Strenuous physical activity and load carriage in extreme environments is an important part of the activities of military personnel. The average infantry combat load in Afghanistan is about 30 kg, but can be as much as 58 kg (9). The effects of these loads on the aerobic function of soldiers have not been fully studied. Also, it is unclear how soldiers of differing body size and composition may be affected by these heavy loads.
There is a significant decrease in the ability of individuals to perform aerobic work when carrying a heavy load (3,8,11,20). However, there has been little research performed on the correlation between performance on load-carriage activities and the unloaded activities traditionally used as indicators of aerobic performance in the military, i.e., distance runs. One study that characterized the connection between unloaded and loaded activities was performed by Fallowfield et al. (10), who found no relationship between a loaded long-distance task and an unloaded maximal test.
Load-carriage tests may be more occupationally relevant than unloaded run times for military personnel (4,10,25–27). Traditional tests of aerobic fitness may be biased toward individuals with smaller body mass (16,25–27). Vanderburgh et al. (27) were able to show that a load of 30 lb was sufficient to remove this body mass bias. Furthermore, the measurement of relative oxygen consumption may underestimate the true fitness levels of larger individuals (5,21). Individuals with a greater mass may outperform their smaller counterparts in aerobic tests that require heavy load carriage because a load of a given absolute mass would be a smaller percentage of the larger person's mass and, thus, an easier challenge for the larger individual to perform (10,16).
Body composition may also affect load-carriage performance (3,16). Among individuals with similar body mass, those with a greater proportion of fat-free mass would be expected to handle a load better than those with a higher proportion of fat mass.
Previous studies have attempted to characterize the physiological effects of load carriage during exercise. They have shown a clear increase in the energy cost and cardiovascular demand (3,8,11,20), decreased mobility (23), and decreased time to exhaustion (TTE) on an aerobic task (3). However, there has been no previous research on the aerobic capacity (
) of an individual while carrying a load. A subject would reach a lower speed and grade combination on an incremental treadmill test when carrying a load, but the muscular and aerobic systems should still be taxed maximally at the point of exhaustion. However,
might be reduced if either pulmonary limitations or local muscular fatigue caused the subject to stop before reaching maximal aerobic ability.
Two studies from the 1980s found that wearing a load decreases measures of pulmonary function (14,17). These changes—decreases in forced expired volume in 1 second (FEV1) and forced vital capacity (FVC) without a change in their ratio—are similar to restrictive disorders, such as obesity. Both of these studies also found that a 10-kg load decreased maximal voluntary ventilation (MVV) but, surprisingly, a 30-kg load did not further decrease MVV. Similar to obesity, carrying an additional load may restrict the ability of the pulmonary musculature to expand the lungs. This reduction in pulmonary function may be a primary cause of reduced aerobic performance while wearing a load. Alternatively, wearing a load may reduce aerobic performance because of increased local muscular fatigue (such as of the back, hips, and legs). One purpose of our study was to compare reductions in pulmonary function measured at rest with those measured during exercise, to assess the impact of pulmonary limitation on aerobic performance.
The purpose of this study was to evaluate the effect of body mass and added mass on both treadmill performance and pulmonary function. We hypothesized that loads increasing from 10 to 30 kg will cause increasing decrements in aerobic performance and capacity associated with increasing decrements in pulmonary function. This decrease in pulmonary function would be because of mechanical restriction of inhalation from the loads carried. We also hypothesized that a traditional unloaded run test would correlate well with an unloaded performance on the treadmill, but less so as load increased.
Experimental Approach to the Problem
To test the hypotheses listed above, each subject participated in 5 tests of aerobic capacity. The first was a 4.8-km run, which is the standard test in the U.S. Marine Corps Physical Fitness Test. The other 4 tests were maximal incremental treadmill tests performed while wearing 4 differing loads (0, 10, 20, and 30 kg). Before each treadmill test, the pulmonary function measures of FEV1, FVC, and MVV were measured wearing the respective load. By performing these 5 tests, it could be observed whether performance on the unloaded run test corresponded with performance on the uphill treadmill tests, as measured by TTE. Also, pulmonary function could be measured wearing the 4 different loads to see if there was significant pulmonary restriction by simply wearing a larger load.
