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Original Research

Effects of Physical Training and Fitness on Running Injuries in Physically Active Young Men

Grier, Tyson L.; Canham-Chervak, Michelle; Anderson, Morgan K.; Bushman, Timothy T.; Jones, Bruce H.

Author Information
Journal of Strength and Conditioning Research: January 2017 - Volume 31 - Issue 1 - p 207-216
doi: 10.1519/JSC.0000000000001487
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Abstract

Introduction

As with recreational and competitive runners, soldiers run to develop and maintain high levels of aerobic fitness. Higher levels of aerobic fitness through running or other aerobic activities have been attributed to several positive health benefits (10,23,40). In a recent systematic review, running was shown to improve aerobic fitness, cardiovascular function at rest, running performance, metabolic fitness, cardiac adaptation, muscular performance, and postural balance (40). Regardless of the reason for running, these benefits come at a cost with approximately 35%–65% of recreational and competitive runners experiencing a running-related injury over a 1-year period (6,29,31,33,35). In the military, 45% of all exercise- and sport-related injuries and 23% of all injuries within a 1-year period were attributed to running and jogging (14).

Since physical fitness is a requirement of the U.S. Army, soldiers are required to perform mandatory unit physical training (PT) which is often performed 5 to 7 times per week and can vary from extreme conditioning programs to Army Physical Readiness Training (2,12). During unit PT, soldiers may run similar or varying number of miles per week, dependent on their specific unit or military occupational specialty. An investigation of Infantry soldiers found that a few over half of soldiers ran less than 10 miles per week and a few less than half ran 10 or more miles per week during unit PT (12). Running mileage is in addition to road marching, calisthenics, resistance training, agility training, sprint or interval training, and PT performed during personal time. Soldiers also spend time performing other types of training such as specialized skills training (eg, airborne) and offensive and defensive tactical training which can be very physical.

Both civilian and military studies have investigated injuries attributed to running. Civilian studies have generally investigated personal risk factors (sex, age, body mass index [BMI]), training-related risk factors (running experience, distance ran per week, ground surface), and health lifestyle risk factors (previous injury, smoking, orthotics) for running-related injuries (21,39,49,50,52,54). However, these studies have not investigated fitness as a potential risk factor for running-related injuries. Conversely, military studies have generally investigated training, physical fitness, and health behavior risk factors for musculoskeletal injuries but not for specifically running-related injuries (12,13,24,27). The purpose of this investigation was to examine the effect of PT and fitness on injury risks for running-related injuries while controlling for personal characteristics.

Methods

Experimental Approach to the Problem

This was a cross-sectional survey of U.S. Army soldiers. Participants were asked to complete a questionnaire about personal characteristics, PT, physical fitness, and their most recent injuries. Injury risk factors were then explored for soldiers who had indicated a running-related injury on the questionnaire.

Subjects

Participants were male soldiers from 2 U.S. Army infantry brigades. The brigades consisted of different types of battalions: infantry, cavalry, field artillery, brigade support, and a brigade special troop battalion. Personnel rosters were requested and obtained for each brigade to verify soldier information and specific demographic characteristics as reported on the survey. Soldiers present for duty were given a questionnaire. This investigation was approved by the U.S. Army Public Health Command Review Board. The study conforms to the Code of Ethics of the World Medical Association (approved by the ethics advisory board of Swansea University) and required soldiers to provide informed consent before participation. Informed consent documents were signed by all soldiers. The age range was 18 to 53 years old.

Procedures

Participants were asked to complete a questionnaire about personal characteristics, health behaviors, PT, previous Army Physical Fitness Test (APFT) scores, their most recent injury occurring within the last 12 months, the activity they were performing when an injury occurred, and number of limited duty days. Questions about PT assessed the frequency of running, calisthenics, resistance (weight) training, agility drills, sprint/interval training, and road marching. Those who ran were also asked about the average distance run during unit and personal training. Injuries were defined on the questionnaire as accidental or on purpose and occurring in 1 of 2 ways: (a) when strong sudden forces are applied to the body (traumatic injuries) or (b) when smaller forces are applied to the body repeatedly, over and over again (overuse injuries). Respondents were also asked the cause of their injury. Only soldiers, who indicated that running caused their injuries on the questionnaire, were considered as having a running-related injury.

