of a physical training program on the time at which a set distance could be completed while carrying an external load. The majority of the external load had to be in a backpack. Studies involving manual material handling were excluded (for a review of these see ). Physical training was defined as a routine and systematic set of physical movements (i.e., exercise) designed to improve one or more of the components of physical fitness (3). Thus, studies were selected if they involved an exercise program designed, in whole or in part, to improve load carriage performance.
The methodological quality of the selected articles was assessed using the checklist of Downs and Black (6). The 5 major areas rated by the checklist were (a) reporting quality, (b) external validity, (c) bias, (d) confounding, and (e) statistical power. The checklist had 27 items, most of which were rated on a 2-point scale as either “yes” (1 point) or “no” (no point). One reporting item (relating to confounding) had a possible score of 2. For the purposes of this review, the single statistical power question was reduced from a possible score of 0–5 to a score of 0 or 1. Thus, the maximum possible score was 28. The 4 authors independently rated each of the selected articles. After the independent evaluation, the reviewers met to examine the other reviewer's scores and to reconcile major differences. The average score of the 4 reviewers served as the methodological quality score. Scores were converted to a percent of the total score by dividing each rater's scores by 28 and multiplying by 100%.
Cohen's d statistic was used to calculate effect sizes for changes in load carriage performance (4,7). The d statistic indicated the amount of the difference in SD units between criterion variable scores assessed at different time points. Thus, a d = 1.00 indicated a change of 1SD from 1 time point to the next. For the purposes of this review, time points could have been pretraining, midtraining, or posttraining. Cohen (4) considered small, moderate, and large effect sizes to be d values of 0.2, 0.5, and 0.8, respectively. To adjust the d statistic, as recommended by Cohen for studies involving the same subjects (i.e., repeated-measures designs), the correlation coefficient between the pretest and posttest measures was required. However, none of the articles in this review provided this statistic. Dunlap et al. (7) have noted that this is the case with most published repeated-measures studies and have suggested that when correlations are not provided, then the d statistic should be calculated from the available means and SDs. This was the method employed in this review. In addition to effect sizes, crude changes (percentage) in load carriage performance were calculated as either: ([posttest mean time − pretest mean time]/pretest mean time) × 100% or ([midtest mean time − pretest mean time]/pretest mean time) × 100%.
Meta-analysis was performed to combine study results and examine the overall effect of various training modes on load carriage performance. Studies were grouped for meta-analysis if they examined the effect of a single training mode (e.g., aerobic exercise alone) or combined training modes (e.g., aerobic training with resistance training). The meta-analysis technique employed here involved the use of effect sizes (d statistic) and sample sizes as described by Petitti (34). This procedure produced a summary effect size (SES) and summary 95% confidence intervals for the combined data. The SES estimated the overall change in load carriage performance for the training mode or combined training modes for all the studies included in the analyses.
A total of 20 publications were identified that involved physical training to improve load carriage performance. Of these, 9 articles were excluded for various reasons. Four studies involved load carriage data from previously published studies (12,36,41,44), and in these cases, the studies with the largest number of subjects or the original study was selected for review (11,42,43). Four studies used measures other than the completion time over a set distance as the criterion for improvement (37-40). One study asked soldiers to complete a course in a prescribed time rather than as fast as possible (33). In one case, there was both a government technical report (25) and a journal article (24) on the same study. The technical report provided some additional and useful methodological information, so both articles were considered in the review to obtain as much information as possible. Thus, there were 11 publications from 10 original studies that were included in this review.
Methodological Considerations in Reviewed Studies
Table 1 shows the methodologies and quality scores for the investigations that met the review criteria. The subjects participating in the studies were diverse and involved active duty soldiers (16,17,24), military recruits (2,42,43), untrained women (11), and recreationally active men (13) and women (14). In one study, the subject group was not clear (23), but they were likely college students. The length of training (training period) ranged from 4 (a midpoint test) to 24 weeks. Training frequencies ranged from 3 to 5 sessions per week, although in 3 studies involving military recruits, the training frequency was not well defined (2,42,43).
The methodological quality scores were generally high ranging from 60% (23) to 80% (11,17) of the total available points. The average ± SD of all ratings combined was 77 ± 5% of the total available points. The lowest scoring study (23) was generally well conducted but, within the Downs and Black (6) “reporting” category, the study failed to provide details on adverse events, confounders, and the subject population from which the sample was drawn.
Criterion Load Carriage Task
A number of studies used a criterion load carriage task requiring the subjects to complete a 3.2-km distance as rapidly as possible; however, in these studies, the loads carried ranged widely, from 15 to 46 kg (11,13,14,23,25,42,43). Other studies involved distances of 0.4 km (13), 2.4 km (2), 5 km (16), and 20 km (17) with loads of 18, 20, 19, and 46 kg, respectively.
