The fitness level of young men entering Finnish compulsory military service has declined with a concomitant increase in body mass (BM) during the last 25 years (30). In order to improve the physical fitness of new conscripts entering military service, a new updated physical training program for an 8-week basic military training (BT) period was established in 1998 (35)." However, the effect of the BT program on physical fitness and performance among current conscripts, who exhibit large interindividual variations in physical fitness levels, is unknown.
Conscripts entering military service are intensively exposed to nontraining stresses because of the totally new environment and the change in their working conditions. Most of these young men will have their first experience of demanding physical training, eating outdoors, and performing overnight exercises in a forest. In addition, the physical training involved in military service may exceed their previous training level, which could contribute to overreaching (OR) in the multistressor environment of the military service. Overreaching is an accumulation of training and nontraining stress resulting in short-term decrements in performance capacity, which may be accompanied by related physiological and psychological signs and symptoms of maladaptation (22). Overreaching could also be an early indicator of the development of overtraining syndrome (OTS) (22,33), which is characterized by an unexpected decrease in performance, despite increased or maintained training load (38). There is some evidence to suggest that the development of OTS is not only solely related to the stress of training but also to nontraining (psychological) stress (15,21).
To monitor responses to training and to avoid OR and OTS, there is still a clear need for reliable tools that facilitate the early diagnosis of OTS (36). Although there are no specific markers of impending OTS, testosterone (an anabolic hormone) and cortisol (a catabolic hormone) have often been used to represent the overall anabolic and catabolic activities in the body (2,37). In addition, serum testosterone and cortisol concentrations have been identified as reliable markers of military training stress (24), and the testosterone/cortisol ratio has been used to represent the balance between anabolic and catabolic activity (1). It has also been suggested that acute exercise-induced changes in serum hormones could be used to monitor training load and to avoid OTS (36). In particular, a diminished response of cortisol to an acute submaximal exercise (39), and reduced basal cortisol levels (31,36) may be useful indicators of exhaustive training loads. In addition, insulin-like growth hormone (IGF-1) has recently been identified as a suitable marker for monitoring training load (23) and investigating OTS. Furthermore, an impaired mood state and subjective complaints are consistently described as sensitive and early markers of OTS (36).
The purpose of the present research was (a) to study effect of an 8-week Finnish military basic training period (BT) on physical fitness, body composition, mood state, and serum biochemical parameters, (b) to determine the incidence of OR during BT among new conscripts, and (c) to evaluate whether the initial levels or training responses of physical fitness, body composition, and serum biochemical parameters differ between OR and noOR subjects.
Experimental Approach to the Problem
Performance tests and physiological and psychological markers to monitor the training load of BT training were selected based on existing literature of OTS and training studies. Performance tests (maximal aerobic fitness O2max test and submaximal test) and physiological tests were performed 3 times, and psychological markers 5 times during the 8-week training period. In addition, acute responses to a submaximal test were studied. For the submaximal test, a marching test was selected because it closely resembles the routine activities of conscripts and enables a test to be performed for a large group of subjects. Furthermore, sick absences were followed over the entire BT, because poor O2max levels have been associated with training related sick absences during military service (17). The experimental protocol is presented in Table 1.
Purposes 2 and 3
No single symptom or set of symptoms have been found to be associated with OR or OTS. However, a decrease in performance and an increase in perceived exertion at the same subjective workload have been found to be related to OR and OTS (13,14,38,39). In addition, impaired general health status, negative mood, and feelings of fatigue have been found to be related to OR and OTS (13,22,38,39). Therefore, subjects in this study had to fulfill 3 of 5 criteria to be classified as OR subjects. In addition, differences between subjects who fulfilled a single OR criteria were evaluated to determine whether initial levels of physical fitness, body composition, and serum biochemical parameters differed among these subjects.
- Criteria 1. A reduced O2max of greater than 5% (13,31) or did not perform the test because of illness. Absence from the test was set as an additional criterion, because all 3 O2max tests were completed by a total of 36 conscripts. It was assumed that illness itself reduces performance. Furthermore, overload training has been reported as a risk factor for upper respiratory track infections (URTIs) (32,39).
