Risk Factors for Injuries During Military Static-Line Airborne Operations: A Systematic Review and Meta-Analysis

We followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines¹⁵  and the review protocol described in this section.

PubMed and the Defense Technical Information Center (DTIC) were searched to find studies on risk factors for airborne-related injuries. For the DTIC search, we examined only unclassified articles (ie, approved for public release, distribution unlimited). Key words were airborne OR parachuting OR parachutes OR paratrooper AND injuries OR wounds OR trauma OR musculoskeletal. The reference lists of obtained articles were searched for articles not found in the retrieval services. The files of a senior researcher (J.K.) with airborne injury experience were also examined. In 3 cases, we contacted authors to clarify data-collection methods. No limitations were placed on the dates of the searches, and the final search was completed in December 2014.

Articles were selected for the review if they (1) were written in English, (2) involved static-line parachute operations, (3) involved military personnel, (4) involved injuries either documented directly on the drop zone or obtained from safety or medical records, and (5) provided a quantitative assessment of any potential airborne-related injury risk factors. Injuries had to occur during parachuting operations from the time personnel exited the aircraft to the time they departed from the landing zone. We examined publication titles and reviewed the abstract if the article appeared to describe injuries during military airborne operations. The full text of the article was retrieved if the abstract suggested that injury risk factors were addressed. Quantitative risk-factor data could be contained within the text of the article, in tables, or in graphs. Data presented in graphs were estimated. If the authors did not explicitly report on a specific risk factor but data were available in the article to calculate it, then we included the article and the data in the review.

We did not include studies that involved (1) free-fall operations or high-altitude, low-opening operations; (2) ejections from aircraft involving parachutes; (3) airborne soldiers involved in other-than-direct parachute operations; and (4) self-reported injuries (eg, questionnaires). Information contained in abstracts was also not included. Abstracts were not often peer reviewed and were difficult to locate because they generally were not included in reference databases.

The summary statistic was the risk ratio (RR) and its 95% confidence interval (CI) that we extracted from each article. The RR was the ratio of the risk of injury in 1 group to that of another (baseline) group. In calculation of injury risk, the numerator was the number of paratroopers injured, and the denominator was the number of aircraft exits. For each potential risk factor, the reference (baseline) group was defined with an RR equal to 1.00. An RR greater than 1.00 indicated a higher risk in 1 group than in the baseline group, and an RR less than 1.00 indicated a lower risk. Data in many studies had to be reanalyzed to obtain RRs. When this was necessary, we used the OpenEpi Calculator (www.openepi.com, Emory University, Atlanta, GA) to calculate RRs and their 95% CIs.¹⁶  We noted in the text when we could not provide these values but the data indicated the direction of a particular factor (ie, increased or decreased risk).

The Comprehensive Meta-Analysis Statistical Package (version 3.2; Biostat, Englewood, NJ) was used to perform meta-analysis on variables when at least 2 studies and sufficient information were available for this calculation. For all meta-analyses, we employed a random model that used RRs and their 95% CIs. The meta-analysis produced a summary RR (SRR) and a summary 95% CI that represented the pooled results from all of the individual investigations. Heterogeneity or homogeneity of the SRR was assessed using the Q and I²  statistics.¹⁷  Heterogeneity was the degree of variability in the RRs used in a particular meta-analysis. The I² statistic indicated the percentage of heterogeneity among studies, with smaller values indicating less heterogeneity and larger values, more heterogeneity. In calculating the I² statistic, negative values were set equal to zero, which indicated very little heterogeneity.¹⁸  Tables always contained meta-analyses with all studies of a particular risk factor included; however, in a few cases, studies that appeared to account for a large portion of the heterogeneity were eliminated from the analysis, and the results were included in the text.

We also used the Comprehensive Meta-Analysis Statistical Package to provide funnel plots and calculate Begg and Mazumdar correlations to examine publication bias. Publication bias based on funnel plots was assessed by examining the symmetry of the distribution of studies around the log of the pooled effect size. We used the Begg and Mazumdar test¹⁹  to calculate the rank-order correlation between the treatment effect and the standard error, with the latter driven primarily by sample size. A correlation suggested that publication bias existed. At least 3 studies were required for funnel plots and Begg and Mazumdar correlations.