Subjects were young adult men and women, who were considered low risk by American College of Sports Medicine guidelines (1). The subjects were recruited from the Old Dominion University student body and local military bases. Subjects selected for this study were required to be highly active individuals, defined as individuals engaging in 2.5 hours of vigorous physical activity per week over the past 4 weeks (24). The university's institutional review board approved this study, and written informed consent was obtained from every subject before data collection.
A total of 48 participants were recruited and completed at least 1 session. Forty-two participants (22 men and 20 women; age range was 19–36 years) completed all 5 sessions for the study and 6 withdrew. Four of these subjects withdrew because of scheduling difficulties, whereas 2 withdrew because of unrelated medical issues. Height and mass were measured with the subject wearing shorts and a T-shirt and standing on a balance scale. These measurements were also used to calculate the body mass index (BMI) of the individual. Body composition was measured using the air-displacement plethysmography method (BodPod; Life Measurement, Concord, CA, USA).
Each subject participated in 5 separate testing sessions within a period of 1–2 weeks. Subjects were instructed to abstain from other strenuous physical activity within 24 hours before each scheduled test. In the first session, subjects performed the 4.8-km run. In the next 4 sessions, the subjects engaged in maximal incremental treadmill tests performed with varying amounts of additional mass (0 kg, 10 kg, 20 kg, or 30 kg) in a counterbalanced order. The 10-kg load consisted of a weighted vest (Ironwear Fitness, Pittsburgh, PA, USA) that simulated Interceptor body armor both in mass and in the fact that it contained hard plastic chest and back plates that restricted flexion and extension of the torso. The vest was adjusted using side straps to ensure a snug but comfortable fit. For the 20-kg and 30-kg loads, subjects wore a backpack of either 10 or 20 kg in addition to the weighted vest.
The following stages were used in the incremental treadmill protocol, each lasting 3 minutes; the first 2 stages were 4.8 kph (3 mph) and 6.4 kph (4 mph) at 0% grade; successive stages were at 6.4 kph with grades of 5, 10, 15, and 20%; thereafter, grade remained at 20%, and speed was increased to 7.2 kph (4.5 mph) and then 8.0 kph (5 mph). During these incremental treadmill tests, the subjects' expired gases were analyzed using a metabolic cart (Vmax 29c; SensorMedics, Yorba Linda, CA, USA) to determine the peak oxygen consumption (
), peak ventilation (VEpeak), and peak respiratory exchange ratio (RERpeak).
is reported relative to body mass. The subjects wore a chest strap heart rate monitor (Polar, Kempele, Finland) for measuring peak heart rate (HRpeak). The highest values obtained over a 60-second period (typically, the final 60 seconds) were designated peak, as opposed to maximal, because it was assumed that muscular fatigue during tests with the heavier loads might prevent the attainment of a true maximal
Pulmonary function was tested at the beginning of each of the 4 visits involving treadmill testing wearing the specific load to be carried that day, using the same metabolic cart. The FEV1, FVC, and MVV were measured. The flow meter and gas analyzers of the metabolic cart were calibrated before each trial for each subject. The pulmonary function tests were conducted with the subject in the standing position so that the entire weight of the load was supported by the torso, and not resting on the thighs as might occur if seated.
A 2-sample t-test was used to determine if men and women differed significantly in age, height, mass, body fat percentage, BMI, or lean body mass (LBM). Comparison of data collected during the treadmill tests of different loads (
, RERpeak, HRpeak, VEpeak, and TTE) was performed using 2-way analysis of variance (ANOVA) (trials and sex) with repeated measures on 1 factor (trials). Similarly, pulmonary data (FEV1, FVC, FEV1/FVC, ans MVV) collected before the treadmill tests while wearing different loads were compared using 2-way ANOVA with repeated measures on 1 factor. Tukey post hoc tests were used to determine which trials differed from one another. Regression analysis was used to correlate performance on the 4.8-km run with TTE on the various treadmill tests. Regression analysis was also used to compare performance on the various tests with body mass and with LBM. Significance for all tests was set at the 0.05 level.
Demographic and anthropometric values for the final 42 participants are displayed in Table 1. Men had greater values for height, mass, and LBM, whereas women had a greater body fat percentage. Time to exhaustion on the treadmill tests was correlated with the speed of the 4.8-km run for the 0, 10, 20, and 30-kg trials. Correlation coefficients were significant for each load, but decreased with increasing treadmill load, especially in the female participants (Table 2). At the 30-kg load, r2 values indicated that 4.8-km run performance explained only 42% of TTE for men and 30% for women.
