In the present study, we examined risk factors for MSDs among male conscripts during a six-month military service. The findings indicated that a low level of physical fitness expressed by 12-minute running (Cooper's test) was clearly associated with MSD with a dose-response relationship, confirming the association of low levels of aerobic fitness and subsequent risk of injury [6
]. Furthermore, we present new findings that poor results in standing long jump, push-up or back lift tests combined with poor result in Cooper's test are strong predictors for MSDs. In addition, higher WC and BMI, earlier musculoskeletal symptoms, poor school success and company were all clearly associated with MSDs elucidating previously equivocal findings. It was also observed that some military tasks specific to the company involve higher risks for MSDI than other tasks. Good entry-level physical fitness, normal BMI and normal WC were protective factors against MSDI in all companies suggesting that these intrinsic and modifiable risk factors are amenable for prevention programmes.
The main finding of the present study was the association between low physical fitness and MSDs. A number of studies have documented the association of low levels of aerobic fitness and subsequent risk of injury [6
], although a conflicting result was reported in a Finnish study of injury hospitalisations [9
]. Poor muscular strength and endurance are also reported to be risk factors for injuries during military training, although not as frequently [7
]. A civilian study among intercollegiate basketball and track athletes clarified these findings by demonstrating that core stability has an important role in the prevention of lower extremity injuries [38
]. The findings of the present study, that poor back lift or push-up test result combined with poor aerobic endurance (Cooper's test) are strong predictors for MSDs, support the importance of core strength and stability to protect against MSDs. Moreover, improved control of the lumbar neutral zone with trunk muscles decreases low back pain among middle-aged men [39
], a common MSD in the present study.
The US Army Physical Fitness Test includes a two-mile (3.2 km) run and push-up and sit-up tests. Hence, the finding that MSDs were associated with poor results in standing long jump and back-lift tests is new. In the present study, a combination of Cooper's test and lower extremity muscle fitness (standing long jump test) proved to be a strong predictor for MSDs with a dose-response relationship. The standing long jump requires efficient motor control of the whole body in addition to measuring power production of the lower limb extensor muscles. Moreover, the standing long jump test is a good marker of lower limb dynamic muscle strength [40
]. The present finding suggests that in addition to good aerobic endurance, motor control and strength of the lower extremities are important factors of physical fitness in the prevention of MSDs during military training. However, criticisms have been raised with regard to army physical fitness tests because they tend to penalise larger, not just fatter, individuals because body weight acts as a load. Larger individuals receive lower scores than their lighter counterparts, although larger persons perform work-related fitness tasks, such as carrying loads, better in a military environment [41
Individuals with lower aerobic capacity probably experience greater physiological stress than individuals with better aerobic fitness during long-term military basic training (marching, running, combat training), which may also predispose to MSDs [1
]. Various hypothetical mechanisms have been presented to explain this association. Conscripts with lower aerobic fitness levels may perceive military training as more difficult and fatigue more rapidly [42
]. It has also been proposed that fatigue leads to changes in gait and kinematics in lower extremities [43
] which may result in musculoskeletal stress in specific body areas and predispose to injuries [45
Low levels of physical activity are associated with injuries in several military studies [3
]. In the present study, low physical activity level during the three months prior to entering military service was associated with the risk of MSDI with a dose-response relationship, but only in the univariate models. This may be due to the fact that results in the final model were adjusted by other physical activity-related variables. Physical activity level before entry into the military service in particular, is associated with overuse injuries [10
] suggesting that untrained conscripts overload their musculoskeletal structures and tissues more often than their active counterparts during military training.
Among young civilians, high exposure to competitive sports participation is associated with a higher risk of injuries [47
], consistent with the findings of the present study. In previous military studies, however, participation in competitive sports was not associated with MSDs [6
]. High running mileage is an evident risk factor for injuries based on several military [1
] and civilian studies [49
], indicating that as the total amount of exercise increases, the injuries decrease first, until a point is reached at which injuries increase disproportionately with changes in physical fitness [49
In the present study, abdominal obesity and high BMI were associated with a higher risk for MSDI and long-term MSDI compared to smaller WC and normal BMI. In earlier studies, higher BMI was linked to an increased risk of injury during military service [6
], although contradictory results indicating no association between BMI and injuries [24
], and an association of lower BMI with injuries [21
] are also reported. Mattila and colleagues [40
] demonstrated that a high proportion of body fat measured by dual-energy x-ray absorptiometry (DEXA) is clearly associated with poor running performance and muscle strength among conscripts and proposed a stricter entry level BMI for Finnish conscripts. Morbidly obese persons might be temporarily discharged from the army in Finland, mainly on the basis of their subjective perception of being able to cope with military service [40
]. Severely obese persons do not meet military entrance standards [7
] in professional armies, which may partly explain the equivocal results from different studies.