- Criteria 2. An increase in mean rating of perceived exertion (RPE) during the submaximal marching test greater than 1.0 (13) from the lowest value at week 1 or week 4 and remaining more than 1.0 above the lowest value, or did not perform the test because of illness (see description above). All 3 submaximal marching tests were completed by a total of 35 conscripts.
- Criteria 3. An increase in somatic symptoms of OTS greater than 15% from week 4 to week 7 and remaining the same or increasing from week 7 to week 8. Subjects were divided into tertiles based on an increase in somatic symptoms of OTS from week 4 to week 7; 15% was the upper third.
- Criteria 4. Admitted feeling physically or mentally overloaded at week 7 or 8.
- Criteria 5. Sick absence more that 10% of daily service (upper third tertile based on the sick absences during BT).
The study took place during winter in Finland, when daily outdoor temperatures ranged from −31 to +1°C, with an average of −13°C (data from the local weather station). The overall physical load of the 8-week BT period was set according to the standard direction of the Defense Command. The intensity level of physical activity in the daily program was low in the first week of BT and increased thereafter. Food and water intake were in accordance with the standard army meal, and water intake was not restricted. However, subjects were not permitted to use any extra nutritional supplements throughout the study. The average sleeping time at the garrison was from 10 pm to 5:45 am. Military education included physically demanding activities such as marching, combat training, and sport-related physical training. The conscripts also carried combat gear weighing 20 kg, including clothing, particularly during marching and combat training. In addition, the training included an overnight field exercise. Garrison training involved theoretical education in classroom settings, material handling, shooting, and general military education, such as close-order drills. Conscripts marched 4 times per day (ca. 5 km in total) for meals and slept in dormitory-type rooms.
During the BT period, practical skills are trained daily. At the beginning of BT, the amount of physical training is approximately 2·h·d−1, increasing to 3-4 hours during weeks 4-7. The BT schedule includes 4 longer (from 2 to 8 hours) marching exercises with battle equipment. However, daily outdoor exercises, including transportation, are often performed by marching. Rifle marksmanship in the garrison is performed twice a week throughout the BT period.
Male conscripts (n = 57, aged 19.6 ± 0.3 years) from the Signal Battalion of Northern Finland participated in the present study. They were selected from a total of 131 subjects who initially volunteered to participate. From these subjects, 47 were discarded based on cardiorespiratory or musculoskeletal disorders and incomplete fulfillment or willingness to perform special duties. The remaining 84 were divided into 3 categories according to their level of voluntary physical activity before military service, which was determined by the International Physical Activity Questionnaire (7). To ensure large variations in initial physical fitness levels, 19 conscripts were randomly selected from each category, making a total of 57 subjects. All subjects were fully informed of the experimental protocol and gave their written consent to participate in the study. They were also advised of their right to withdraw from the investigation at any time. The study protocol was approved by the Finnish Defence Forces, and the Ethical Committees of the University of Jyväskylä and the Kainuu region of Finland.