The inclusion and exclusion of publications at each stage of the literature search are shown in Figure 1. The search resulted in 4175 total citations in PubMed, 13 497 in the DTIC search, and 12 from other sources. The 12 citations originally obtained from other sources (available files of the senior researcher) were also found in the PubMed search. After reviewing titles and abstracts, we obtained 71 full-text publications from PubMed and 22 from DTIC. Of the 22 DTIC publications, 18 were technical reports that were also published as peer-reviewed journal articles and found in PubMed. The other 4 DTIC technical reports did not contain risk-factor information, so no DTIC technical reports were included in the review. After reviewing the 71 full-text articles, we observed that 25 studies fully met the review criteria. However, 3 studies^5,20,21  used a database to which data were added progressively over time. Given that these reports^5,20,21  contained the same injury risk factors, only the most recent study with the largest number of aircraft exits was considered.⁵  Therefore, we included 23 studies in the review. One study was published in 2 journals.^22,23  Three selected studies^5,20,24  involved the authors of this report, indicating a possible conflict of interest.

The 23 investigations included in this review were observational cohort investigations. Injury data were recorded as normal airborne operations were ongoing. Table 1 provides the injury case definitions, military units, types of paratroopers, dates of data collection, total jumps recorded, and quantitative risk factors included in the study. Injury case definitions differed, but many investigations included any type of physical damage to the body that occurred during jump operations.^* Two studies^28,31  included most injuries sustained during jump operations but excluded minor injuries, such as contusions, abrasions, and lacerations. Five investigations^{27,32,36,39,42}  included only injuries that resulted in limited duty, during which the soldier was excused from performing specific tasks by medical personnel due to the injury. A few studies^36,38,43  included only specific types of injuries, such as lower extremity injuries and fractures³³  and ankle injuries. One study²⁹  did not provide an injury definition, and another²⁷  examined only injuries that occurred at altitude (before ground contact) and resulted in limited duty.

Most authors^{22,26–28,31–33,36–44}  collected injuries from available safety or medical records. Other researchers recorded injuries directly on the drop zone,^{5,20,24,25,30,35}  often with follow-up by examining medical records.^5,24,25,35  In 1 case,²⁹  how injury data were obtained was unclear. Most studies involved paratroopers from the United States.^† Other studies involved service members from Australia,^31,39,40  Belgium,^28,29  Brazil,⁴¹  Israel,²⁶  and the United Kingdom.^25,30 

The publication dates of the studies ranged from 1946 to 2014, a 68-year period. The total number of aircraft exits in the studies ranged from an estimated high of 1 115 860³⁸  to a low of 554,⁴⁰  with an estimated total of 2 775 567 jumps. This is an underestimate of the total number of jumps because 1 study³³  did not report the total jumps and another²⁵  noted that a small number of jumps may not have been properly recorded. Numerous potential risk factors were recorded in the studies (Table 1).

The 2 most studied airborne-related injury risk factors were time of day and jumps with or without equipment. Investigations examining these variables are shown in Table 2. Only 1 of the 8 studies examining time of day indicated that daytime jumps had a higher injury risk than night jumps.²⁵  Early studies^25,26,28  appeared to have more variability than later ones.^{5,24,30,34,42}  The SRR from the meta-analysis indicated an almost doubling of injury risk, but both the Q and I²  statistics indicated high heterogeneity in this estimate. Little publication bias was noted, with funnel plots (Figure 2A) showing studies generally distributed symmetrically about the pooled effect size, and the Begg and Mazumdar correlation was low.

Table 2 also showed that the carriage of additional equipment elevated injury risk in all 7 studies examining this factor. The SRR from the meta-analysis suggested that the injury risk was more than doubled when paratroopers jumped with extra equipment, but again, high heterogeneity existed in this estimate, as indicated by the Q and I² statistics. The funnel plot (Figure 2B) indicated that studies were generally distributed around the mean effect size (with 1 outlier) and the Begg and Muzumdar correlations were low and not significant.

The associations between airborne injuries and various weather variables, including wind speed, temperature, humidity, heat index, and wind direction are demonstrated in Table 3. We could not perform meta-analyses because different investigations had different strata (eg, wind-speed groupings), did not report denominators (number of aircraft exits), or lacked 95% CIs. Recordings of wind speed differed (average, lowest, maximal), but higher winds increased the injury risk. Some researchers^28,30  suggested a dose-response relationship between wind speed and injuries, but others^5,24,25  reported little difference in injury risk until a critical speed was exceeded. Pirson and Verbiest²⁸  found little difference in injury risk until the temperature exceeded about 25°C, but Knapik et al⁵  suggested that higher temperatures or a higher heat index modestly increased the injury risk in a dose-response manner. The effects of humidity were not clear. Lillywhite³⁰  noted that winds from the rear of the aircraft elevated the injury risk.

