Changes in Cardiovascular Performance during an 8-Week Military Basic Training Period Combined with Added Endurance or Strength Training

INTRODUCTION

It is well recognized that obesity in the youth and also among military personnel has increased, while aerobic capacity has decreased during the last decades in the western countries.1-6 Regular physical activity is essential for improving physical fitness and preventing obesity. This phenomenon was presented as a big challenge for military training and in the recruitment of capable soldiers. Modern military operations are still physically demanding for both soldiers and their leaders despite technological developments. Besides military skills, a successful operation requires particularly good physical and mental readiness.

Dyrstad et al.7 observed that poor improvement in soldiers' endurance performance and muscular strength is due to an inadequate amount of high-intensity endurance or strength training during military service. Studies have shown that endurance, resistance, or concurrent endurance and resistance training has a positive influence on physical performance and body composition.8,9 However, improvements are specifically related to the type of training performed.8 Although improvements in both strength and endurance performance have been observed through concurrent training,8,9 the magnitude of improvements is smaller than either resistance or endurance training alone.9 Therefore, additional high-intensity strength or endurance training may positively influence these characteristics in military personnel above that achieved by current military training.

Some reports have demonstrated a significant increase in the soldiers' aerobic capacity, especially, during the first 6 to 12 weeks of the basic training (BT) season.7,8,10,11 On the contrary, some studies have found no changes or even a reduction in V02max, especially after the BT season.1216 A high initial aerobic fitness level and an inadequate amount of high-intensity aerobic training have been the main reasons for the lack of improvement in V02max.7 Dyrstad et al.7 have also reported that soldiers with initial V02max of >54.9 mL * kg^sup -1^ * min^sup -1^ did not achieve additional benefit for aerobic fitness during the BT. When endurance and strength training are combined, it is important to take into consideration that combined training, even with a low frequency, can lead to some interference in explosive strength development due to the limitations of rapid voluntary neural activation of the trained muscles.17

In general, the main purpose of BT (6-12 weeks) is to prepare soldiers mentally and physically for the military environment. In addition, the aim is to increase soldiers' aerobic capacity and muscle strength to prepare them for the demanding military tasks. However, it is not well known how to incorporate additional activities (endurance and strengdi training) to regular military training to achieve greater gains and minimize the risk of injuries. Therefore, the purpose of the present study was to examine changes in cardiovascular performance during an 8-week BT combined with emphasized endurance or strength training. Thus, we examined whether it was possible to achieve extra benefits for the conscript's aerobic capacity by increasing the proportions of the endurance training. Also, we examined the effects of these training modes on maximal strengdi development of leg extensors to study possible interference of BT and endurance training.

METHODS

Subjects

A total sample size was 72 male conscripts who were divided into an emphasized endurance training (ET; n = 24), strength training (ST; n = 24), and standardized basic training (normal training (NT); n = 24) groups. There were nine dropouts (ET, four persons; ST, three persons; and NT, two persons) due to cessation of military service, illnesses lasting >14 days, missing training diaries information, and a change of garrison. The subjects were randomly selected for the study at the beginning of their military service and their detailed initial characteristics are given in Table I.

The subjects voluntarily participated in the study after passing a medical examination. They were carefully informed about the design of the study with special information on possible risks and discomfort that might occur. Subsequently, they signed informed consent before the experiment. This study was conducted according to the declaration of Helsinki 1975 and was approved by the Ethical Committee of the Central Finland Health Care District and the University of Jyväskylä (Jyväskylä, Finland).

Experimental Design

The present BT season lasted 8 weeks including a total of 300 hours of military training. The BT program for all groups consisted of military-related physical training, sport-related physical training, and other military training (Table II). During marching and combat training, the subjects carried combat gear weighing 15 to 25 kg. BT also contained field exercises lasting from 4 hours up to 3 days for a total of 1 to 2 weeks. The military training of the BT standard program was mainly performed within aerobic limits. The overall amount of the sport-related physical training (SRPT) was planned to be 50 hours during BT.

The SRPT programs of BT of each group differed with regard to specific training sessions. The SRPT program of ET consisted of a higher proportion of ET, which included Nordic walking, walking, running, cycling, and some other endurance exercises. The ET group had three endurance sessions a week with a duration of 60 to 90 minutes. The training intensity was mainly aerobic but every third exercise session included intensity that exceeded the anaerobic threshold.