To determine O2max (ml·kg−1·min−1), the conscripts performed a maximal treadmill test in week 1 (before training) and after 5 (week 5) and 8 (week 8) weeks of the BT period. The follow-up tests for each subject were always performed at the same time of the day between 8.30 am and 5 pm, after having consumed a similar diet before each test. The start of the test involved walking for 3 minutes at 4.6 km·h−1 and walking or jogging at 6.3 km·h−1 (1% slope) as a warm-up. Thereafter, exercise intensity was increased every 3 minutes to induce an increase of 6 ml·kg−1·min−1 in the theoretical O2max demand of running (2). This was achieved by increasing the initial running speed of 4.6 km·h−1 by a mean of 1.2 km·h−1 (range 0.6-1.4 km·h−1), and by increasing the initial grade of 1 deg by a mean of 0.5 deg (range 0.0-1.0 deg) up to the point of exhaustion (2). Pulmonary ventilation and respiratory gas exchange data were measured on-line using the breath-by-breath method (Jaeger Oxygen Pro, VIASYS Healthcare GmbH, Hoechberg, Germany), and mean values were calculated at 1-minute intervals for statistical analysis. The analyzer was calibrated before each test according to manufacturer specifications. Heart rate (HR) was continuously recorded at 5-second intervals using a telemetric system (Polar810i, Polar Electro Oy, Kempele, Finland). Rating of perceived exertion was assessed at the end of each workload. The criteria used for determining O2max were a lack of increase in O2max and HR despite an increase in workload of the treadmill, a respiratory exchange ratio higher than 1.1, and a postexercise blood lactate value (determined 1 minute after exercise completion from a fingertip blood sample using a lactate analyzer; LactatePro®, Arkray, Japan) that was higher than 8 mmol·L−1 (2). All participants fulfilled these criteria.
Submaximal Marching Test
The 45-minute submaximal marching test (exercise) was performed after 1 week of adjustment to military service at the end of the first week (before training), and after 4 (week 4) and 7 (week 7) weeks of the BT period. The time between the tests was 3 weeks, and the follow-up exercise for each subject was always performed at the same time of the day between 9 am and 12 pm. Heart rate was recorded in 5-second intervals during the entire exercise to define the mean HR. Rating of perceived exertion was assessed 3 times, every 15 minutes using a 6- to 20-point scale. Rating of perceived exertion was calculated as the average of these 3 values. The aim of determining marching speed was to divide subjects into 3 groups so that each group involved subjects marching at approximately 70% of their individual maximal workload. Exercise of this intensity and duration has been shown to result in increased serum cortisol levels (34) and to be tolerable for untrained subjects. Firstly, theoretical individual maximal workload (ml·kg−1·min−1) was calculated based on the treadmill speed and grade in the O2max test using the American College of Sports Medicine (ACSM) estimation formula for O2 in running (2). Secondly, because in load carriage tasks (∼20 kg) the relationship between relative O2max and endurance time has been shown to be lower compared with an absolute O2max value (3), theoretical individual maximal workload as ml·kg−1·min−1 was converted into L·kg−1. Thirdly, the individual speed corresponding to 70% of maximal workload when carrying the 20-kg combat gear during the test on even terrain was determined. It was assumed that carrying the extra load required a similar level of effort to carrying extra body weight. Because the ACSM formula for the estimation of O2max in walking yields unrealistically low values for brisk walking (above 5 km·h−1), a new equation for marching was determined that combines the ACSM equations for walking and running between 5 and 8 km·h−1. The equation is based on the earlier findings that walking and running are metabolically equivalent at a velocity of approximately 8 km·h−1 (20). Fourthly, the individually estimated marching speeds were rounded to the closest even number. Twelve subjects were categorized into the group marching at 6 km·h−1, 43 subjects at 7 km·h−1, and 4 subjects at 8 km·h−1. An accurate and even marching speed was ensured by the group leader who walked in front of the group without extra weight and wore the Polar S625 (Polar Electro Oy) running computer, which was equipped with an individually calibrated speed-measuring device.
Body composition measurements (BM, fat-free mass [FFM], fat mass [FM], and percentage of body fat [F%]) were performed at the same time points as the marching tests using 8-point bioelectrical impedance (Inbody720, Biospace Co. Ltd, Seoul, Korea). For each subject, the measurements were performed at the same time between 6 am and 7 am after an overnight fast and after avoiding, with no exercise for 12 hours before the test. The physical activities in the daily program were planned to be of a low intensity on the day preceding each measurement, and fluid status was estimated to be in balance based on the dietary records of the subjects. The subjects were barefoot and wore T-shirts and trousers. Body height was measured to the nearest 0.5 cm using a wall-mounted stadiometer. Body mass index (BMI) was calculated as BM (kg) divided by height (m) squared. Recent studies have shown that 8-polar bioelectrical impedance is a reliable method to detect changes in FFM (ΔFFM) and FM (ΔFM) when performed under strict standardized conditions (16). Accordingly, ΔFFM and ΔFM during the study were calculated using this method.
