A 15-Year Longitudinal Study of Body Composition in Finnish Military Divers

ABSTRACT

Introduction

Finnish military divers perform a great variety of tasks all year round, all of which require good physical health and fitness. Poor physical fitness can hinder the ability to cope with different situations. A high body fat percentage has been suggested to increase the risk of decompression sickness, whereas low muscle mass has been associated with an increased risk of musculoskeletal disorders and trauma. However, a low-fat composition may render divers vulnerable to cold and hypothermia during a dive in cold Arctic conditions. The aim of this study was to evaluate possible changes in body composition of Finnish military divers during a 15-year follow-up period (2007-2021).

Materials and Methods

We used body composition measures from military divers’ fit-to-dive evaluations from the Finnish Defence Forces’ Diving Medical Centre. Measurements were taken with two bioimpedance devices that function based on the electrical conductivity properties of the human body. The data from both devices were compared with Bland–Altman plots to show that the devices produced congruent data and the results from both devices could be included in the study. Possible changes in body composition were evaluated with a linear mixed model with random intercepts and slopes for each variable.

Results

Results from the two bioimpedance devices showed no significant differences. This allowed us to combine the results in the same data analysis. There were no apparent differences in the age of the divers between the years during the follow-up. The majority of the divers were between 25 and 30 years of age. Age correlated significantly with most measures, the clinically most significant being a higher fat percentage in older divers. However, all measures were within target values defined by the Diving Medical Centre. The divers were able to retain sufficient muscle mass in all age groups.

Discussion and Conclusions

According to this study, Finnish military divers have managed to maintain a surprisingly good muscle mass in all age groups despite body composition changes due to aging in older subjects. A slight increase in fat mass can be observed with increasing age; nevertheless, the values have remained within fit-to-dive target levels. The current training routines of Finnish military divers are adequate for maintaining solid physical fitness and body composition over the course of the diving career.

INTRODUCTION

The Finnish Defence Forces (FDF) and Border Guard divers (later in this article referred together as military divers) perform a great variety of operations all year round, all of which require good physical health and fitness. Poor physical fitness can hinder the ability to cope with different situations, and poor physical fitness and high body fat percentage has been suggested to increase the risk of decompression sickness (DCS).^1,2 Moreover, poor physical fitness, especially together with low muscle mass, is associated with an increased risk of musculoskeletal disorders and trauma.^3–5 Although a low fat mass may indicate a high physical and cardiovascular condition, it may render divers vulnerable to cold and hypothermia during a dive in cold Nordic conditions. The ideal fat percentage for a military diver in Arctic conditions remains debatable.

Body composition has shown to be significantly linked to cardiorespiratory fitness.⁶ A higher fat mass and fat percentage are associated with slower performance times in military firefighting simulation tests.⁷ Aerobic and anaerobic capacities as well as body composition are the strongest predictors of performance determinants in these physically challenging tests.⁷ Military tasks are typically physically moderate to high in intensity.^8,9 Some tasks even include periods of very high physical intensity (60-100% of the maximal aerobic level).¹⁰

In Finnish conscripts, the median body mass index (BMI) has increased since 1993 and on the contrary, the endurance fitness, estimated from Cooper test results, has decreased since the 1970s.¹¹ Although the body composition of the average Finnish conscript has changed significantly in the past decades in accordance with all western societies, Finnish military divers are healthy and highly motivated to maintain good physical fitness. It is likely that a similar evolution would not have occurred in military divers, but there is no research to confirm this.

Military divers are trained at the Finnish Navy Coastal Brigade at a yearly Navy Diver’s Course. The course is attended by both conscripts and employed personnel, and it is divided into two rotating specialization lines—in even years combat divers and in uneven years explosive ordnance disposal divers are trained. The selection criteria for the course are very strict, and only a few of the applicants are selected for the course. Challenging physical fitness tests exclude most of the applicants.¹² Many trainees discontinue the course due to health issues or excessive physical burden. For these reasons, military divers are a highly selected group of overall physically fit individuals.

Military divers are examined every second year at the Diving Medical Centre (DMC) located at the Coastal Brigade (Kirkkonummi, Finland). These examinations include a common assessment of work ability and a fit-to-dive (FTD) evaluation. The interval can be shorter than 2 years depending on the diver’s age, medical conditions, and maximal oxygen uptake (V0₂max). A body composition analysis with an InBody body composition analyzing device is included in the evaluation.