The SRPT program of ST contained a special ST program. The ST group had three strength training sessions a week with a duration of 60 to 90 minutes. A whole-body ST program consisted of gym training or circuit training and each training session always included two exercises for the leg extensor muscles. During the first 3 weeks, the subjects trained with loads of 30 to 50% or 60 to 70% of 1 RM for two to three sets and 10 to 15 or 20 to 40 repetitions (muscle endurance), during weeks 4 and 5 with loads of 60 to 80% of 1 RM for two to four sets and 6 to 10 repetitions (hypertrophy), and finally during weeks 6 to 8 with loads of 80 to 100% for five to seven sets and one to six repetitions (maximal strength).

The NT group served as a control group. The SRPT program was performed in accordance with the standard BT program, which consisted of endurance-type activities such as ball games, fitness training, and orienteering.

Measurements

The subjects were tested on two occasions using identical protocols. The initial tests were performed during the first week of BT and the second during the ninth week of service.

Body Composition

Body mass (BM) was measured using a bioimpedance device (InBody 720, Seoul, South Korea) and body height by a commercial scale. Waist circumference (WC) was measured by a tape measure.18 Body fat (BF) was estimated by measuring skin fold thickness at seven different sites.19

Aerobic Performance

Maximal oxygen uptake (V02max) was measured using a bicycle ergometer (Ergotine GmbH, Ergotine, Germany). The initial work load of the test was 50 W, and it was increased by 25 W every second minute until exhaustion (Fitware Oy, Mikkeli, Finland). Oxygen uptake (VO2) was measured continuously using a gas analyzer (SensorMedics, Yorba Linda, California). The blood samples were taken from a fingertip every second minute to measure blood lactate concentrations using the Biosen C line Sport (EKF Diagnostics, Barleben/ Magdeburg, Germany). Heart rate was recorded continuously using a heart rate monitor (Polar Electro, Kempele, Finland).

Volitional exhaustion was the main criterion indicating that VO2max was achieved, and the highest mean VO2 over 1 minute was set as V02max. The subject's exhaustion was ensured by the following two basic criteria (1) a respiratory exchange ratio above 1.05 and (2) perceived exertion >17.

Muscle strength

The subjects were carefully familiarized with the testing procedures of voluntary force production of the muscles tested. Maximal isometric force of the bilateral leg extensor muscles was measured using an electromechanical dynamometer.20 In this test, the subjects were in a sitting position so that the knee and hip angles were 107° and 110°, respectively. The subjects were instructed to exert their maximal force as fast as possible. A minimum of three trials was completed for each subject and the best performance with regard to maximal force was selected for the subsequent statistical analysis.

Questionnaire

All of the subjects answered questions with regard to their education, smoking and alcohol drinking habits, sleeping behavior, sick leaves, and a previous level of leisure time physical activity. The single-item question on leisure time vigorous physical activity (SIVAQ) on this questionnaire was, "Think about a previous 3-month period before the military service and consider all leisure-time physical activity with duration of at least 20 minutes. What was die frequency of your physical activity?" The alternatives were (1) less tìian once a week, (2) no vigorous activities but light or moderate physical activity at least once a week, (3) vigorous activity once a week, (4) vigorous activity twice a week, (5) vigorous activity three times a week, and (6) vigorous activity at least four times a week.21 In a further analysis, the alternatives 1 and 2 were combined as an inactive group, 3 and 4 a moderate activity group, and 5 and 6 an active group.

Statistical Analysis

Standard statistical methods were used for the calculation of means, standard deviations, and Pearson product moment correlation coefficients. The data were then analyzed using multivariate analysis of variance with repeated measures. Probability adjusted t tests were used for pairwise comparisons when appropriate. The criterion for statistical significance was set at p < 0.05.

RESULTS

Body Composition

In the beginning of the BT season, no initial differences between the groups were observed with respect to age, height, body mass, physical activity, muscle strength, or aerobic capacity. The results of body characteristics are shown in Table III. Both BM and body mass index (BMI) remained unaltered in the ET group during the 8-week training period, whereas WC (2.6%; ? < 0.01) and BF (5.2%; ? < 0.01) decreased. Correspondingly, in the ST group, all of the body composition variables of BM (1.1%; ? < 0.05), BMI (1.1%; ? < 0.05), BF (16.3%; ? < 0.001), and WC (2.9%; ? < 0.001) decreased. The NT group showed a decrease in their BF by 9.5% (p < 0.01). However, no significant differences between the groups in the body composition variables were observed after the 8-week training period.