The APFT consists of 3 events: a 2-minute maximal effort push-up event, a 2-minute maximal effort sit-up event, and a 2-mile run performed for time. Army Physical Fitness Test variables (number of push-ups and sit-ups and 2-mile run times) were converted into quartiles (Q), with Q1 representing the lowest performance and Q4 representing the highest performance. Predicted V̇o2max was estimated from 2-mile run times using the following equation: predicted V̇o2max = 99.7 − (3.35 × [2-mile run time]) (37).

Statistical Analyses

The Statistical Package for the Social Sciences (SPSS), Version 19.0, was used for statistical analysis. Frequencies and percent distribution were calculated for personal characteristics, health behaviors, PT, physical fitness, injury variables, and limited duty days. Mean values and SDs were calculated for age, BMI, PT, limited duty days, and 2-mile run times. Body mass index was calculated as weight in kilograms divided by height in squared meters (kg·m−2). Injury incidence was calculated as the number of soldiers with one or more injuries (overall injuries), and soldiers with running-related injures were divided by the total number of soldiers surveyed. Running-related injury risk ratios and 95% confidence intervals (95% CI) were calculated for personal characteristics, health behaviors, PT, and physical fitness variables. A chi-square test was also run on these same variables to identify any linear trends. Potential risk factors for running-related injuries (p ≤ 0.05) were explored using a multivariate logistic regression which only includes subjects with complete data for the specific variables chosen. Correlations between the variables selected for the multivariate models were also examined for any covariance. Odds ratios (OR) and 95% CIs were calculated for each potential risk factor (independent variables). An analysis of variance with a post hoc Tukey's test was used to determine whether there was covariance between PT variables, age, BMI, and average 2-mile run times (a measure of aerobic fitness). An estimated sample size calculation was performed assuming a baseline risk of 15% injured and risk ratios/OR between 1.3 and 1.5 based on previous experience. The sample size needed to show significance ranged between 1,380 and 4,718 soldiers (SPSS is a registered trademark of IBM Corporation.).

Results

Soldiers (n = 4,236) were an average age of 26.4 ± 5.8 years with an average BMI of 26.1 ± 3.6 kg·m−2. For fitness performance, soldiers had an average 2-mile run time and estimated V̇o2max of 14.9 ± 1.7 minutes and 49.6 ± 5.6 mL·kg−1·min−1, respectively. Overall injury incidence for the previous 12 months was 42% (n = 1,788), with 14% (n = 583) of injured soldiers (one-third of the injured soldiers) attributing their injuries specifically to running. Strains and sprains accounted for 59% of the running-related injuries. The most common body parts injured were the knee (31%), ankle (20%), lower leg (11%), foot (7%), and lower back (6%). Limited duty profiles were given to 65% of the soldiers (n = 379) injured while running, with an average of 54 ± 68 limited duty days. Limited duty days by body part injured were 60 ± 85 days for the knee, 55 ± 62 days for the lower leg, 53 ± 47 days for the foot, 47 ± 38 days for the lower back, and 40 ± 60 days for the ankle. Forty-two percent of soldiers had limited duty profiles for less than 30 days, 24% between 30 and 60 days and 34% with a limited duty profile greater than 60 days.

Table 1 displays the univariate logistic regression results for risk factors associated with running-related injuries and various measures obtained from the questionnaire. Soldiers who were older, had a BMI equal to or greater than 25 kg·m−2, ran more than 16 miles per week during unit PT, and had low muscular endurance (sit-ups) were at a higher risk of a running-related injury. However, soldiers who ran more miles per week during personal PT, performed a greater frequency of resistance and agility training, and had faster 2-mile run times were at a lower risk of incurring a running-related injury. It is interesting to note that unit and personal PT running mileage injury rates moved in opposite directions. Significant linear trends were found for all of the variables in Table 1, except for cigarette use, calisthenics, sprint training, and road marching. When combining overall mileage run per week for both personal PT and unit PT running, soldiers ran a mean distance of 14.5 ± 9.7 miles per week.

Table 1.
Table 1.:
Unadjusted risk factors associated with running-related injuries in U.S. Army soldiers.