Four studies reported on the reliability of their criterion load carriage task. Kraemer et al. (25) noted a test-retest reliability of 0.92 for their 3.2-, 45-kg carriage task (n = 11). Knapik and Gerber (17) reported an intraclass correlation of 0.89 for 13 women who completed 2 pretraining 5-km distances while carrying 19 kg. There was very little difference in average ± SD completion times between the 2 trials (44.9 ± 3.3 and 44.4 ± 2.6 minutes, p = 0.36). Harman et al. (11) found a test-retest reliability of 0.82 for 3 women who completed two, 3.2-, 34-km carriage tasks administered 5 months apart. Finally, Kraemer et al. (23) reported a test-retest reliability coefficient of ≥0.95 for a 3.2-km, 34-kg load carriage task.
Physical Training Modes
The most commonly studied modes of physical training included aerobic exercise, resistance training, interval training, and load-carriage exercise; other modes of training were also included, as discussed below. Some investigations had a number of experimental groups perform various training mode combinations (13,14,23–25). Other studies had only a single mixed-mode training group that was measured before and after training (2,11,17,42,43). One study used the same mixed-mode training program for all the subjects but manipulated the frequency, intensity, and duration of a load-carriage exercise (16). Most studies provided some form of progressive overload to gradually and systematically improve specific components of physical fitness (30).
For aerobic training, many studies used long-duration running, with progressive overload achieved by increasing running distance (14,16,17,23,24). Some studies included interval training on flat surfaces or hills to challenge both aerobic and anaerobic energy systems (14,16,17,24,43). In the studies using interval training, progressive overload was achieved by increasing the number of intervals, reducing rest time between repeats, and manipulating distances (14,16,17,24). Two studies had subjects run a standard 3.2-km distance throughout the training program (11,13) but included interval training within the program. One of these latter 2 studies had subjects perform progressive interval training throughout the program (13), whereas the other incorporated intervals beginning at week 14 of the 24-week program (11). One study had subjects perform not only running but also cycling and stair stepping (23). Two studies did not provide details on the exact aerobic training performed (2,42).
Types of resistance training were highly varied, both within and between studies. Several investigations used linear resistance training programs in which the number of sets and repetitions were the same throughout the training period and progressive overload was achieved by increasing the amount of weight lifted (13,16,17,24). Other studies used periodized resistance training programs in which the number of repetitions and loads was manipulated to emphasize muscular endurance (higher repetitions, lower resistance) at some points in the training program, and muscle strength (lower repetitions, higher resistance) at other points (11,14,23). Two studies examined upper body (UB) only resistance training (23,25), whereas most included both UB and lower body (LB) training (11,13,14,16,17,23,25,43). Kraemer et al. (23) compared the effectiveness of resistance training programs that emphasized power (faster weight lifting movements) to programs emphasizing hypertrophy (slower movements with more repetitions). Many studies used exclusively (13,14,17,23,24) or primarily (43) free weights for resistance training; other studies used machine (e.g., Universal® and Nautilus®) devices (16,43), sand-bag lifts and carries (11,16), partnered resisted exercises (23), heavy box lifting (11,42,43), and band-resistance exercises (23). Two studies included LB plyometrics (11,23) to improve leg strength and power (29). Some studies did not provide details on the resistance training performed (2,42).
Other forms of training in these studies included calisthenics (16,23,42,43), movement drills for agility and balance training (13), and sports activities (42,43).
Importantly, several studies included specific load carriage tasks within their training programs (2,11,13,16,42,43). Two studies held speed and distance constant and progressively increased the load (intensity) up to that carried on the criterion load carriage task (11,13). One study held the aerobic and resistance training constant and manipulated frequency of the progressive load-carriage exercise (16). Three studies conducted in British Army basic training noted that load carriage was part of the program but did not describe the features of that training (2,42,43).
Changes in Load Carriage Performance
Table 2 shows the training groups in each investigation and the changes in the criterion load carriage performance task expressed as (a) the mean change in performance time (minutes), (b) the crude percent changes in performance from the pretest to the midtest or posttest, and (c) the effect size (Cohen's d statistic). One investigation (16) reported negative effect sizes because posttest measures were slower than pretest measures, presumably because of elevated environmental temperatures and longer rest breaks on the posttest. Three studies employed 2 separate load carriage tasks within their studies (13,42,43). Three studies not only examined changes in performance at the conclusion of the study but also at a midpoint in the study (2,11,14).