Aerobic Capacity

During the 8-week training period, the ET group increased their VO2max by 8.5% (p < 0.05). The ST group increased its VO2max by 12.0% (p < 0.01; Fig. 1) and the NT group by 13.4% (p < 0.001). In the total subject group, VO2max improved by 10.5% (p < 0.01) during the 8-week training period. In addition, a significant correlation (r = -0.56, p < 0.001) was observed in the total group of subjects between the initial level of VO2max and its changes during the 8-week training period (Fig. 2).

Correspondingly, the ET, ST, and NT groups also increased their maximal aerobic cycling power (W^sub max^) by 4.2, 5.5, and 4.8% (p < 0.01-0.05), respectively; Table IV). At the submaximal working loads of 125 to 225 W, significant increases (p < 0.01-0.05) in VO^sub 2^ were observed in the NT group, whereas no changes were observed in the ET and ST groups. Significant decreases in blood lactate accumulation were observed in the ST (p < 0.01-0.05) and NT (p < 0.05) groups at the submaximal working loads of 100 to 150 W, whereas no significant changes were observed in the ET.

Muscle Strength

The ET and ST groups increased their maximal voluntary bilateral isometric leg extension force by 12.9% (p < 0.01) and 9.1% (p < 0.05), respectively, while no significant increase occurred in the NT group (5.2%; p = 0.45; Fig. 3). These relative increases did not, however, differ significantly between the ET and ST groups. In the total group of subjects, an increase of 8.9% (p < 0.01) in muscle strength was recorded.

Associations between Physical Activity, Aerobic Capacity, and Sick Leaves

Before the military service (previous 3 months) the proportion of the subjects with no leisure time physical activity was 14.3% (SIVAQ alternatives 1 and 2), with moderate physical activity 44.4% (alternatives 3 and 4), and with physical active ones 41.3% (alternatives 5 and 6). The mean initial VO^sub 2^max among the inactive subjects (n = 9) was 41.3±6.7 mL.kg^sup -1^ * min^sup -1^, whereas it was among the subjects with moderate physical activity (n = 28) 43.6±8.0 mL*kg^sup -1^ * min^sup -1^ and with physically active ones (n = 26) 47.0 ± 9.2 mL*kg^sup -1^ * min^sup -1^. Inactive subjects increased their VO^sub 2^max during BT by 19.4% (p < 0.001), while the subjects with moderate physical activity increased VO^sub 2^max by 10.9% (p < 0.001) and the physically active subjects by 7.8% (p < 0.05; Fig. 4). The difference in VO^sub 2^max between the inactive subjects and the active ones was only 1.8% after the 8-week training period, whereas before the training period it was 12.1%. However, there were no differences between the groups in terms of self-reported physical activity at the beginning of BT.

Physical activity seemed to be associated with health status. Mean VO^sub 2^max was 46.9 ± 9.1 mL * kg^sup -1^ * min^sup -1^ among subjects with no sick absence days (n = 32), while the mean VO^sub 2^max was 40.5 ± 5.1 mL * kg^sup -1^ * min^sup -1^ in the subjects with at least three sick absence days (n = 19). However, no differences between the training groups with regard to the total number of the sick absence days were observed.

DISCUSSION

The present 8-week ST and ET programs in association with the standardized BT, including physical, combat, and march training, significantly improved V02max in both training groups as well as in the NT group. However, no significant differences were observed between the groups. The ET and ST groups showed greater improvements in the time to exhaustion than the NT group. In addition, positive changes were observed in body composition, since BF and WC decreased in all groups during BT, even though the subject's initial values were already quite low. In addition, both ST and ET groups increased their maximal leg extension strength, whereas no significant change occurred in NT.

Higher previous physical activity (SIVAQ), over a period of 3 months before the military service, of the subjects was associated with better initial cardiorespiratory functions and smaller improvements in VO^sub 2^max during BT. Accordingly, the inactive subjects showed the greatest improvements (12%) in their VO^sub 2^max values. In addition, the difference in the VO^sub 2^max values between the active and inactive subjects reduced by 10% compared to the initial values. Thus, inactive subjects obtained almost the same fitness level as active ones during the 8-week training period. Interestingly, the present subjects with higher initial VO^sub 2^max also had fewer sick absence days than those ones with lower VO^sub 2^max. This is well in line with earlier findings that showed a link between low aerobic capacity, muscle fitness, and obesity, which are risk factors for musculoskeletal injuries among the conscripts.22-24