The association between airborne-related injuries and the type of aircraft is illustrated in Table 4. Jumps from fixed-wing aircraft resulted in a higher injury risk than jumps from balloons in all 3 studies^25,28,30  that examined this association. The SRR demonstrated high heterogeneity primarily due to the much larger RR in 1 study.³⁰  Excluding that study from the meta-analysis resulted in an SRR of 1.53 (95% CI = 1.14, 2.07), Q statistic P value of .88, and I² value of 0. In the full analysis (all 3 studies^25,28,30 ), interpreting the funnel plots was difficult due to the small number of investigations, but the studies were distributed on both sides of the mean effect (Figure 2C). The Begg and Mazumdar correlation was relatively high but not different due to the small number of investigations. When comparing fixed- and rotary-wing aircraft, the latter demonstrated a lower risk in 2 of 3 studies.^5,30  The study showing a higher risk in rotary-wing aircraft⁴²  involved a very small number of aircraft exits (777 in this comparison). Excluding that study⁴²  from the meta-analysis resulted in an SRR of 6.98 (95% CI = 3.60, 13.54), Q statistic P value of .89, and I²  value of 0. In the full analysis, the funnel plots were difficult to interpret with only 3 studies^5,30,42 ; however, the studies were distributed on both sides of the mean effect (Figure 2D), and the Begg and Mazumdar correlation was low, suggesting little publication bias (Table 4). The C130 Hercules aircraft had a lower injury risk than other aircraft in 2 investigations^41,42  but not in a third.⁵ 

The association between airborne injuries and various types of landing zones is presented in Table 5. The wide variety of landing-zones precluded meta-analysis. Landing zones described as “rough” resulted in a higher injury risk than those described as “sand dunes” or “flat/grassy.”^26,41  Landing strips^22,32  resulted in a higher injury risk than those described as “fields”³²  or “paved runways.”²²  Water landings resulted in a lower risk than ground landings.⁴⁰  Several landing zones were described in 1 study, with little difference in risk for all but 1 zone called Geronimo.⁵ 

The association between airborne injuries and soldiers' personal characteristics, including sex, age, body weight, and height, is shown in Table 6. We could not perform meta-analyses because 1 of the 2 studies on sex reported odds ratios rather than RRs,³³  the 2 studies reporting on body weight^29,40  used different strata, and only 1 study²⁹  reported on height. Both studies reporting on sex^33,37  indicated that women had a higher injury risk than men. The association between injuries and height was not clear. Greater body weight appeared to elevate the injury risk of ground landings, but the risk during water landings was not clear due to the extremely limited data. The 2 studies involving age produced contradictory results.^34,37 

Table 7 demonstrates the association between airborne-related ankle injuries and the parachute ankle brace (PAB). Not wearing the PAB almost doubled the risk of an ankle injury. The elevated risk was relatively consistent among studies, as indicated by the Q and I² statistics, which both suggested little heterogeneity. We noted little publication bias; the funnel plot (Figure 2E) showed the studies were generally distributed symmetrically about the pooled effect size, and the Begg and Mazumdar correlation was relatively low.

The association between airborne injuries and a variety of other potential risk factors is provided in Table 8. Parachutes with larger canopies reduced the injury risk.^5,20,28  Staggered exits from opposite sides of aircraft with 2 doors reduced injuries at altitude.²⁷  Exits that were not simultaneous³¹  or exits from tailgates⁵  reduced the injury risk. Entanglements among paratroopers substantially increased the risk of injury.⁵  Students in basic airborne courses had a lower injury risk than soldiers in refresher courses or on military exercises.²⁶  Soldiers familiar with a parachute system tended to have a lower injury risk than those learning a new parachute system, but the number of jumps was small.⁴²  Military officers had a lower injury risk than enlisted soldiers.⁵  The order in which paratroopers exited the aircraft had little influence on injury risk,⁵  but the injury risk was higher when the number of paratroopers exiting the aircraft was greater.³⁰ 

We found factors that increased injury risk during military static-line airborne operations, including night jumps, jumps with extra equipment, higher wind speeds, higher environmental temperatures, jumps from fixed-wing aircraft (compared with balloons and rotary-wing aircraft), jumps onto certain types of terrain, female sex, greater body weight, not using the PAB, smaller parachute canopies, and simultaneous exits from both sides of an aircraft. Other factors that appeared to increase risk but were examined in only 1 investigation included a higher heat index, winds from the rear of the aircraft, entanglements, less experience with a particular parachute system, enlisted rank (compared with officers), and jumps involving a greater number of paratroopers.

