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00005768-201404000-0002000005768_2014_46_795_moore_marksmanship_4miscellaneous-article< 109_0_13_6 >Medicine & Science in Sports & Exercise© 2014 American College of Sports MedicineVolume 46(4)April 2014p 795–801The Effects of Acute Hypoxia and Exercise on Marksmanship[APPLIED SCIENCES]Moore, Chelsea M.1; Swain, David P.1; Ringleb, Stacie I.2; Morrison, Steven31Department of Human Movement Sciences, Old Dominion University, Norfolk, VA; 2Department of Mechanical and Aerospace Engineering, Old Dominion University, Norfolk, VA; and 3School of Physical Therapy and Athletic Training, Old Dominion University, Norfolk, VAAddress for correspondence: David P. Swain, Ph.D., Department of Human Movement Sciences, Old Dominion University, Norfolk, VA 23529; E-mail: dswain@odu.edu .Submitted for publication April 2013.Accepted for publication August 2013.ABSTRACTPurpose: This study evaluated the effects of acute hypoxia and physical exertion on marksmanship.Methods: At each of five simulated altitudes (162 m, SL; 1015 m, 1K; 2146 m, 2K; 3085 m, 3K; 3962 m, 4K), subjects performed four shooting trials: at rest, immediately after a 60-s run with load, and twice more separated by 30-s rest. Arterial oxygen saturation (SaO2), HR, and ventilation rate (VR) were recorded.Results: Both increasing altitude and exercise significantly (P < 0.05) decreased marksmanship. The shooting scores at 4K were significantly lower than those at all other altitudes. There was a likely trend for scores at 3K to be lower than those at SL and 1K (P = 0.06 and 0.07, respectively). The shooting score at rest was significantly greater than that in all trials after exercise. Partial recovery of marksmanship after exercise occurred. Altitude and exercise both significantly reduced SaO2 and increased VR. HR did not change with altitude but increased after exercise. There was a strong positive correlation (r = 0.84) between marksmanship and SaO2. There was a strong inverse correlation (r = −0.72) between marksmanship and VR, and a modest inverse correlation (r = −0.54) between marksmanship and HR.Conclusions: Increasing altitude impaired marksmanship, with a threshold at 3000–4000 m. The decreased marksmanship was closely related to decreased arterial oxygen saturation and increased ventilation, the latter increasing movement of the chest wall.Expeditionary warfighters in the military must be ready to deploy and effectively complete their mission on short notice even if deployed to altitude. Well-known cardiorespiratory effects of acute exposure to the hypoxia of altitude include a decrease in arterial oxygen saturation (SaO2) with compensatory increases in ventilation and HR (10). However, the decline in function is not simply limited to these physiological systems. Of particular note, hypoxia decreases cerebral oxygenation during exercise (29), impairs postural balance (33), and impairs the performance of military pilots during flight simulation (31). Decrements in any of these areas could lead to a reduction in shooting accuracy.Despite the obvious implications for impaired performance resulting from altitude effects, only two studies have looked at the effects of different altitudes on marksmanship (24,32). Tharion et al. (32) compared marksmanship at sea level with that at 4300 m. The sea level test was performed under resting conditions, whereas the test at altitude was performed after the subjects jogged to the top of Pike’s Peak (4300 m). Marksmanship was reduced at the initial altitude test and was still reduced when retested after 2–4 d at altitude. Muza et al. (24) tested marksmanship immediately after a 10-min lift–carry task and found that shooting performance was poorer at an altitude of 2743 m compared with that measured at sea level. However, it is important to note that marksmanship was not tested in that study under resting conditions. Although both studies observed decreased marksmanship at increasing altitude, each examined only one altitude above sea level, and the conditions of rest and exercise were not evaluated in a comprehensive manner.Independent of altitude, an acute bout of vigorous exercise has been shown to impair marksmanship (12,30). When subjects performed a shooting task immediately before and after a 200-m shuttle run while carrying a load, the marksmanship score was reduced by 18% after the run and returned to normal during a brief recovery period (30). Similar results were obtained by Evans et al. (12).The purpose of the current study was to evaluate the independent and combined effects of acute altitude exposure and vigorous physical exertion on rifle marksmanship. We hypothesized that increasing altitudes and physical exertion will both cause