Blood Samples and Analyses
Blood samples were drawn from an antecubital vein after an overnight fast at rest (basal), 2 hours after a light breakfast before exercise (pre-exercise) and immediately after exercise (postexercise). Basal samples were taken for serum total testosterone, cortisol, and serum sex hormone-binding globulin (SHBG); pre-exercise samples for serum total testosterone, cortisol, and total insulin-like growth factor-1 (IGF-1); and postexercise samples for serum testosterone and cortisol. Fingertip lactate (La) samples were taken pre-exercise and postexercise. Circadian variability in blood parameters was minimized by collecting basal samples at the same time in the morning between 6.30 am and 7.30 am and by collecting individual pre-exercise and postexercise samples at the same time of day between 9 am and 12 am after similar patterns of food ingestion. The standardized breakfast before the test had a total energy content of 1,860 kJ, 61 g of carbohydrate (56% of total energy intake), 16 g of protein (14% of total energy intake), and 15 g of fat (30% of total energy intake). All blood samples were taken in the same position (seated). Subjects were instructed not to ingest alcohol, coffee, tea, chocolate, cola drinks, or bananas on the morning of the measurements or the previous evening.
The percentage change in plasma volume (%ΔPV) was calculated from changes in hemoglobin and hematocrit according to the method of Dill and Costill (8). Serum postexercise values were adjusted for changes in plasma volume: Postexercise adjusted = postexercise + (postexercise × %Δplasma volume/100). Exercise-induced relative changes in serum hormonal concentrations were calculated using the equation (postexercise adjusted − pre-exercise)/pre-exercise *100, and expressed as Δ%. La was analyzed from fingertip blood samples (Lactate Pro). Testosterone, cortisol, and SHBG were analyzed by Immulite 1000 (Diagnostics Products Corporation, Los Angeles, CA, USA) using commercial chemiluminescent enzyme immunoassays IMMULITE®/IMMULITE 1000 (Diagnostics Products Corporation). The intraassay coefficients of variance for these assays were <5.7%, <4.8%, and <2.4%, respectively, and sensitivities of variance were 0.5, 5.5, and 0.2 nmol·L−1, respectively. Hematocrit and hemoglobin were analyzed using a Sysmex KX 21N-analyzer (Sysmex Co., Kobe, Japan). Insulin-like growth factor-1 was analyzed with an R&D duoset enzyme-linked immunosorbent assay kit (R&D Systems, Minneapolis, MN, USA). The intra and interassay coefficients of variance and sensitivity for these assays were <6.4%, <9.1%, and 26 pg·mL−1, respectively.
Questionnaires and Sick Absence
The questionnaire to assess the somatic symptoms of OR and OTS was formulated based on previously reported symptoms of OTS (12,15,22,36). The somatic symptoms were subjective ratings of well-being; URTIs (Cronbach's alpha 0.82), flu-like symptoms (Cronbach's alpha 0.74), digestive disorders and reduced appetite (Cronbach's alpha 0.45), musculoskeletal disorders (Cronbach's alpha 0.70), and physical complaints and sleep difficulties (Cronbach's alpha 0.61). The subjects rated how severely they experienced a list of symptoms during the last week of training using a 5-point Likert scale; 1 = not at all, 2 = 1 day, 3 = 2-3 days, 4 = 4-5 days, and 5 = 6-7 days. The sum of the symptoms associated with OTS was determined as the sum of the symptom scores. In addition, the question “Do you feel physically or mentally overloaded?” was asked. Questionnaires were administered 1 day before and 1 week after the marching test at the same time of day in standardized conditions to avoid pre-exercise vs. postexercise and morning vs. evening variations (22). Sick absence was defined as an attendance or non-attendance in daily service because of illness or injuries examined by a physician.