The individual studies and meta-analyses showed that both night jumps^{5,24,26,28,30,32,42}  and jumps with extra equipment^{5,24,28,30,31,39,42}  increased the injury risk; however, considerable variability existed in the RRs of the individual studies. Publication bias was minimal, and the effects of both risk factors were very robust. In the 1 study²⁵  that showed little difference between day and night jumps, the author noted that the night jumps were generally conducted in “good weather conditions.” In the few investigations^24,30  in which multivariate analyses were performed, both night jumps and jumps with extra equipment were independent risk factors for injury when considered with other risk factors, such as wind speed and temperature. During night jumps, individuals are less able to see the ground, perceive distance and depth, and appreciate the direction of horizontal drift. These and other factors possibly contributed to less controlled landings and a reduced ability to see obstacles on the drop zone, which may be associated with the higher injury rates.

Jumps with extra equipment typically involved loaded rucksacks, special weapons, or weapon containers.^{5,24,28,31,39}  This equipment can substantially increase the total weight on the parachute and result in a faster descent rate, leading to greater impact forces on ground contact that could increase injury risk.

Higher wind speeds increased injury risk, and the few authors^5,24,30  who performed multivariate analysis indicated that higher wind speed was an independent risk factor for injury. Combining data from studies was not possible for reasons cited earlier (different recording methods, different strata or levels of a variable, and lack of information about the number of aircraft exits). Nonetheless, careful examination of the data suggested little difference in injury risk until wind speeds exceeded 8 to 11 knots.^5,24,30  In the US Army, airborne training operations are discontinued when winds are greater than 13 knots.⁴  Wind increases horizontal drift and parachute oscillations. When horizontal velocity from drift and oscillation are added to the vertical descent velocity, ground impact forces are elevated, and landing control may be compromised; these factors may lead to a higher injury risk. Winds can push a jumper away from preplanned drop zones into obstacles, rougher terrain, or trees. High winds can also drag paratroopers on the ground after they land and before they have time to collapse their parachute canopies.

Whereas the effect of humidity was not clear, higher environmental temperatures or a higher heat index (a value calculated from temperature and humidity) modestly increased the injury risk.^5,28  In 1 multivariate analysis,⁵  higher temperature was clearly an independent injury risk factor when considered with night jumps, extra equipment, wind speed, and other variables. Assuming similar humidity and barometric pressure, increases in temperature decrease air density,⁴⁶  resulting in faster parachute descent rates and greater ground impact velocities.

Lillywhite³⁰  reported that winds from the rear of the aircraft increased injury risk. The author hypothesized that, in this situation, aircraft-exit dynamics cause the jumper to drift backward. If the jumper does not correct the backward drift by pulling on the canopy risers, he or she will be forced into a backward landing. A backward-drifting soldier has difficulty executing proper landing procedures and is likely to impact the ground sequentially with the feet, buttocks, and head, which may lead to a higher injury risk.³⁰  Proper landing procedures consist of executing a parachute landing fall (PLF). Since the early 1940s, the United States and many European paratroopers have used the PLF to dissipate the forces of ground contact because it appeared to reduce the injury risk compared with earlier landing methods.^47,48  A PLF begins with the feet and knees together, toes pointed toward the ground, and knees slightly bent and rotated to the side. The upper extremities are raised with the forearms held tightly in front of the face, and the chin is tucked. As the feet contact the ground, the paratrooper rolls sideways and sequentially onto the outer side of the legs, thighs, buttocks, and trunk. The soldier then rolls onto his or her back to complete the PLF.