Among the lifestyle characteristics, smoking, alcohol intake and frequency of drunkenness were clearly associated with MSDs in univariate models, but after further adjustments the associations weakened. The present finding that high frequency of drunkenness prior to the beginning of military service is a risk factor for MSDs has, to our knowledge, not been reported before. Risk taking behaviour and cognitive deficits are more common among smokers, which may partly explain the altered risk for MSDs in adjusted models [1
]. Moreover, smoking and alcohol intake are strongly associated with each other among young men [53
] which is consistent with the present data. This interaction attenuated the association between MSDs and predictive variables when both variables were placed in the same model. Altarac and colleagues [19
] reported that cigarette smoking is associated with exercise-related injuries sustained during basic military training. After controlling for other factors, the adjusted odds ratio for smokers experiencing an exercise-related injury during basic military training was approximately 1.5-fold compared to non-smokers. Similar findings have also been reported in other military studies [3
]. Although among young smokers, the aerobic capacity is similar to non-smokers [7
], smoking may be associated with MSDs in many other ways. Smoking causes a deficit in bone density [55
]. This effect may be detected even in young healthy persons [56
]. Several studies have concluded that smoking hampers wound and fracture healing and impairs fibroblast function [57
]. Overuse injuries are known to result from repetitive microtrauma leading to inflammation and local tissue damage [59
]. There is no clear evidence, however, of the association between smoking and bone fractures among military recruits, because the underlying mechanisms are thought to depend on long-term exposure [19
]. Overall, alcohol and smoking are probably indicators for risk-taking behaviour rather than causal risk factors for MSDs among the young during military training.
The finding of the present study that lower school success, a combination of educational level and grades in school, was associated with MSDs is concordant with some previous studies [12
]. These studies reported lower educational level as a risk factor for foot injuries [12
] and military discharge [60
], but in general the association of poor school success and MSDs has not been investigated in the army setting. Lower grade of mental ability, however, is reported to be associated with acute musculoskeletal injuries [61
] and severe low back pain [62
] among young men.
It is well established that previous injury history is associated with a higher risk of injury during basic military training [3
]. In the present study, chronic impairment or disability due to earlier musculoskeletal injury and prior sports injury during the month before military entry were also associated with a higher risk for MSD. On the other hand, a past training injury may be a marker of past physical activity [20
]. Musculoskeletal symptoms during the three months before military entry were strongly associated with MSDs in the present study. This predictive association is not generally investigated in the army environment, but musculoskeletal complaints are associated with a higher risk for premature discharge from military service [28
The results of civilian [63
] and military [3
] studies suggest that modification of running distance, frequency and duration may be effective toward preventing lower extremity injuries. A recent study by Finestone and Milgrom [17
] reported a promising 60% decrease in stress fractures by reducing cumulative marching and by assuring a minimum sleep regimen in the Israeli army. Similar findings were reported in a previous study of soldiers in the US Army [4
]. Both studies reported that these changes in military training did not lower the soldiers' combat readiness or physical fitness test results. The key element in military weight-bearing training to avoid overuse related MSDs is to gradually increase the distance, frequency and duration of training [3
]. A study of the Singaporean army, however, demonstrated that a formal pre-training conditioning programme may be more effective toward reducing attrition than training with a gradual increase in pace, which extended the basic military training by one month [64
]. Similar findings from the US Army showed that pre-conditioning of low-fit recruits resulted in lower attrition and a tendency towards lower injury risk [65
]. In the Finnish Defence Forces, as well as in other mandatory armies in Nordic countries, the proportion of conscripts with low physical fitness and obesity has increased dramatically over recent decades. This phenomenon may cause serious health problems in the future. In addition, the phenomenon forces military training programmes to adapt to these changes in mandatory armies [32
A recently published randomised controlled trial from the Danish conscription army revealed that an exercise programme enhancing muscular strength, coordination, and flexibility based on intrinsic risk factors identified in previous studies was not effective in reducing the incidence of lower extremity overuse injuries [66
]. This study was the first randomised, placebo-controlled study investigating the preventive effect of concurrent exercise programmes on overuse injuries in the military environment. The intervention was speculated to be more effective in situations with a more gradual increase in load [66
The present study has several strengths. First, the definition of MSD is clear. Moreover, the data regarding MSDs was collected using electronic patient files, which guaranteed a high coverage of MSDs because all patients who entered the garrison clinic were recorded in the computerised system. Second, the participation rate was high (98%). Furthermore, the design of the study was a prospective follow-up of two successive cohorts of conscripts with the aim of providing information on the risk factors of MSDs in an army environment during one entire year. The study limitations arise from the fact that, after the initial eight weeks of basic training, training programmes diverged depending on the company. Although the physical training was maintained at approximately the same level in different companies, the military training tasks were different. The presented associations between risk factors and MSDs were, however, adjusted by the company. In addition, because the threshold for seeking medical care may vary between individuals, some conscripts may have been more inclined to seek professional care than others.
The present study provides a wide spectrum of modifiable risk factors for MSDs. Although association does not indicate causality, increased knowledge of the risk factors and injury mechanisms is an essential component when planning intervention programmes. An appropriate intervention based on the results of the present study would be to increase both aerobic and muscular fitness prior to conscript training. Attention to appropriate waist circumference and BMI would strengthen the intervention programme. Well-planned randomised controlled studies are needed to provide more evidence from effective interventions before large-scale prevention programmes are initiated in a military environment.