a significant decrease in marksmanship. By examining the shooting responses at several altitudes between sea level and 4000 m, we wished to determine whether there was a threshold altitude that demonstrated a decrement in marksmanship. We also hypothesized that the decrement in marksmanship would be related to changes in arterial oxygen saturation (SaO2), HR, and ventilation rate (VR).METHODSSubjectsParticipants were healthy men and women, age 18 to 31 yr (allowable age range was 18–44 yr), who were considered low risk for adverse cardiac events by the American College of Sports Medicine guidelines (1). None were taking any medications that might affect blood pressure or HR. No subjects had substantive exposure to altitude in the previous 6 months (one subject had spent 6 d at a moderate altitude 2 months before participation in the study; no other subjects had any exposure). A total of 17 volunteers were recruited from Old Dominion University’s ROTC program and from local military bases. Participants were required to be qualified on the M-4 or M-16 rifle to ensure they were trained in rifle marksmanship. Fifteen participants (13 male and 2 female) completed the study and two withdrew. One subject withdrew because of scheduling difficulties, while the other withdrew because of an unrelated medical issue. Before participation in the study, written informed consent was obtained. The study was approved by the University Institutional Review Board. For the 15 subjects who completed all testing, they were on average 26 ± 1 yr old, with a height of 177 ± 2 cm, a mass of 85.8 ± 3.8 kg, and V˙O2peak of 49.6 ± 1.9 mL·min−1·kg−1.Testing protocolSix separate testing sessions were completed within a period of 2 to 3 wk. Subjects were asked to refrain from ingesting caffeine or performing strenuous exercise 2 h before each test. A familiarization trial was completed in the first session, followed by an incremental exercise test to measure peak oxygen consumption (V˙O2peak). During this session, the participant was instructed on the use of a laser-modified AR-15 rifle and fired five shots from a distance of 5 m at an image of cross-hairs for calibration. Then a sequence of five shooting scenarios was presented, in which the participant fired at one or two targets in each scenario at simulated distances of 15 to 50 m. The targets were either stationary or moved laterally. For this portion of the familiarization only, the participant was informed where the target would appear and what it would do. (See below for a description of the target and shooting simulator.) After this initial practice, the subject was instrumented as he or she would be for later experimental trials (see below) and breathed at a simulated altitude of 162 m. This instrumentation included wearing a weighted vest of 10 kg and backpack of 20 kg for men and 10 kg for women (for a total load of 30 kg for men and 20 kg for women). A final practice trial of shooting–running–shooting was then completed, as would be performed in subsequent experimental trials. The load of 30 kg for men was selected on the basis of previous research that measured the fighting load of infantrymen in Afghanistan (10). The load of 20 kg for women was based on conversations with female combat veterans. These loads were also used in our previous study on military training and marksmanship (30).An incremental exercise test was performed to measure peak oxygen consumption (V˙O2peak) while wearing the weighted vest and backpack and without the hypoxia facemask. Participants were fitted with a chest strap HR monitor, a mouthpiece, nose clip, and head support for the collection of exhaled air. The treadmill test consisted of 3-min stages, beginning at 4.8 km·h−1 and 0% grade, then 6.4 km·h−1 and 0% grade, followed by 5%, 10%, and 15% grade while maintaining 6.4 km·h−1. Gases were analyzed by the V˙max metabolic cart (V˙max 29c; SensorMedics, Yorba Linda, CA), which was calibrated with known concentrations of O2 and CO2 before each test. Peak oxygen consumption (V˙O2peak) was determined as the highest V˙O2 over three consecutive 20-s periods. The subjects were verbally encouraged to exercise as long as possible. Because of expected muscular fatigue while carrying the load up a steep grade, the criteria for attainment of a true maximal V˙O2, such as achieving a RER of at least 1.10, were not used.At each of five experimental sessions, different mixtures of air that simulated different altitudes were inhaled: 162 m (approximately sea level, SL), 1015 m (1K), 2146 m (2K), 3085 m (3K), and 3962 m (4K). The simulated altitudes were accomplished by breathing through a facemask attached to a hypoxia generator (Hypoxico altitude training systems; Hypoxico Inc., New York, NY). The