The significant decreases found in BF and WC in all groups might have been a consequence of daily energy expenditure that was higher than energy intake, leading to an energy deficit. Gomes-Merino et al.25 reported that energy expenditure could exceed even to 5,000 kilocalories (kcal) a day during military field exercises. In this study, the subjects' daily energy intakes were ~3,000 kcal, served by the garrison dinning hall, and during the field exercises >3,500 kcal.26 In the present study, the mean BM decreased in the ST group, while no changes in BM were observed in the ET and NT groups during the 8-week period. This could be explained by a higher reduction of fat mass in the ST group as compared to the two other groups. Gordon et al." have shown similar findings in their 1-year intervention in the recruits of the South African Defence Forces. Although the aerobic capacity of the recruits improved by 8% and BF decreased during the first 10 weeks, thereafter values remained at the same level or even reduced.

Figure 2 shows that higher initial VO^sub 2^max values were related to lower changes in VO^sub 2^max induced by the 8-week training period. In addition, the statistical analysis revealed that the subjects with higher than 55.5 mL . kg^sup -1^ . min^sup -1^ of VO^sub 2^max did not improve their maximal aerobic capacity. In this case, it is possible that the intensity of the military training was not high enough for positive responses in VO^sub 2^max. These findings were supported by the study of Dyrstad et al.7 They have explained the poor improvements of the soldiers' aerobic capacity and muscular strength by the low amount of high-intensity ET and ST during military service. Rosendahl et al.27 have reported that a 12- week training period including marching, general conditioning (mostly running), and military specific training led to 14% improvements in a 12-minute running test and 11% in VO^sub 2^max among untrained soldiers, whereas no change occurred in the well-trained groups. In the British Army, BT improved VO^sub 2^max of the recruits by 6. 1 % and BF reduced by 2.8%,28 while in another study of British recruits VO^sub 2^max increased by only 3.6%. I5 The present results strongly support these earlier findings.

Furthermore, the special ST program combined with BT positively enhanced the aerobic capacity of the conscripts. Thus, strength training three times a week did not lead to interference with the development of VO^sub 2^max. This is well in line with previous data, for example, by Bell et al.,29 who observed that the increases in VO^sub 2^max were similar between concurrent strength and ET and ET only during a 12- week training intervention. Maximal voluntary bilateral isometric leg extension force improved in both the ST and ET groups, but not in the NT group. Thus, the overall training load during BT in NT was not a sufficient stimulus to increase maximal strength of the lower extremities. The increase in maximal strength observed in ET may be attributable to the fact that a part of the present ET was carried out using cycling, which is known to load the lower extremity muscles.30 However, since the increase in strength in ST was not higher than in ET, it is possible that the overall BT program, including a large amount of ET interfered with strength development in ST. It is known that an interference effect on strength development may occur when the overall volume and/or frequency of training is high,31 whereas low-frequency concurrent ST and ET lead to large gains in maximal strength.17

Strength development is accompanied with significant increases in maximal voluntary neural activation and enlargements in both the muscle cross-sectional area and the size of individual muscle fibers.17 Since no measurements to record muscle activation or hypertrophy of trained muscles were made in the present study, it is not possible to evaluate the mechanisms behind the present strength increases in the ST or ET groups. Possible mechanisms an interference effect on strength development may be the high proportion of militarybased ET,31,32 lower daily sleeping hours (~2 hours per day less than before military service), and particularly lower caloric intake. For example, Nindl et al.33 observed that after 8 weeks of hard military training involving high-energy deficit, maximal lifting strength, and power output in Rangers declined by ~20%. These changes were mainly consequences of losses in BM and fat-free mass.

In conclusion, the present BT program in the Finnish Defence Forces, including a high amount of ET (physical, combat, and march training), improved cardiovascular performance of the conscripts. However, no further improvements in VO^sub 2^max were obtained by specific ET three times a week. BT also positively influenced body composition by decreasing BF and WC, but BT alone was not a sufficient stimulus to increase the maximal strength of leg extensors. BT, including a large amount of ET, may have interfered with strength development in the ST group. To what extent improvements in endurance capacity can be obtained throughout the entire military service needs further experimental research work.

ACKNOWLEDGMENTS

We also express our special thanks to Ms. Inna Lisko, Ms. Mari Mattila, and Mr. Henry Lehto for their help during the measurements.

This study was supported in part by grants from the Scientific Committee for National Defence, the Ministry of Education, Finland, the Foundation of Sport Institute, and the Foundation of Werner Hacklin.

© 2008 Association of Military Surgeons of the United States Provided by ProQuest LLC. All Rights Reserved.