Individuals of smaller mass did not perform better than individuals of larger mass, as demonstrated by the lack of significant correlation between body mass and the 4.8-km run speed for men and women. For body mass vs. TTE, second degree polynomial regression lines were the best fit model, but only the one for men in the 30-kg test was significant. Figure 1 displays the regression for men in the 30-kg treadmill test, which shows that the optimal body mass for achieving the best performance was 92 kg. Results were similar at all loads when correlating performance with LBM, and the optimal LBM for men with the 30-kg load was 82 kg.
Within-subjects effects of load on FEV1, FVC, and MVV were significant. All 3 measures decreased significantly with increasing load (Table 3). For all 3 of these pulmonary measures, there was also a significant between-subjects effect for sex, with males significantly higher than females. FVC/FEV1 ratio was not affected by load, and did not differ between sexes.
Time to exhaustion, HRpeak,
, and VEpeak decreased significantly with increasing load (Table 4). Time to exhaustion,
, and VEpeak also showed a significant between-subjects effect for sex, with males significantly higher than females. There was no significant effect of load on RERpeak or the VEpeak/MVV ratio.
Our main finding was that increasing loads worn on the upper torso decreased pulmonary function and resulted in parallel decreases in aerobic performance and capacity. We also determined that performance on a traditional unloaded 4.8-km run predicted performance on an uphill treadmill task more poorly as load increased. This was especially true for the female subjects, who saw a much greater drop in correlation from the 0-kg to the 30-kg tests. This is similar to the findings of Treloar and Billing (23), who found that women were affected more than men when performing anaerobic tasks while carrying a load. Because women are smaller than men (62 kg vs. 73 kg in this study), absolute external loads pose a greater relative challenge to women. Moreover, women have a greater body fat percentage than men (25% vs. 11%), resulting in an even smaller overall fat-free mass. Fat-free mass has been shown in other studies to be correlated with increased performance on load-carriage tasks (3,16).
Our final purpose was to evaluate the effect of body mass (as well as LBM) on performance during the loaded tasks, and we found less of a relationship than hypothesized. However, the added mass of an external load did significantly decrease both aerobic performance (TTE) and pulmonary function (FEV1, FVC, and MVV). Decreased performance on the treadmill tasks with increasing load supported the finding in past literature that load increases the energy cost of walking and running (3,11,13,18).
The most common test of aerobic fitness in the U.S. military is an unloaded distance run. One of the purposes of this study was to determine whether a level 4.8-km run, as used by the Marine Corps, is a useful indicator of loaded aerobic performance on an uphill task. A significant correlation was found at all loads. However, as hypothesized, the strength of the correlation decreased with increasing load. Only 42% of the variance in treadmill performance while carrying a 30-kg load can be explained by 4.8-km run speed for men and only 30% for women. We theorize that this correlation would be even lower, and possibly no longer significant, at combat loads approaching 60 kg, as used in recent military operations (9). Our findings lend further support to the position that unloaded run tests may be inappropriate for assessing the aerobic ability of military personnel and other occupations requiring load carriage (25).
Regarding the effect of body mass, in contrast to our hypotheses, neither body mass nor LBM was significantly related to performance in any of the aerobic tasks, other than by men on the 30-kg treadmill task. This finding was surprising, given that smaller individuals are well known to have greater body mass–relative aerobic capacity (
in ml·min−1·kg−1) than larger individuals, and better performance on body mass–related tasks, such as running without a load (2,22). In this group of subjects,
on the unloaded treadmill test did not show this classic relationship, suggesting that the subjects were not highly enough trained to observe this effect. Other reasons for the lack of a greater body mass effect in this study could be the relatively narrow range of body masses within each sex and the need for an even greater sample size. Nonetheless, the finding that larger men had better performance than smaller men with the heaviest load (30 kg) lends some support to the hypothesis that larger individuals will perform better with absolute loads, given that these loads represent a smaller percentage of their mass (12,19).
One of the limitations of this study was that participants had a wide range of fitness levels, as observed in the wide range of
measurements (34.1–73.7 ml·min−1·kg−1 in the 0-load treadmill test). Researchers attempted to control for fitness by selecting only participants who self-reported at least 2.5 hours of vigorous physical activity each week. However, this amount varied greatly with some participants reporting as much as 12 hours and others reporting just over 2.5 hours. If a sample could be used in which all participants had a similar fitness level, correlations on the different tasks might be more significant. Future research should recruit participants of a more uniformly high fitness level, such as individuals in military special operations teams or endurance athletes.