Physiological and psychological responses to training (before, and after 4 and 7 weeks of BT) and exercise (pre and postsubmaximal marching test) were assessed using repeated-measures ANOVA. Least significance difference (LSD) as Post hoc analysis was used to identify significant differences. In addition, the effect of training and exercise and their interaction were calculated. Pearson product-moment correlations were used to observe associations between variables. These analyses were performed for the 35 subjects who completed all submaximal marching tests and thus had all results.
Purposes 2 and 3
Independent samples T-tests were used to identify significant differences between OR and noOR subjects. In addition, groups of subjects satisfying each single OR criteria (1-4) were evaluated independently by independent samples T-tests. For OR criteria 5 (sick absence), 1-way ANOVA was used to identify significant differences between the groups. During the study, there were 42 military service days, and the average number of sick absences was 3 days, ranging from 0 to 10 days (0-24%). Subjects were divided into tertiles based on sick absence; 0% sick absence (n = 20), 2-7% sick absence (n = 19), 10-24% sick absence (n = 18). All data are presented as mean ± SD. Where assumptions for normality were not met, the data were log transformed before statistical analysis. The untransformed values are shown in the text, tables, and figures for more meaningful comparison. Statistical analyses were performed using SPSS (Version 14.0.1. 2005; SPSS Inc., Chicago, IL). The level of statistical significance was set at p ≤ 0.05.
Purpose 1. The Effect of the Basic Training Period
All 3 submaximal marching tests and O2max tests were completed by a total of 21 conscripts. In 65% of cases, the reason for not participating in the test was URTI. There was a main effect of training for O2max (p < 0.001), postexercise lactate (p < 0.05), and maximum HR in O2max tests (p < 0.01). Conscripts showed positive responses to training with an 11% increase in O2max after 5 weeks of training (before 45 ± 7 vs. week 5 49 ± 5 ml·kg−1·min−1, p < 0.001). From week 5 to week 8, O2max did not change significantly. In addition, the individual O2max values before training were negatively correlated with individual Δ%O2max after 5 (r = −0.61, p < 0.001) and 8 (r = −0.74, p < 0.001) weeks of training. Postexercise lactate was lower after 8 weeks of training (9.8 ± 2.1 mmol·L−1) compared with 5 weeks of training (11.1 ± 2.1 mmol L−1, p < 0.05) and before training (11.6 ± 2.3 mmol·L−1, p < 0.01). Maximum HR was lower after 5 (p < 0.01) and 8 (p < 0.001) weeks of training compared with before training values (before 195 ± 7 b·min−1, week 5 190 ± 7 b·min−1, and week 8 188 ± 6 b·min−1).
The initial average body height was 178.2 ± 7.9 cm, BMI 24.7 ± 4.5 kg·m−2, and F% 19.3 ± 7.5%. The values before and after 4 and 7 weeks of training for BM, FM, and FFM are presented in Table 2. From one subject, body composition measurement by 8-point bioelectrical impedance was not available at week 7; therefore, the number of the subjects is 34 in Table 2. Both BM and FM exhibited a main effect of training (p < 0.001), decreasing from week 4 to week 7 (p < 0.001 and p < 0.01, respectively), with lower values compared with the before training values (p < 0.001). All subjects exhibited a decrease in FM after 7 weeks of training compared with the before training values (range from −28.0 to −0.4%). In contrast, the mean FFM was higher after 7 weeks of training compared with the before training values (p < 0.05) (Table 2).
Basal Serum Hormone Concentrations
There was a main effect of training for serum cortisol, SHBG, and IGF-1 (p < 0.001) and for the testosterone/cortisol ratio (p < 0.05). Basal testosterone levels were similar at all 3 time points (Figure 1A). Basal SHBG increased, whereas basal cortisol and pre-exercise IGF-1 decreased from week 4 to week 7 (p < 0.001), being significantly different from the before training values (p < 0.05-0.001) (Figure 1B-D). Serum basal testosterone/cortisol ratio increased from week 4 to week 7 (p < 0.01) (before 0.034 ± 0.013, week 4 0.037 ± 0.016, week 7 0.040 ± 0.012). However, 46% of the subjects showed an initial increase in basal testosterone during the first 4 weeks of training. Thereafter, basal testosterone decreased from week 4 to week 7 in these subjects. In contrast, 23% of the subjects showed increases in basal testosterone throughout the study.