The divers strive for the highest V0₂max scores possible. In Finland, military divers are required to have a V0₂max >45 mL min^-1 kg^-1 measured in a clinical stress test on a treadmill. Some specific operators are required to have 50 or 55 mL min^-1 kg^-1. However, values >50 mL min^-1 kg^-1 are recommended for all military divers. Moreover, divers should not be obese. Their body fat percentage should preferably be <23% and their BMI should be <30. The absolute body fat percentage upper limit is 28%. There is no lower limit for body fat percentage, although experience in practice has shown that individuals with a low-fat composition tend to suffer from cold during long dives especially in winter.

The physical demands and protocols of FTD evaluations for Finnish military divers are comparable with those of many North Atlantic Treaty (NATO) countries.^13–16 On the other hand, published data on military diver’s health, physical fitness, and body composition is limited. Research on recreational and sport divers show that while diving has become more popular, increasing numbers of individuals with obesity and multiple health issues engage in the sport.^17,18 However, the civil diving community is not fully comparable with military divers, but trends in the general community may reflect trends in military communities as well.

FTD evaluations are performed with great precision and with standardized methods. Therefore, the data can be utilized in scientific studies. For being able to proactively recommend changes in training, working routines and the frequency of FTD evaluations, which all aim to ensure the military diver’s long-term performance ability, it is considered important to identify possible changes in health variables.

The aim of our study was to identify possible changes in the body composition of military divers during a 15-year follow-up period (from 2007 to 2021), and to discuss reasons for possible changes. Findings from this study could then inform potential changes to the program or recommendations for revised standards.

METHODS

Subjects

All the data in this study were from Finnish military divers (FDF and Border Guard divers), who were examined as a part of their occupational FTD examination at the DMC (Kirkkonummi, Finland). Most divers were examined every second year, but a minority were evaluated more frequently due to age, medical conditions, alarming increase in body fat mass or low V0₂max results. A research permit was granted by the FDF Defence Command (permit identification number: AS7019). The FDF does not report the exact number of active military divers—therefore one condition for granting research permission was not to report the number in this article. Written consent, for the scientific use of their employees’ data, was obtained from the Finnish Border Guard Command. Ethical approval was granted by the Ethical Committee of Tampere University Hospital (R21093/6/2021). The study adhered to the Declaration of Helsinki.

Procedure

The data were retrospectively retrieved from the occupational medical records of the DMC. All data were transferred to an Excel-file for further processing.

Body composition values were measured during FTD evaluations with the InBody body composition analysis device. This is a bioimpedance device that functions based on the electrical conductivity properties of the human body.¹⁹ Two different InBody devices were utilized during the analysis period, up to 6/2017 with the InBody 720 (data exported from the device with Lookin´Body 3.0 and saved as Excel files) and from 1/2018 with the InBody 770 (data exported from the device with Lookin´Body 120 and saved as Excel files) (Biospace Ltd., Seoul, South Korea). The measured body composition values are reported in Table I.

The data were combined into one Excel file with the personal identification of divers, information on the time of examination and the body composition values (A—W) for each time point. The measures that were available only from Lookin´Body 120, but not from Lookin´Body 3.0, were excluded from this study. Finally, the personal identification of the divers was replaced with randomized diver numbers.

Comparability of Data from Two Different Measuring Devices

In 2018, the DMC’s InBody body composition analysis device was updated to a newer version by the manufacturer. Both devices have the same working principle, but because of the sensitivity of the measuring method, an additional validation project was carried out to evaluate the comparability of the two devices. In 2020 FTD evaluations (n = 20) of body composition were analyzed using both the old and the new device. These data were used to ensure the comparability of the data provided by the two devices and to allow for the pooling of the data into the same evaluation series.

Statistics

First, to evaluate possible differences between the two InBody devices, we validated similar findings across the two machines by comparing the standard deviations between the replicates and the machines. We further investigated the differences between the machines using Bland–Altmann plots, where if the plots do not show any significant differences, the devices would be considered comparable.

We present the observations per age and study year using bar plots and show the distribution of age in different years using box plots. We further investigated the effect of age and year on the measurements by fitting linear mixed effects models with random intercepts and slopes for each variable. Both the age and year of the test were modeled using restricted cubic splines to allow non-linear effects, age with four knots and the year of the test with three knots. We tested the overall effect of age/year using F-tests and show the effects of age/year against each variable using plots.

P-values below .05 were considered to be significant. All analyses were done with R version 4.2.2 using packages nlme^20,21 and ggplot2.²²

RESULTS

Comparability of Data from Two Different Measuring Devices

According to Bland–Altmann plots, no significant differences between the two InBody devices were found. The difference between the replicates was roughly the same as the difference between the devices when compared using standard deviations. Because of these findings, we concluded that the data were comparable and no adjustments needed to be made before conducting further analyses.