lowest altitude the generator could produce was 162 m; thus, this was chosen as the “sea level” condition. The order of air being breathed during the five experimental sessions was counterbalanced. The first subject’s order was SL, 4K, 3K, 2K, and 1K. Subsequent subjects started with the next altitude, so that all altitudes were the first trial for 20% of the subjects, all were the second trial for 20%, etc. A much greater number of subjects would have been needed for full counterbalancing, but this order provided no altitude with an advantage over any other. Subjects performed one condition on each testing day and were blinded to the altitude. Marksmanship score was determined by the computerized system, and therefore, the investigators were not blinded.During each experimental session, participants were first fitted with the weighted vest and backpack, pulse oximeter (Oxi-GO; Oximeter Plus. Inc, Roslyn, NY) on a finger of the nonshooting hand, hypoxic mask, and Bioharness (Zephyr, Annapolis, MD) around the chest for measurement of HR and VR. Once instrumented, the shooting simulator was calibrated with five shots aimed at an image of cross-hairs. SaO2, HR, and VR were then recorded during 5 min of seated rest. After the 5-min rest, the subject performed the first of four courses of fire (COF 1, described below). Then the subject straddled the treadmill while treadmill speed was increased to 12.1 km·h−1, at which point, the subject stepped onto the treadmill and ran for 1 min, quickly stepped off the treadmill, picked up the rifle, and COF 2 immediately commenced. A 30-s standing rest period was completed before COF 3, and a second 30-s rest was completed before COF 4. The treadmill run with load was designed to mimic strenuous physical exertion in a combat environment. A similar task was used in our previous marksmanship study (30).The marksmanship test used a civilian AR-15 rifle fitted with an in-barrel laser and a computerized projection system (Cognitive and Physiological Testing Urban Research Environment, CAPTURE; Advanced Anti-Terror Technologies, Clermont, FL). A projector was positioned 5 m from a wall and projected torso silhouette targets that were 60 cm wide and 100 cm tall at a distance of 5 m, and were proportionally smaller for greater simulated distances; i.e., the target was 20 cm wide when representing a distance of 15 m and 3 cm wide when representing a distance of 100 m. Seven concentric rectangular scoring rings encompassed the width of the target, with points ranging from 10 for the center ring to 4 for the outermost ring. The subject fired from a distance of 5 m at simulated distances of 15–100 m. In each COF, the subjects fired once at each of 10 targets (maximum possible score of 100 points). The targets appeared single or doubly every few seconds. Each COF took 30–49 s. The targets appeared in a continuous sequence without explanation, and the order of targets varied. A total of four target orders were used. Each subject faced the same order of targets, but they were unaware of which specific target would appear during testing.StatisticsWithin-subjects two-way (trial and altitude) repeated-measures ANOVA was performed on marksmanship scores, shooting SaO2 (the average of pre and postshooting values), shooting HR (recorded throughout the trial), and shooting VR (recorded throughout the trial). One-way ANOVA was performed on the baseline resting values of SaO2 (average of five values recorded once per minute), HR (recorded continuously over 5 min), and VR (recorded continuously over 5 min). Correlations between marksmanship scores and SaO2, HR, and VR were performed. One-way ANOVA on peak HR and peak VR recorded during the treadmill runs at each altitude and on postrun SaO2 was done. For analyses where the ANOVA revealed significant F-ratios, planned contrasts were used to determine which means differed from others. All statistical analyses were performed using SAS statistical software, 9.1 (SAS Institute Inc., Cary, NC), and statistical significance was set at P < 0.05. All data are presented as mean ± SE.RESULTSThere was a significant (P < 0.001) main effect of altitude on shooting performance (Fig. 1). Planned contrast analyses demonstrated that the shooting score at 4K was significantly less than that at all other altitudes (P ≤ 0.001). The score at 4K during trial 1 averaged 17.2% ± 5.6% less than that at SL. There were also likely trends for the score at 3K to be less than that at SL and 1K (P = 0.061 and 0.074, respectively).FIGURE 1. Results of shooting performance (mean ± SE). There were significant main effects of altitude and exercise trial on marksmanship score. *Trend for 3K < SL, 1K; **4K < SL, 1K, 2K, 3K; a trial 1 > trials 2, 3, 4; b trial 2 < trial 3.There was a significant (P < 0.001) main effect of the physical exertion trials on shooting performance, in which exercise reduced marksmanship from rest, and it somewhat improved after the recovery trials (Fig. 1). The score in trial 1 was significantly greater than that in trials 2 (P < 0.001), 3 (P = 0.003), and 4 (P = 0.005), and the score in trial 3 was significantly more than that in trial 2 (P = 0.046). The decrease in shooting score from trial 1 to trial 2 was 14.9% ± 3.8% at SL, and although this value rose to 24.3% ± 6.0% at 4K, the interaction between altitude and physical exertion task on marksmanship did not reach statistical significance (P = 0.983). The decrease in shooting score in going from a resting state at SL (trial 1) to immediate postexercise at 4K (trial 2) was 34.9% ± 6.1%.As shown in Figure 2, SaO2 during each shooting trial (i.e., the average of the pre- and postshooting SaO2 values) was significantly reduced by increasing altitude (main effect, P < 0.001). The SaO2 at each altitude was significantly different than that at all other altitudes (P < 0.001 for all comparisons). SaO2 during shooting was also significantly affected by the physical exertion task and subsequent recovery (main effect, P < 0.001). SaO2 in trial 1 was significantly greater than that in trial 2 (P < 0.001) and trial 3 (P = 0.007). Trial 2 was significantly less than trials 3 and 4 (P < 0.001 for both). Trial 3 was significantly less than trial 4 (P = 0.007). In other words, SaO2 decreased after the run and recovered in the subsequent trials, reaching the same value in trial 4 as before the run in trial 1. There was also a significant (P < 0.001) interaction between altitude and shooting trial on SaO2. The decrease in SaO2 after the run (i.e., SaO2 at trial 1 minus that at trial 2) was greater with increasing altitude, dropping 1.7% ± 0.6%, 3.4% ± 1.0%, 6.3% ± 1.0%, 7.7% ± 0.8%, and 9.1% ± 1.0% at altitudes from SL through 4K, respectively.FIGURE 2. Arterial oxygen saturation (SaO2) during shooting (mean ± SE). There were significant main effects of altitude and exercise trial on SaO2. There was a significant interaction between altitude and trial. *SL > 1K > 2K > 3K > 4K; atrial 1 > trials 2, 3; btrial 2 < trials 3, 4; ctrial 3 < trial 4; #decrease from trial 1 to trial 2 increased with altitude; specifically, SL < 2K, 3K, 4K; 1K < 2K, 3K, 4K; 2K < 4K.HR during shooting was not affected by altitude (main effect, P = 0.504), but it was by the physical exertion task and recovery (main effect for trials, P < 0.001) (Fig. 3). The HR in trial 1 was significantly lower than that in all other trials (P < 0.001). Trial 2 was significantly greater than trials 3 and 4, and trial 3 was significantly greater than trial 4 (P < 0.001 for all comparisons). Thus, HR was increased after the physical exertion task, and recovered somewhat in the subsequent trials, but did not recover all the way to that observed in the first trial. There was no significant interaction between altitude and trial on HR (P = 0.753).FIGURE 3. HR during shooting (mean ± SE). There was a significant main effect of exercise trial on HR. Altitude did not have a significant main effect on HR. aTrial 1 < trials 2, 3, 4; btrial 2 > trials 3, 4; ctrial 3 > trial 4.Increases in altitude caused an increase in VR during shooting (main effect, P = 0.001) (Fig. 4). Specifically, VR at 1K was significantly less than 2K (P = 0.026), 3K, and 4K (P < 0.001 for both). Also, VR at SL was greater than 1K (P = 0.019). The physical exertion task also significantly affected VR during shooting (main effect for trials, P < 0.001), with trial 1 lower than all other trials (P < 0.001), trial 2 greater than trials 3 and 4 (P < 0.001 for both), and trial 3 greater than trial 4 (P = 0.016). Thus, as with HR, VR increased after the run and decreased in recovery, but did not decrease all the way to that observed in the resting trial. There was no significant interaction of altitude and physical exertion on VR (P = 0.902).FIGURE 4. VR during shooting (mean ± SE). There were significant main effects of altitude and exercise trial on VR. *SL > 1K; **1K < 2K, 3K, 4K; atrial 1 < trials 2, 3, 4; btrial 2 > trials 3, 4; ctrial 3 > trial 4.During the initial 5 min of rest, SaO2, HR, and VR were recorded. SaO2 at rest was 98.3% ± 0.5%, 97.4% ± 1.0%, 95.8% ± 1.0%, 94.3% ± 1.8%, and 91.4% ± 2.2% at SL through 4K, respectively. All altitudes were significantly different from each other in SaO2, except for SL and 1K. Resting HR values were not significantly different between any altitudes. Average VR was significantly increased with altitude (main effect, P = 0.034). Values of HR, VR, and SaO2 recorded with the 60-s run are presented in Table 1. There were no differences