Responses to Submaximal Marching Tests
There was a main effect of training for HR and RPE during submaximal marching (p < 0.001). Heart rate was lower after 4 weeks (p < 0.001) compared with the before training values, and after 7 weeks of training compared with the week 4 and before training values (p < 0.001) (Table 3). Rating of perceived exertion was lower after 7 weeks of training compared with the before training (p < 0.001) and week 4 values (p < 0.01) (Table 3). Blood La exhibited a main effect of training (p < 0.001) and exercise (p < 0.001), and a training and exercise interaction (p < 0.001) by increasing because of exercise before (179 ± 211%, p < 0.001) and after 4 weeks of training (68 ± 91%, p < 0.001), but not after 7 weeks of training (13 ± 49%, n.s.). In addition, postexercise La was lower after both 4 (p < 0.001) and 7 (p < 0.001) weeks of training compared with the before training values (Table 3). Serum testosterone and cortisol showed a main effect of exercise (p < 0.001). In response to exercise, serum testosterone increased significantly at each time point (p < 0.05-0.001) (before training 20 ±23%, p < 0.001; week 4 12 ± 27%, p < 0.01; week 7 11 ± 25%, p < 0.05). After 7 weeks of training, Δ%testosterone induced by exercise was lower compared with the before training values (p < 0.05) (Figure 2). Serum cortisol increased significantly only after 4 weeks of training (before 5 ± 44%, n.s.; week 4 16 ± 35%, p < 0.01; week 7 8 ± 31%, n.s.). Individual variations in cortisol responses were large, ranging from −48 to 107% (Figure 2).
The somatic symptoms associated with OTS exhibited a main effect of training (p < 0.001), whereby symptoms increased toward the end of BT. The greatest increase was observed between week 4 and week 7 (p < 0.01) (Figure 3). Concurrently, the proportion of subjects who answered “yes” to the question “Do you feel physically or mentally overloaded?” increased from 41% to 80% (Figure 4).
Purpose 2 and 3. Incidence of Overreaching and Differences between overreaching and no-overreaching Subjects
From a total of 57 subjects, OR criterion 1 was detected among 42% of subjects, criterion 2 32%, and criterion 3 13%. According to the measured results before training, the subjects who fulfilled OR criteria 1, 2, or 3 were not significantly different from those who did not fulfill any of these OR criteria.
Criterion 4 was detected among 83% of subjects. Those who admitted to feeling physically or mentally overloaded in week 7 or 8 had lower O2max (3.3 ± 0.4 vs. 3.9 ± 0.5 L·min−1, p < 0.001), FFM (61.9 ± 0.8 vs. 68.7 ± 5.6 kg, p < 0.05) and higher HR in the submaximal marching test (152 ± 11 vs. 141 ± 10, p < 0.05) before training compared with those who answered “no.”
Criterion 5 was detected among 32% of subjects. The subjects who had sick absences of more than 10% had lower O2max (46 ± 6 vs. 40 ± 7 ml·kg−1·min−1, p < 0.01), basal testosterone (18.6 ± 4.3 vs. 14.9 ± 4.1 nmol·L−1, p < 0.05), and testosterone/cortisol ratio (0.036 ± 0.015 vs. 0.026 ± 0.007, p < 0.05) before training compared with those who had no sick absences. In addition, subjects with more than 10% sick absence also had lower basal testosterone after 4 and 7 weeks of training (21.1 ± 7.5 vs. 17.0 ± 4.7 and 20.0 ± 6.1 vs. 15.5 ± 5.6 nmol·L−1, p < 0.05) (Figure 5A-C).