Table 2 displays the results of a multivariate logistic regression for risk factors associated with running-related injuries in U.S. Army soldiers. Analysis revealed that soldiers who were older and ran more than 16 miles per week during unit PT (marginally significant, p = 0.06) were at higher risk of a running-related injury. Higher frequency of resistance training and higher aerobic endurance as measured by 2-mile run times were associated with a lower risk of a running-related injury. Since 2-mile run time (a measure of cardiovascular endurance) and sit-ups were correlated with BMI, a second model was used limited to PT variables while controlling for personal characteristics (age and BMI). The results of the PT model (Table 3) are similar to the first model with a few exceptions. Overweight and obese soldiers have a greater risk of a running-related injury, similar to the findings in the univariate model (Table 1). Soldiers running more than 16 miles per week for unit PT have a significant (p < 0.01) higher risk of a running-related injury, and soldiers running more than 5 miles a week for personal PT exhibit a lower risk of a running-related injury.

Table 2.
Table 2.:
Adjusted risk factors associated with running injuries in U.S. Army soldiers (n = 2,979).
Table 3.
Table 3.:
Adjusted physical training risk factors associated with running injuries in U.S. Army soldiers (n = 3,402).

The relationships between age, BMI, PT, and 2-mile run times were further explored along with V̇o2max estimations (Table 4). For age, soldiers' run times remained similar until reaching the oldest age group (≥31 years) where average 2-mile run time performance slowed significantly and V̇o2max declined. For BMI, average 2-mile run time performance slowed successively and V̇o2max decreased as the BMI categories increased from normal to obese. For unit distance running, soldiers who ran more miles per week during unit PT had similar 2-mile run times compared with those who ran equal to or less than 5 miles per week. However, soldiers who ran greater than 5 miles per week for personal PT had faster 2-mile run times than those running less than or equal to 5 miles per week or performing no personal running. Their V̇o2max was also higher. For unit resistance training, average 2-mile run times progressively improved (got faster) as the frequency of resistance training increased and V̇o2max was higher.

Table 4.
Table 4.:
Physical fitness performance measured by 2-mile average run times (in minutes and fractions of a minute) and estimated V̇o 2max by age, BMI, distance run and resistance training.*
Table 4-A.
Table 4-A.:
Physical fitness performance measured by 2-mile average run times (in minutes and fractions of a minute) and estimated V̇o 2max by age, BMI, distance run and resistance training.*

Discussion

This investigation examined personal characteristics, PT, and physical fitness risk factors for running-related injuries. One of the major findings was that physical fitness had a greater influence on running-related injuries than PT and BMI when run in the same model. Also, the more mileage run during personal PT resulted in greater aerobic fitness (i.e., faster run times). For injury risk, this analysis revealed that soldiers who were older, had higher BMIs, ran longer distances during unit PT, and had lower cardiovascular endurance as measured by the 2-mile run were at a higher risk of a running-related injury. However, soldiers who ran more than 5 miles per week for personal PT and performed resistance training during unit PT 3 or more times per week were protected against running-related injury.

Older soldiers had a higher risk of a running-related injury than younger soldiers. Several previous investigations have reported that age is not associated with risk of injury or there is limited evidence supporting higher or lower age affecting the risk of a running-related injury (19,33,53,49,52). However, most of these are civilian studies investigating recreational, competitive, and marathon runners who have the ability to choose their own intensity, frequency, and duration. This probably enables them to modulate their training according to how they feel. Also older runners who have become injury prone may stop running and perform other types of aerobic activities. Soldiers, however, are generally required to perform PT 5–7 days a week in groups (12). This may place soldiers with lower cardiovascular endurance at a higher risk of injury. Previous studies have shown that lower cardiovascular endurance as measured by run time is associated with a higher risk of injury (12,13). In this study, older soldiers had lower aerobic fitness (slower 2-mile run times) than younger soldiers. Previous studies investigating age and aerobic fitness have shown that aerobic fitness decreases with age (18). More specifically, physiological factors such as heart rate, ejection fraction, and cardiac output have been shown to decrease during exercise in healthy men as they age (45). Therefore, age itself would be expected to be a risk factor for running-related injures because of a physiological decline in aerobic fitness. Older soldiers may also have a greater likelihood of being in staff or supervisory positions, which tend to be more sedentary and could contribute to lower aerobic fitness. Another risk factor for older soldiers is a history of injuries. It is possible that older soldiers may have experienced a larger number of previous injuries which could predispose them to a greater risk of a running-related injury (32,49,52). Therefore, older soldiers may be at a higher risk of a running-related injury because of a combination of lower levels of aerobic fitness, higher likelihood of a sedentary/supervisory job, and the effects of previous injuries.