in peak HR between altitudes. Peak VR was significantly greater at 3K than 1K (P = 0.026). SaO2 recorded immediately postrun decreased significantly with each higher altitude (P < 0.001 for all).TABLE 1 Peak HR, peak VR, and postrun SaO2 at each altitude (mean ± SE).The mean values for shooting score in each trial at each altitude were compared with the respective means of the cardiopulmonary variables. There was a strong positive correlation (r = 0.838, P < 0.001) between SaO2 and marksmanship scores (Fig. 5). There was a strong inverse correlation (r = −0.724, P < 0.001) between VR and marksmanship score and a modest (r = −0.540) yet significant (P = 0.014) inverse correlation between HR and marksmanship score.FIGURE 5. Correlation between arterial oxygen saturation (SaO2) and marksmanship scores.DISCUSSIONThe aims of this study were to (a) assess the effect of acute altitude exposure on marksmanship and (b) to evaluate the effect of a physical exertion task at altitude on marksmanship. Our key findings were that both increasing altitude and the physical exertion task significantly impaired marksmanship. Altitude appeared to have a threshold effect, with marksmanship decreasing at 3000–4000 m.While ability to perform precision motor tasks at altitude is clearly compromised, there have been only two previous studies that have specifically addressed the effects of altitude on marksmanship (24,32). However, both studies only investigated a single altitude higher than sea level. Tharion et al. (32) found marksmanship was reduced at 4300 m, and Muza et al. (24) found it was reduced at 2743 m. Our findings were similar in that we observed a strong trend for marksmanship to decline at 3K and a significant decrease at 4K. We found no decrease in performance at either 1K or 2K, which are altitudes that have not previously been examined in any study.A key difference in our study from the previous two was evaluating the effect of an exercise challenge at each altitude. When at sea level, Tharion et al. (32) measured marksmanship only at rest and measured it at altitude immediately after exercise (their resting measure at altitude was performed after 2–4 d of acclimation). Muza et al. (24) did not measure marksmanship at rest at either sea level or altitude but, instead, only measured it immediately after a physical challenge. In contrast, we measured marksmanship at rest, immediately after exercise, and after recovery from exercise, at all of the altitudes tested.As predicted, the physical exertion task resulted in a significant decrease in marksmanship at all altitudes, i.e., scores in trial 2 (immediately after the 60-s treadmill run) were lower than scores in trial 1 (before the run). This result is consistent with previous research by Swain et al. (30) who used a similar physical exertion task and shooting scenario. Swain et al. used a 200-m shuttle run that took approximately 60 s, as opposed to a 200-m continuous run of 60 s on a treadmill used in this study. Although a shuttle run is more demanding than a continuous run of the same distance and duration, we used the somewhat easier, continuous task to ensure that subjects could complete it even at the highest altitude tested. Moreover, we needed to use a treadmill to maintain the attachment of the subject to the hypoxic equipment. In both studies, male subjects carried a 30-kg load, whereas female subjects carried a 20-kg load during both running and shooting. Swain et al. (30) observed an 18% decrease in marksmanship immediately after the run, whereas the current study found a similar 15% decrement after the run performed in the sea level trial. This decrement increased to 24% at 4K, although this was not significantly greater.This study did not evaluate what effect the load itself may have had on marksmanship. Clearly, the load contributed to the physical exertion task, but did the load affect marksmanship while standing? Because the load was distributed over the front and back of the chest (though not equally), the added mass may have improved marksmanship by dampening body sway. Alternatively, the load may have impaired marksmanship by putting the shooter in a biomechanically challenging position. A separate study of marksmanship with and without a load, in which center of pressure motion is recorded via force plate, would be needed to address this question.As expected, the simulated altitudes caused a decrease in arterial oxygen saturation at rest. Our values (98%, 97%, 96%, 94%, and 91% at SL, 1K, 2K, 3K, and 4K, respectively) were quite similar to those obtained in other studies of terrestrial altitude from sea level to approximately 3000 m (24,25), although values observed at 4300 m were about 10% less than we obtained at 4K (24,25). This difference could be due to the device used to create the normobaric hypoxia (NH) in the current study. However, the further decrease in SaO2 caused by exercise was similar to that seen in Muza et al.’s study (24) using terrestrial altitude. Muza et al. reported that a 10-min lift-and-carry task reduced SaO2 by 7% from resting values when at 2743 m and by 12% when at 4300 m. The decreases in percent SaO2 after exercise in the current study were 2%, 4%, 8%, 10%, and 9% at SL, 1K, 2K, 3K, and 4K, respectively. Strenuous exercise can impair oxygen loading in the lungs via the effect of a low pH on the oxygen–dissociation curve.The use of NH to simulate the hypobaric hypoxia (HH) of higher altitudes is a limitation in this study. There appear to be some differences in the physiological responses to HH and NH at the same inspired O2 pressure (21,27), although not all experts in the field agree (23). For example, the increase in ventilation is reportedly greater with NH, and there is a greater work of breathing with NH due to the higher air density at a higher barometric pressure (21,27). Transcapillary fluid flux and lymph flow are affected by air pressure (18), which could potentially cause differences in NH and HH. It is not known if the NH used in the current study may differentially affect marksmanship as compared with HH, as neither we nor others have performed a comparative study. The primary effects of hypoxia are likely due to reduced SaO2, which certainly occurs with NH.HR did not vary between altitudes in the current study. It is generally recognized that HR at rest and during low-intensity exercise is elevated with increasing altitude, whereas maximal HR is reduced (20). Studies performed at approximately 3000 m have found only small (13,16) or nonsignificant (6,34) increases in resting HR. Thus, the lack of an HR effect at moderate elevations in altitude at rest in this study is consistent with previous reported findings. Moreover, given that maximal HR is typically reduced by altitude, it is reasonable that HR in this study after a brief bout of near-maximal exercise was unaffected.Acute exposure to simulated altitude impairs cognitive function. Common measures that have been negatively affected are arithmetic tasks, reaction time to visual stimuli, psychomotor skills, and short-term memory (26). Although impairment is most consistently reported at simulated altitudes above 3000 m (5,19,26,31,35), impairment has sometimes been reported at altitudes in the range of 2100–2700 m (26). Moreover, exercise at altitude exacerbates the detrimental effect (2,26). Ando et al. (2) found a significant correlation between impairment in reaction time to a visual stimulus and decreased cerebral oxygenation during exercise at a simulated altitude of only 2200. Reaction to a visual stimulus was an important performance aspect in the current study.Vision itself is impaired by hypoxia, although the effect is not pronounced at the moderate altitudes simulated in this study (26). Decreased visual sensitivity under photopic (daylight) conditions is substantial at high altitudes (such as the study of Horng et al. (14) at 7620 m) but may be limited to impaired peripheral contrast sensitivity at moderate altitude (9). Under mesoptic (twilight) conditions, moderate altitudes as low as 2438 m impair several aspects of visual acuity and chromatic sensitivity (7,8). The current study used photopic conditions; thus, little effect on visual acuity was expected. However, we hypothesize that the decreased marksmanship observed in this study may be exacerbated under mesoptic conditions, which are common in ground warfare and warrant further investigation. We observed a decrease in SaO2 with simulated altitude, exacerbated by strenuous exercise, that could impair marksmanship due to diminished cognitive function and psychomotor skills, as well as by impaired balance (33), which was not measured in this study. Indeed, we found a significant correlation between the decrease in marksmanship and the decrease in SaO2, which has not been previously reported.The minimum duration of hypoxic exposure required to produce cognitive impairment is not known. Several studies have used exposures of an hour or more (19,26,35). However, the testing protocol of Ando et al. (2) began after only 10 min of hypoxic exposure, and Temme et al. (31) measured decreased performance on a flight simulation task during the third through eighth minute of hypoxic exposure. Similarly, studies of vision under hypoxia have used acute exposures, 15 min before testing in Connolly’s studies at moderate altitude (7–9), and only 1–3 min of total exposure time (including testing) at high altitude (14). These durations are comparable with the current