At least 3 of the 5 criteria were detected among 33% of the subjects, all of whom were classified as OR subjects. The remaining 67% were classified as noOR subjects. At the beginning of BT, only basal SHBG differed between OR and noOR subjects (p < 0.01) (Table 4). Similarly, OR subjects had higher basal SHBG than noOR subjects after 4 (p < 0.01) and 7 weeks of training (p < 0.05). Furthermore, OR subjects had higher basal serum cortisol than noOR subjects after 7 weeks of training (p < 0.05). Moreover, OR and noOR subjects differed in terms of the response of the basal testosterone/cortisol ratio to training from week 4 to week 7 (p < 0.05), and the maximal La/RPE ratio in the O2max test from baseline to the end of BT (Table 4).
The purpose of the present study was to evaluate the training load of an 8-week Finnish military basic training period among new conscripts, to determine the incidence of OR during BT, and to assess whether initial levels or training responses differ between OR and noOR subjects. The subjects' physical fitness characteristics in this study were representative of young Finnish men (30). However, there was a high incidence of dropouts from the performance tests; only 63% of the subjects completed all submaximal tests and 38% both submaximal and maximal tests. The dropouts did not mean that these subjects had more sick absence. However, serum testosterone, testosterone/cortisol ratio, and O2max were lower in “high incidence of sick absence” subjects than in “no sick absence” subjects. Furthermore, subjects who admitted to feeling overloaded demonstrated low O2max, FFM and high submaximal test HR before training. These observations confirm the earlier finding that poor aerobic fitness is a risk factor for sick absence during military service (17) and highlight the importance of good physical fitness for overall well-being. A new finding was that testosterone had an impact on sustaining stress and maintaining health during the 8-week military training period.
For those conscripts who were able to perform all the tests, the increase in O2max indicates an improvement in aerobic capacity. In addition, decreases in HR, postexercise La, and serum testosterone during the 45-minute submaximal marching test indicate decreased exercise-induced metabolic stress (28,40) during the first 4 weeks of BT. Furthermore, as expected, our data showed an inverse relationship between initial O2max and relative changes in O2max, indicating that the training was most effective for the least fit subjects. This is comparable with studies performed in the Polish (9) and USA (28) armies.
During the second half of the 8-week training period, a stagnation of increase in O2max was observed. Concurrently, basal serum IGF-1 decreased, and basal serum SHBG and testosterone/cortisol ratio increased with no change in basal serum testosterone. Furthermore, basal serum cortisol decreased with a concomitant increase in the number of subjects who felt overloaded. Previously, serum total testosterone concentration has been shown to decrease in response to demanding training periods (6,36), and during extremely stressful military training periods (23,27), and to increase with overload training (37). In contrast to the present study, basal SHBG has been found to increase (19,26) and IGF-1 to decrease (23,25-27) after heavy training. A decrease in IGF-1 is suggested to indicate a diminished capacity for protein deposition. In addition, IGF-1 is sensitive to both energy intake and dietary protein content, and demonstrates significant decreases with inadequate dietary intake (25,26). Furthermore, a 7-week intensive endurance overload training program (37) and combined endurance and strength overload training for 6 weeks (5) have been found to decrease testosterone/cortisol ratios. Even the biochemical changes of testosterone and testosterone/cortisol ratio are controversial compared with previous findings, a stagnation of increase in O2max in this study indicates that the training load during the second half of the 8-week training period may have been too exhaustive. This is also supported by a decrease in maximal postexercise lactate and HR (14,22,38), and the fact that 46% of subjects showed a decrease in basal serum testosterone from week 4 to week 7.
Classification of overreaching (OR) or overtraining syndrome (OTS) is challenging, and it is rarely classified by a single criterion. In this study, OR subjects had to fulfill 3 of 5 criteria, and OR was detected among 33% of the subjects. The only previous study concerning OR or OTS among young men who entered military service and performed an 8-week BT training period was by Chicharro et al. (4). They found a 24% incidence of OTS during BT, determined by absolute decreases in the free testosterone/cortisol ratio, a classical criterion for OR/OTS (1,5,19). Similarly, in this study, OR subjects differed in response to training compared with noOR subjects. OR subjects exhibited a tendency toward a decrease in basal Δ% testosterone/cortisol ratio, whereas noOR subjects exhibited an increase. Snyder et al. (31) defined 5 criteria for OR, one of which was a decreased maximal La/RPE ratio. In the present study, the maximal La/RPE ratio tended to decrease during the latter part of BT in OR subjects.