In the Hendrickson et al. study (14), the control group, which merely continued their pretest recreational training, improved their posttest carriage performance by about 7%, compared with about 13–14% for the other 3 training groups. As Hendrickson et al. point out (14), this suggested a strong familiarization effect and that improvements because of physical training in this study might be lower than indicated. If the posttest measures are recalculated by subtracting the change in the control group (−2.6 minutes), then the posttraining effect sizes (d values) become 0.45, 0.45, and 0.46 for the resistance training only, aerobic training only, and combined resistance and aerobic training groups, respectively.
Meta-Analysis of Training Effect Sizes
Table 3 shows the meta-analysis demonstrating the effects of various training modes or combinations of training modes on load carriage performance. The SESs express the magnitude of the changes in load carriage performance from the results of the combined studies. In some cases in which midpoint training periods were available, these were selected to more evenly equate studies on the length of the training periods. In other cases, this was not possible because there was only 1 posttest measurement period. One study (16) was not included in the meta-analyses because the posttest was confounded by different environmental temperatures and longer self-directed rest breaks on the posttest.
For aerobic training alone, 2 studies (23,25) showed virtually no training effect on load carriage performance, whereas the Hendrickson et al. study (14) showed a large training effect. This resulted in large confidence intervals around the SES. For UB and LB resistance training alone, the SES was larger than that for aerobic training alone, but there were only 2 studies, 1 with a small to moderate effect size and the other with a large effect size. Again, the confidence intervals were wide around the SES.
When resistance training was combined with aerobic training (resistance-aerobic training), the SES was large. The SES was almost identical for resistance training involving the UB and LB and for resistance training involving the UB alone. When load-carriage exercise was included as part of training programs with resistance-aerobic training, the SES was more than twice as large compared with resistance-aerobic training without load carriage.
Field-based training that included load-carriage exercise also resulted in large improvements in load carriage performance. It should be noted that most of these field-based training studies involved trainees in British Army basic training. Basic training entails a very great variety of physical activities besides routine physical training. Finally, periodized resistance training appeared to have larger training effects than did linear resistance training programs.
The results of this review indicated that some combined modes of physical training can considerably improve load carriage performance. Substantial training effects were apparent when progressive resistance training was combined with aerobic training and that training was conducted at least 3 times per week over at least 4 weeks. When progressive load-carriage exercise was part of the training program, much larger training effects were obtained. Field-based training that combined a wide variety of training modes and included progressive load-carriage exercise was also very effective in improving load carriage performance. Aerobic training alone or resistance training alone had smaller and more variable effects, depending on the study.
The largest overall improvements in load carriage performance were found when once weekly progressive load-carriage exercise was part of the training program (2,10,11,42,43). This follows from the exercise principle of specificity whereby gains in physical performance are the greatest when individuals systematically exercise with the task in which performance improvements are desired (8,27,28,32). Progressive load-carriage exercise likely involved the skills, muscle groups, energy systems, and related components of fitness that were important for the performance of this task (35). Nonetheless, physical training without load carriage was also successful in improving load carriage performance, albeit to a lesser extent. Effective gains in load carriage performance were seen with just resistance-aerobic training (for the most part, running), suggesting that the combination of strength and cardiorespiratory endurance are important fitness components of an overall program to improve load carriage performance.
An interesting observation was that UB resistance-aerobic training was almost as effective in improving load carriage performance as combined UB and LB resistance-aerobic training. In load carriage using backpacks, most of the weight rests on the shoulders (15,26). Well-designed and properly worn hip belts can be effective in moving about 30% of the load to the hips (26), but the UB musculature still assumes most of the pack load and assists in stabilizing the upper torso during locomotion (18). It appears that resistance training that improves the strength and muscular endurance of the UB musculature is more important for enhancing load carriage performance than LB strength-muscular endurance exercise. This suggests that the resistance training portion of a program to improve load carriage performance should focus primarily on the UB.
One of the more conflicting findings in this review was in regard to aerobic training alone. Two early studies by Kraemer et al. (23,25) found virtually no improvements in load carriage performance with exclusive use of aerobic training. On the other hand, Hendrickson et al. (14) found substantial performance gains with aerobic training alone. Because the criterion load carriage tasks involved completing a fixed distance as rapidly as possible, improving the cardiorespiratory endurance component of physical fitness might have been expected to result in faster load carriage times. The fact that cardiorespiratory endurance was enhanced in all 3 studies was verified by improvements in maximal effort unloaded 3.2-km run times (14,23,25) or peak V̇O2 (14). The use of interval training cannot account for the differences because one study using interval training showed no gain in load carriage performance (25), whereas the other showed substantial gains (14). Resolution of these conflicting findings will have to await further investigation.