It was hypothesized that FVC, FEV1, and MVV would all decrease with increasing load. The reasoning for this hypothesis was that it would be more difficult for the individual to inflate the lungs when wearing the load on the chest. The observed results supported this hypothesis. Because the hypotheses stated that FEV1 and FVC would both decrease with increasing load, it was expected that FEV1/FVC would not change significantly, which was also supported by the data. These findings are consistent with the findings of previous research showing decreases in FEV1 and FVC, with no change in FEV1/FVC (6,7,14,15,17). Decreased FEV1 and FVC with no change in their ratio is similar to the effects of mild restrictive pulmonary disease. One example of restrictive pulmonary disease is the decreased pulmonary function due to obesity. In this case, extra body mass restricts the inflation of the lungs. Our results indicate that excess mass in the form of a load carried on the chest results in a similar effect on pulmonary function to obesity. In this study, the subjects experienced a decreased pulmonary function with load, even before they began exercise. This restrictive decrease in pulmonary function would likely decrease aerobic performance even without local muscular fatigue or mechanical limitations.
decreased with increasing load, as hypothesized. There was also a significant decrease in HRpeak, further supporting the idea that participants were not able to achieve maximal aerobic exertion when carrying a load, possibly due to local muscular fatigue. Respiratory exchange ratio is generally considered a good indicator of the relative intensity of exercise for an individual, with values ≥1.10 considered an indication of maximal aerobic effort. Surprisingly, the mean value for RERpeak was above 1.10 for all loads despite the decrease in
with load. Although some research has reported RER while carrying a load (20), no previous studies have reported RERpeak or RERmax. Future studies should further examine the divergence of RERpeak and
in loaded exercise.
One possible explanation for a lack of decrease in RERpeak is that increasing load resulted in more lactic acid buildup in the skeletal muscle due to local muscle fatigue. Increased lactic acid from extensive use of the anaerobic energy system would require more blood buffering, increasing the amount of carbon dioxide in the blood to be filtered out by the lungs and thereby increasing RER (i.e., a muscular limitation). However, the decrease in
with increasing load might be explained by the decreased pulmonary function.
VEpeak decreased with increasing load, as hypothesized. The ratio of VEpeak to MVV was assessed to determine if the primary cause of decreased performance was because of pulmonary constraint or if there were other factors also in play. VEpeak/MVV did not change with load. Similar decreases in MVV and VEpeak suggest that a primary limitation in pulmonary function imposed by wearing an external load on the chest (as indicated by decreased MVV) carried over to ventilatory function during exercise with the load (as indicated by decreased VEpeak). Further research should investigate the effect of differing distributions of weight on these pulmonary measures, to determine if a more ideal fit would cause less decrease in pulmonary function, as suggested by the work of Legg and Cruz (14,15).
Although other studies have investigated the effects of load carriage on aerobic performance (3,4,8,10,11,13,16,18–20,23,26), this is the only study, to our knowledge, that has measured peak aerobic capacity, demonstrating a load-induced decrease. This study also used more subjects than other studies on the topic, most with fewer than 20 subjects participating. Only a minority of studies investigated load carriage in both men and women (15,23). This study has the benefit of a relatively large sample size (N = 42) including both men and women. However, it appears that an even larger sample size may be required to determine a relationship between performance and body mass. Further research should also be performed to characterize the effects of sex on load-carriage performance.
This study shows that carrying an external load, such as a combat load, on the chest results in decreased pulmonary function. This decrease is greater as the load is increased (up to 30 kg in this study). The load also decreases uphill aerobic performance, and this decrease is associated with a decrease in the highest attainable
. Although local muscular fatigue cannot be ruled out as a reason for the decreased performance, the decrease in pulmonary function is likely to be at least partially responsible. To decrease the effects of pulmonary restriction, different distributions of load should be investigated. Performance on a 4.8-km run is strongly correlated with unloaded uphill treadmill performance. However, this correlation falls as load on the treadmill task increases, suggesting that the distance run is not a good measure for evaluating fitness for loaded tasks. Based on these results, fitness professionals in situations that require uphill loaded aerobic tasks, such as firefighting and military operations, should consider alternative testing and training procedures for their clients to assess aerobic fitness and preparedness.
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