Univariate analysis showed a significant trend of increasing running-related injury risk with increasingly higher BMI. Several factors may account for the association of higher BMI and higher risk of weight-bearing, running-related injuries. When running or walking, the body's weight is supported by each leg and foot during the gait cycle. As running and walking speed increases, the muscles of the lower extremities contract with a greater force to move the body forward. Previous studies have suggested that excessive weight may place additional biomechanical stress on weight-bearing joints, produce greater muscular force (leading to early fatigue), and altered kinematics (gait disturbance) which may all contribute to an increased risk of musculoskeletal injury (1,7,30,32,43,46). Studies investigating running-related injuries and BMI have found conflicting results (33,48,51,34,47). In 2 reviews of running-related injuries and BMI, it was concluded that BMI has no effect on running-related injuries (49,52). This was probably because most runners (not recreational runners) are not overweight. In studies investigating elite, marathon, or competitive runners, the average BMI tends to be less than 25 kg·m−2 (35,17,50). Therefore, the difference between these elite, marathon, or competitive runners and soldiers who perform running are higher than average BMIs (the average BMI in the current evaluation was 26.1 kg·m−2). As a result of having higher BMIs and being less fit, soldiers with elevated BMIs may be at a greater risk of experiencing a running-related injury. Also, there is likely to be a greater range of BMIs among soldiers and “average” male citizens than among distance runners.

Although BMI was not a significant risk factor in the first multivariate model (Table 2), which contained PT and physical fitness variables, it was a significant risk factor for a running-related injury when physical fitness was not entered into the second multivariate model (Table 3). This could mostly be attributed to the relationship between 2-mile run time performance and BMI. Previous studies and this study have shown that as BMI increases, average 2-mile run time performance decreases (12). Because higher BMI as this article indicates is associated with slower run times and higher risk of injury, it should not be discounted as a risk factor for “average” exercise participants.

One of the most commonly cited training errors leading to injury is excessive mileage run per week (19,33,53). It has been estimated that 60% of running injuries are attributed to training-related errors with half of those errors being excessive running mileage (20). In a study of runners in a 10,000-m race, running-related injury rates increased with distance run per week from approximately 14% when running less than or equal to 10 miles per week, 29% when running 10 to 19 miles per week, 37% when running 20 to 29 miles per week, and 48% when running 30 to 39 miles per week (19). This and several similar studies show that risks of injury per unit of lifetime (days, months, years) go up as weekly running mileage increases (25,29,33,35). Civilian studies have also shown higher running-related injury rates than the current investigation where soldiers ran about 15 miles per week on average (29,33,35). From a different perspective, a few studies have investigated or noted that risk of injury decreases per mile of training (25,35,53). This investigation may shed light on this seeming contradiction.

In the current investigation, soldiers who ran greater than 16 miles per week for unit PT were 65% more likely to experience a running-related injury when compared with soldiers running 1 to 5 miles per week for unit PT. This is in agreement with previous literature where the higher the mileage run, the greater the risk of a running-related injury (29,35). However, soldiers running more than 5 miles per week for personal PT were 30%–46% less likely to experience a running-related injury compared with those running 1 to 5 miles per week for personal PT. Although this seems contradictory to the literature, it may be that the protective effect of greater aerobic fitness in soldiers who run more during personal time may outweigh injury risks associated with higher running mileage. It was shown that aerobic fitness (as measured by the 2-mile run) increased with mileage run in the personal PT running group. Higher aerobic fitness has consistently been shown to have a protective effect against injuries among soldiers (15,24,26–28). This study suggests that in an environment where most individuals are required to do some running as a unit, as well as other weight-bearing activities, such as road marching, it may be that the additional aerobic fitness gained through extra personal running miles is more protective than the additional running miles are injurious.