Also, it is not clear if resistance training alone will improve load carriage performance. Meta-analysis suggested that the overall effect of resistance training alone was similar to that of resistance-aerobic training; however, there were only 2 studies that had examined resistance training alone, and they had very different effect sizes, which resulted in a wide confidence interval (Table 3). In the Kraemer et al. study (25), push-up and sit-up performance was significantly improved in the resistance training group suggesting an increase in muscular endurance of the UB and trunk; in the Hendrickson et al. study (14), 1-repetition maximum performance of the squat and bench press was improved suggesting an increase in strength of the UB and LB. Further study is also warranted here.
Several studies have examined the changes in load carriage performance in response to British military basic training (2,42,43) or the physical training program that is currently used for U.S. Army Basic Combat Training (BCT) (13). For the purposes of this review, this type of training was called “field-based” because it was primarily designed for use in outdoor areas (i.e., the field), involved a large number of participants, and used a minimum of equipment. In these studies, load-carriage exercise was combined with a great number of other physical training modes (e.g., aerobic training, sand-bag lifting, plyometrics, agility training, hill running, manual material handling, sports), and these combined training modes more effectively improved load carriage performance than resistance-aerobic training without load carriage exercise. As discussed above, the specificity of the load-carriage exercise likely played a large role in the performance improvements.
Reports on the British field-based programs (2,42,43) did not provide very specific descriptions of the training regimes and military basic training involved a great deal of physical activity outside of regularly scheduled exercise sessions. The additional activity was a potential confounder in trying to interpret the effects of field-based training, although this additional activity can also be interpreted as further training mode variety. On the other hand, the Physical Readiness Training (PRT) program (20) investigated by Harman et al. (13) involved subjects who were not in BCT, and so this study examined a training program without the other activities normally undertaken in BCT. When the PRT group was compared with a group using a resistance-aerobic training program, the 2 groups demonstrated similar improvements on both a 3.2- and a 0.4-km carriage task. These data suggest that in untrained or moderately trained men and women, the current Army physical training program can result in significant improvements in load carriage performance and that these improvements are similar to those achieved with resistance-aerobic training.
Meta-analysis suggested that periodized resistance training programs may offer greater improvements in load carriage performance than linear resistance training programs. The SESs were somewhat larger for the periodized programs than for the linear training programs. The periodized training programs manipulated sets, repetitions, and exercises in either in “blocks” (mesocycles and microcycles) lasting several weeks (13,23), or in an undulating manner with weekly or even daily alterations in intensity and volume (14). On some days, higher weight and fewer repetitions were prescribed to emphasize strength, and on other days, lower weight and more repetitions were prescribed to emphasize muscular endurance. Also, different exercises for different muscle groups were prescribed on different days. It is likely that the emphasis on different components of fitness (strength and muscular endurance), combined with the greater variety of trained muscle groups may have provided an advantage in improving load carriage performance. Linear resistance training programs involved an unchanging number of sets and repetition and exercised the same muscle groups throughout the program. This more restrictive routine, while still effective, appears to result in somewhat lower performance gains. This comparison is somewhat confounded by the subject groups examined. Linear resistance training programs examined soldiers or recreationally active men (13,17,24), whereas periodized programs examined untrained women (11,14,23). Nonetheless, none of these groups had previously been involved in systematic resistance training.
This review had some limitations. First, the studies reviewed here involved a wide array of subject samples, training periods, training frequencies, criterion load carriage tasks, and modes of training. This introduced some difficulties in generalizing findings across studies. However, the use of effect sizes allowed comparisons that assisted with judgments about types of training regimes that provided greater improvements in load carriage performance. Most load carriage tasks involved similar distances but different loads and in all but one case (16), load carriage performance was improved with physical training, with the single exception apparently because of pre-post changes in weather conditions. Second, most of the studies involved relatively short distances, the majority ≤3.2 km (2,11,13,14,23,25,42,43); only 2 of the 10 studies reviewed involved distances >3.2 km (16,17). Finally, only 3 of the 11 studies reviewed here had the specific purpose of improving load carriage performance (11,16,25). Other studies had broader goals like improving general tactical or operational military performance (13,14,23,24), or improving material-manual material handling capability (17,42,43). Nonetheless, virtually all of these studies provided support for the concept that physical training can improve load carriage performance.
Military trainers can take advantage of this review. The results indicated that specific modes of physical training can substantially improve the speed at which loads can be carried over distances. Effective programs appear to be of 2 types. The UB resistance-aerobic training conducted at least 3 times per week that incorporates once weekly progressive road marches can substantially improve load carriage performance. Also, field-based training conducted at least 3 times per week that incorporates a wide variety of training modes and includes 1 weekly progressive load carriage can also successfully increase load carriage speed, at least in untrained or recreationally trained men and women. Further research is needed on aerobic training alone and resistance training alone to determine the effects of these training modes on load carriage performance.
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Keywords:Copyright © 2012 by the National Strength & Conditioning Association.
meta-analysis; aerobic training; resistance training; intervals; periodization; plyometrics