study in which testing began after the fifth minute of exposure.There are numerous physiological variables that could affect marksmanship. For example, the ability to minimize limb tremor is a critical control issue for precision shooting tasks of this nature (17). However, previous research has shown that acute hypoxia (15) and exercise-induced fatigue (22,28) can negatively affect limb tremor. Furthermore, the ability to minimize postural motion, which is often exacerbated with exercise and altitude, is also essential for optimal shooting performance (4,11,33). VR is another variable that could affect marksmanship, as the movement of the chest wall has the potential to affect one’s ability to aim a rifle. As expected, VR increased with altitude and increased with physical exertion, and there was a strong inverse correlation between VR and marksmanship. Military personnel are taught to pause their breathing while in the act of firing. During slow fire, they pause at the end of an exhalation and then squeeze the trigger before beginning the next inhalation; during rapid fire, they pause at any moment within the breathing cycle to squeeze the trigger (3). Greater concentration on this technique may be needed at altitude.Although HR was unaffected by altitude, the correlation between HR and marksmanship was important to examine because HR is a valuable index of physical as well as emotional stress. Military personnel may have to run to a position of concealment and then begin firing right away. We observed a significant inverse correlation between HR and marksmanship, a result consistent with our previous work (30). Swain et al. (30) argued that the relationship was associative, not causal. In the current study, we found that the correlation of SaO2 with marksmanship (0.84) and of VR with marksmanship (−0.72) were much stronger than the correlation between HR and marksmanship (−0.54). We suggest that HR is a useful marker of factors that affect marksmanship, but SaO2 and ventilation are more directly related. However, these latter two variables cannot be readily measured in a practical environment.In addition to the use of NH, there were other limitations in this study. Although a snug fit of the breathing mask was used, there was no test to determine whether any leakage occurred. Moreover, there was no independent verification of the inspired O2 concentration indicated by the manufacturer of the hypoxic equipment. One other limitation is that we did not specifically address shot tightness in our measure of marksmanship, although it was included in the scoring system. We converted each shot into a score based on closeness to the center of the target and then derived a single score for each participant over the entire course of fire. This was done to evaluate shooting ability in a dynamic, combat environment, engaging multiple targets at relatively close range using the offhand (i.e., standing) shooting posture; we did not evaluate static marksmanship under slow, controlled fire, as is done in competitive target shooting.CONCLUSIONOverall, marksmanship decreased at increasing simulated altitudes, with an apparent threshold at 3000–4000 m. Furthermore, physical exertion reduced marksmanship at all altitudes, from sea level to the highest altitude tested (4000 m). The decreased oxygen availability due to both altitude and strenuous exercise reduced arterial oxygen saturation, which appears to have reduced marksmanship through multiple mechanisms, one of which being increased VR. This finding reinforces the need for marksmen to control breathing when firing. Therefore, a training protocol to assist individuals to control breathing should be developed and evaluated. Further research should also evaluate the effects of acclimation to altitude on these results.We thank the subjects for their participation in this demanding study.There are no conflicts of interest for any of the authors.There was no funding received in support of this study.The results of the present study do not constitute endorsement by the American College of Sports Medicine.REFERENCES1. American College of Sports Medicine. ACSM’s Guidelines for Exercise Testing and Prescription. 8th ed. Philadelphia (PA): Lippincott Williams & Wilkins; 2010. p. 22–8. [Context Link]2. Ando S, Hatamoto Y, Sudo M, Kiyonaga A, Tanaka H, Higaki Y. The effects of exercise under hypoxia on cognitive function. PloS One. 2013; 8 (5): e63630. [CrossRef] [Medline Link] [Context Link]3. Army Field Manual 3-22.9, Rifle Marksmanship [Internet], p. 4–22 [accessed July 22, 2013]. Available from: http://armypubs.army.mil/doctrine/DR_pubs/dr_a/pdf/fm3_22x9.pdf . [Context Link]4. Ball K, Best RJ, Wrigley TV. 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