A wide variety of aircraft have been examined.^{5,25,28,30,41,42}  Jumps from fixed-wing aircraft appeared to have about twice the injury risk of jumps from balloons. However, Essex-Lopresti²⁵  noted that jumps from balloons were generally conducted “in better weather.” Nonetheless, all studies^25,28,30  comparing fixed-wing aircraft with balloons demonstrated less injury risk with the balloons. Jumps from rotary-wing aircraft usually demonstrated a lower injury risk than jumps from fixed-wing aircraft in 2 studies^5,30  but not in 1 study⁴²  involving only 777 jumps. Jumps from rotary-wing aircraft were typically conducted at higher altitudes and off tailgates instead of out of side doors. Higher altitudes allow jumpers more time to control the canopy and prepare for ground landings. In tailgate exits, static lines are attached to cables in a way that is less likely to produce static-line injuries. More space between jumpers likely reduces entanglements and injuries at altitude.

Landing-zone characteristics had a large influence on injury risk.^{5,22,26,32,40,41}  Jumps onto landing zones described as “rough” were about 3 times more likely to produce injuries than jumps onto sand or “flat/grassy” areas.^26,41  This was probably due to uneven ground and obstacles, such as rocks and bushes, on the rougher landing zones that make the proper execution of a PLF difficult.

Jumps onto land had about 4 times the risk of jumps into water,⁴⁰  possibly due to the shock-absorbing quality of water. However, in unplanned water landings, the possibilities of parachute entanglements and difficulties doffing heavy equipment that can pull the jumper deeper into the water also exist.³²  Two investigations^26,32  indicated that jumps onto landing strips had a higher injury risk than jumps onto “field” areas. Areas outside the narrow landing strip tend to have very uneven terrain, embankments, drainage ditches, and other hazards that may be encountered on ground contact. Landing strips are important targets for capture during airborne operations.³² 

Knapik et al⁵  noted a considerably higher injury risk at a landing zone called Geronimo than other drop zones. Possible reasons were not addressed in that article⁵  but were addressed in another report²⁰  using some of the same data. The higher injury risk was likely associated with this single operation involving a night jump and combat loads (factors known to elevate risk) onto a landing zone that was unfamiliar to a large number of participating soldiers. The fact that landing zone was not an independent risk factor for injury in either study^5,20  at least partly supports the hypothesis.

Overall, women had a higher airborne-related injury risk than men even though men likely weighed more than women and body weight appeared to modestly elevate risk. We found no data on the actual body weights of airborne-qualified men and women, but across the entire US Army, the average body weight was 79 kg for men and 62 kg for women, with men weighing 1.27 times more than women.⁴⁹  During airborne operations, the higher injury risk for women was especially apparent for lower extremity injuries and fractures.³³  In the US Army, men and women have similar overall fracture incidences,⁵⁰  but women tend to be in military occupational specialties with lower overall injury risks.⁵¹  When men and women performed similar activities in recruit training, women had more than twice the risk of fractures⁵⁰  and lower extremity injuries.⁵²  Women have a lower bone-section modulus; a lower bone-strength index (ratio of section modulus to bone length); and a thinner and narrower cortical area, which provides less bone strength.⁵³  These factors may increase females' susceptibility to fractures on ground impact during parachuting.

Interestingly, from 1985 to 1994, airborne injury incidences among women were declining and approaching those of men.³³  The opening to women of military occupational specialties that require regular airborne training may have influenced this trend.³³  As noted, when their physical activity is similar, women are more likely to be injured than men.^52,54  Aerobic fitness appears to account for a large portion of this difference,⁵⁵  and the aerobic-fitness level of women entering the US Army has improved over time.⁵⁶  The increase in the amount of airborne training and improvements in aerobic fitness may account for at least portions of the temporal decline in airborne injuries among women.

Height did not appear to be strongly associated with injury risk, but only Pirson and Pirlot²⁹  examined this variable. Trends toward a higher injury risk with increasing body weight during ground landings were suggested in the only 2 studies^29,40  examining this association. Greater body weight would result in faster descent velocities and greater forces on ground impact, which would be expected to increase the injury risk. The association between injuries and body weight in water landings is not clear.

For age, contradictory data were reported in 2 studies that used similar data from emergency department records, with 1 investigation³⁴  showing a higher risk in younger paratroopers and the other³⁷  showing a higher risk in older paratroopers. More research is needed on personal risk factors.