Soldiers who performed resistance training 3 or more times per week were approximately 50% less likely to experience a running-related injury. Most studies investigating running and resistance training assess the combined or individual effects on physical performance. Some of these studies showed that endurance training concurrent with resistance training increases both short- and long-term endurance performance in sedentary and trained individuals (16,44). These improvements may be a result of increased running economy or improved neuromuscular characteristics to counteract fatigue (4,8). In the current investigation, we also found that the soldiers who performed unit resistance training had higher aerobic fitness (as seen by 2-mile run times) than soldiers not performing any unit resistance training. As mentioned earlier, higher aerobic fitness has been shown to have a protective effect against injuries (15,24,26–28). It could also be that soldiers who take the time to perform regular resistance training develop more well-rounded fitness. Increased running economy and improved neuromuscular characteristics as a result of resistance training, along with higher aerobic fitness, may have contributed to the decrease in running-related injuries for those who perform resistance training 3 or more times per week.

Running-related injury risk for soldiers with the slowest 2-mile run times was higher than for those with the fastest 2-mile run times. These findings are consistent with other studies that have shown slower run times are associated with higher risks of training-related injuries among Army trainees and soldiers (12,15,24). Soldiers with lower aerobic fitness may experience greater physiological stress and/or fatigue during tasks, such as running, because of exercising at a higher percentage of their aerobic capacity in comparison with soldiers exhibiting higher fitness levels. Soldiers of lower aerobic endurance capacities will not only be exercising at a higher percentage of their aerobic capacity during PT to accomplish the same task as the more fit soldier but also perceive the task as more difficult (11). This greater physiological stress and fatigue may lead to a higher risk of injury. Studies of fatigue have demonstrated decrements in proprioceptive ability (41), a decrease in joint stability (38), alterations in muscle activity (41), changes in gait (5,56), balance (22,9), low frequency fatigue (3), neuromuscular function (55), and ligament laxity (42). Aerobic fitness was shown as one of the strongest predictors of running-related injuries in this investigation, with the slowest running group having twice the risk of a running-related injury while having a protective effect for the fastest runners.

From a PT perspective, unit and personal running had different influences on aerobic fitness. Although running mileage performed during unit PT had little effect on aerobic fitness, increased running mileage performed during personal PT improved aerobic fitness. It is interesting that aerobic fitness levels remained similar regardless of the distance run during unit PT. Because unit distance running is mandatory and performed in groups, the pace or intensity set by the group may be at levels too low to have any additional impacts on aerobic fitness regardless of the mileage run. Probably, the most important factor with unit PT is that it is performed in large groups and the group as a whole experiences similar benefits and risks. However, soldiers who chose to perform personal running greater than 5 miles per week accrued a personal benefit in the form of higher aerobic fitness. This suggests that the protective effects of higher aerobic fitness gained through personal running may outweigh the injurious effects of increased running mileage during personal training.

There are some limitations to consider in this investigation. Injuries were self-reported for a previous 12-month period. This may have missed more minor or unmemorable injuries that had not occurred recently. Also, physical characteristics such as height and weight were used to calculate BMI and APFT scores obtained by the survey. However, an investigation comparing self-reported and measured heights and weights as well as APFT scores among soldiers found high correlations between self-reported and actual measurements (36).

Practical Applications

In a population of exercising young adult males, those who ran further for personal training exhibited higher levels of aerobic fitness. It appeared that these higher levels of fitness were more protective than the higher mileage was injurious. Resistance training equal to or more than 3 times a week also appeared to add a protective effect for these physically active young men. These findings suggest that for general fitness, a combination of aerobic and strength training will enhance fitness and reduce running-related injuries. Other strategies to prevent running-related injuries may also include reducing excess body fat and monitoring unit PT running mileage.

Acknowledgments

Citations of commercial organizations and trade names in this report do not constitute an official Department of the Army endorsement or approval of the products or services of these organizations. The views expressed in this article are those of the author(s) and do not necessarily reflect the official policy of the Department of Defense, Department of the Army, U.S. Army Medical Department, or the U.S. The authors declare that there are no conflicts of interest.

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Keywords:

injury risk factors; physical training; exercise; V̇o2max; APFT; military

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