The year of the test did not have a significant effect on age (P = .39) when we modeled it using linear regression using restricted cubic splines with three knots. In other words, the mean age of divers remained almost the same every year (Supplementary Data 1). The annual range of mean age was between 32 and 36 years. The percentages of divers in different age groups are presented in Figure 1. The majority of divers were between 25 and 30 years of age. The percentage of observations in this study in different years is presented in the Supplementary Data(Supplementary Data 2). Figure 2 presents the most relevant variables when they are modeled with age of the divers. Figure 3 presents the same variables when modeled with study years. Table I shows the significance level of variables compared to age and year.

DISCUSSION

The novelty of this research is the finding that this highly select group of military divers maintained their physical fitness and avoided degeneration of their body composition and increases in their BMI that is the opposite for other groups of soldiers and the general population.

This study provides valuable insight into the development of body composition in a select group of professional military personnel. The data were gathered with a standardized method for a 15-year period. The authors have not found any similar studies in the literature.

The divers were able to retain sufficient muscle mass in all age groups. Most parameters showed age-dependent body composition changes, which can be explained with physiological aging. Noteworthy was a significant increase in BMI and fat mass with increasing age. Nevertheless, the values stayed within the target levels defined for Finnish military divers and would unlikely have been of great relevance to their ability to execute military diving operations. More relevant to the ability to execute operations is adequate muscle mass, which was well retained in our military divers.

During the study period, the age distribution underwent no significant changes due to the year of the observations, with the majority of divers being between 25 and 35 years of age. However, the number of observations per year did show a decreasing trend, which can be explained by a temporarily prolonged examination interval from 2 to 3 years in 2018-2019 for divers with V02max scores over 58 mL min^-1 kg^-1.

The results indicate that our current diver selection protocol and training routines are sufficient for retaining adequate body composition in Finnish military divers over the course of their career. Based on this research, there is no need to recommend changes in the training routines.

Our study has some limitations. First, the subjects in this study belong to a very specific group of highly specialized professionals. Therefore, the results of this study cannot automatically be generalized to other divers or military professionals. These results are specific to Finnish military divers.

Second, in this study, the reported body composition data were measured with two bioimpedance devices that function based on the electrical conductive properties of the human body. Even better, for evaluating body composition, would have been to use a gold standard method such as a dual-energy X-ray absorptiometry or quantitative magnetic resonance imaging.²³ On the other hand, dual-energy X-ray absorptiometry or quantitative magnetic resonance imaging were not used in the DMC’s FTD evaluations, and the use of these methods could be realistic only in pre-planned studies with great resources.

CONCLUSIONS

Finnish military divers consistently maintained solid muscle mass as age increased despite slight increases in BMI. A slight increase in fat mass can be observed with increasing age; nevertheless, the values remained within target levels.

There was no clinically significant change in the body composition of divers during the 15-year follow-up period. Although research on Finnish military conscripts has shown a decrease in physical fitness and an increase in BMI, the same trends were not observed in these military divers.

The current training routines of Finnish military divers are adequate for maintaining good physical fitness and sufficient body composition over the course of the diving career.

ACKNOWLEDGMENTS

The statistical analysis by biostatistician Mitja Lääperi, M.Sc., is greatly appreciated.

SUPPLEMENTARY MATERIAL

SUPPLEMENTARY MATERIAL is available at Military Medicine online.

FUNDING

The study was funded by the Finnish Defence Forces Joint Healthcare.

CLINICAL TRIAL REGISTRATION

The study has not been registered in a data base. The study was approved by the Finnish Defence FDF Defence Command (permit identification number: AS7019).

INSTITUTIONAL REVIEW BOARD (HUMAN SUBJECTS)

Ethical approval was granted by the Ethical Committee of Tampere University Hospital (R21093/6/2021).

INSTITUTIONAL ANIMAL CARE AND USE COMMITTEE (ANIMAL STUDIES)

Not applicable.

INDIVIDUAL AUTHOR CONTRIBUTION STATEMENT

R.V.L., R.S., K.I.P. and T.K.W. designed the study, reviewed, and edited the manuscript. R.S. collected the data. R.V.L., R.S. and T.K.W. analyzed the data with the help of a biostatistician. R.V.L. drafted the original manuscript. All authors read and approved the final manuscript.

CONFLICT OF INTEREST STATEMENT

None declared.

DATA AVAILABILITY

The data are owned by the FDF and can be considered to be delivered for scientific purposes on request from the corresponding author. The FDF does not report the exact number of active military divers—therefore, one condition for granting the research permission was not to report the number in this article.

INSTITUTIONAL CLEARANCE

Or does not apply.

REFERENCES

Author notes