Use of the PAB clearly reduced the risk of ankle injuries during airborne operations. In an older systematic review focusing on the PAB, Knapik et al⁵⁷  also concluded that the PAB reduced ankle injuries, especially ankle fractures and ankle sprains. Our study updates this earlier review⁵⁷  by adding 1 investigation⁴³  unavailable at the time of the earlier publication. Studies involving both basic airborne students^24,35,38,43  and trained paratroopers³⁶  showed a reduced risk among individuals wearing the PAB. The RRs were similar among studies, as reflected by the low heterogeneity (Q and I²  statistics), and little evidence of publication bias existed among studies. These data are consistent with research on athletes showing a reduction in ankle injury risk when an ankle brace was worn prophylactically.^58–60  The PAB likely reduces injury risk by preventing excessive ankle inversion on ground impact. Some anecdotal observations had suggested that the PAB increased the risk of lower body injuries exclusive of the ankle, but Knapik et al²⁴  reported that this was not the case.

Other injury risk factors included smaller parachute canopies, certain types of aircraft exits, entanglements, military rank, and the number of paratroopers exiting the aircraft. Larger parachute canopies reduced descent velocity, which reduced ground impact forces.^5,20,28  The design of the T-11 parachute system resulted in fewer oscillations than the T-10 system, which would also reduce horizontal velocity and further reduce ground impact forces.^5,20 

The risk of injury was lower during nonsimultaneous aircraft egress, whether jumpers exited the aircraft by 1 door or off a tailgate or had 1-second staggered exits from opposite sides of the aircraft through right and left doors (eg, C-130 or C-17).^5,27,31  The reduced injury risk was at least partly due to the reduced risk of entanglements.²⁷  Entanglements occur when the equipment of 2 or more jumpers becomes intertwined during descent. During exits from aircraft with 2 doors, high-altitude entanglements can occur with simultaneous exits from both sides of the aircraft.²⁷  Whereas a 1-second delay is now required for jumps out of 2-door aircraft,²⁷  this timing is difficult to maintain if a jumper on 1 side rushes the door or hesitates at it. In addition to high-altitude entanglements, midaltitude entanglements (ie, after full canopy deployment) are possible if 1 jumper drifts into another or if 1 jumper's parachute is directly on top of another. In the latter case, the higher jumper can land on top of the lower parachute. During training, parachutists are instructed to pull on their parachute risers to direct their parachutes away from other jumpers. However, midaltitude entanglement may occur rapidly, and the jumper may not be aware until the situation occurs. Entanglements can substantially increase injury risk. Depending on the nature of the entanglement and the posture of the paratroopers when the entanglement occurs, the situation can result in less controlled landings on ground contact.

New airborne students appeared to have a lower injury risk than paratroopers in refresher courses or on military exercises. The lower injury risk may result from new students in basic courses better adhering to established procedures they have just learned and having training personnel thoroughly check and monitor them. The lower injury risk for officers than enlisted service members may be related to officers generally exiting the aircraft first, exiting with less equipment on equipment-loaded jumps, and not rushing to exit the aircraft before it passes beyond the drop zone. Therefore, they are more likely to make a correct and stronger exit and have more air space, a better view of the landing zone to prepare for landing, and lower ground impact velocity. In addition, officers tend to have higher educational levels than enlisted soldiers,⁶¹  and individuals with higher educational levels may have a lower injury risk. A graded relationship appears to exist between injury-related morbidity and mortality and educational achievement or various measures of intelligence in both military^62,63  and civilian^64,65  studies. Greater educational achievement may be associated with behaviors conducive to injury prevention⁶⁶  or the ability to more effectively process information relating to risk reduction.

Injury risk appeared to be elevated if more paratroopers were exiting the aircraft. This may be associated with the fatigue induced by longer waiting times in preparation for jumps and while standing and waiting to exit the aircraft. In addition, on jumps with more paratroopers, more crowding occurs on the aircraft, leading to discomfort and difficulty maintaining staggered exits, which could result in high-altitude entanglements.³⁰ 

Our study had limitations. First, data were collected over a long period (68 years). Technological improvements, including developments in parachutes, aircraft, and protective devices, have occurred during this time. Second, all studies were observational cohort investigations. This type of research is invariably affected by confounders, including the technological developments that may vary among studies. Third, we limited our analysis to military operations but found that very few investigators^67,68  had quantitatively examined risk factors for parachuting outside of military operations. The analyses offer insights into risk factors that may be applicable to sport parachuting and safety (eg, smoke jumpers) and rescue operations.

We identified risk factors associated with military static-line airborne operations. Trainers, operators, and medical personnel should recognize and appreciate these factors and include them in their injury risk evaluations. Understanding and considering these factors during specific airborne maneuvers will increase the probability that the largest number of paratroopers will arrive safely at the battleground ready for their operational missions.