To our knowledge, no previous studies have a reported relationship between basal serum SHBG and the incidence of OR. In the present study, OR subjects had higher levels of SHBG, both before training and after 4 and 7 weeks of training. Although basal SHBG has been found to increase after heavy training (19,26), the mechanism underlying the present findings is still not fully understood. Testosterone did not differ between noOR and OR subjects. However, most of the testosterone is bound to SHBG, which in turn decreases bioavailable testosterone in serum, and may thereby have an influence on training responses.
OR subjects had higher basal serum cortisol at the end of BT, which is in line with the OR scenario presented by Steinacker et al. (33), where an increase in basal cortisol was observed in the OR state. However, a decrease in basal serum cortisol during the second half of BT was observed among the 35 conscripts who performed all submaximal tests. This may be a marker of overtraining, along with lower psychosocial stress (29,31,33,36). Basal serum cortisol has been found to increase during extremely stressful military training periods (23,27), but no change (6,19,37), or a decrease (29,31) has been observed in endurance based overtraining studies. However, in response to strenuous resistance training, both no change (11,18) and an increase (10) of basal serum cortisol have been reported. Glucocorticoids, like cortisol, exert many beneficial actions in exercising humans: increasing the availability of metabolic substrates for the energy requirements of muscles, maintaining normal vascular integrity and responsiveness, and protecting the organism from an overreaction of the immune system in response to exercise-induced muscle damage (29,33). Differences in cortisol behavior between military and overtraining studies, and incoherent training responses of basal serum cortisol in this study, may be explained by the differences in energy availability, training mode, and fitness levels of the subjects. The military training studies were performed under conditions of nutritional and sleep deprivation, in combination with high-energy expenditure and physiological strain. Most of these studies involved relatively short (from 5 to 14 days) training periods, and the subjects were soldiers. In the overtraining studies, food and sleep were ad libitum, and the subjects were athletes or at least had a good fitness level. Furthermore, the training period in the overtraining studies ranged from 2 to 7 weeks, consisting mostly of endurance or strength training. The subjects in the present study were low to moderately fit conscripts in the compulsory army, and training consisted of both strength and endurance training.
Recently, it has been proposed that the best indicator of OR and OTS is the physiological response to an acute stress, such as an exercise test (22). Only 47% of the OR subjects performed all submaximal tests because of sick absences. This may be the reason for the nonsignificant difference in cortisol responses to exercise between OR and noOR subjects. However, among the conscripts who were able to perform all submaximal tests, the exercise-induced increase in cortisol in this study was only significant after 4 weeks of training. It has been suggested that increased basal levels of cortisol contribute to exhaustion of the hypothalamic-pituitary-adrenal axis, thus preventing an adequate cortisol response to such an acute stress (10). In the present study, the attenuated cortisol response before training could be explained by the higher basal values, although this seems unlikely as pre-exercise cortisol levels were not associated with cortisol responses to the submaximal 45-minute marching test. In the compensated stage during training, adreno-corticotropic hormone (ACTH) and cortisol increase in response to metabolic stress; during overreaching, the cortisol response is blunted along with an augmented ACTH response (33). Overtraining is characterized by a decrease in ACTH and cortisol responses (33), and exercise-induced decreases in cortisol have been linked to intensive training (36). Thus, in this study, the increased response of cortisol to exercise after 4 weeks of training among the 35 conscripts who completed all tests indicates an improvement in performance and an attenuated response after 7 weeks is indicative of overreaching.
The fitness level of young Finnish men has decreased with a concurrent increase in BM (30). Currently, approximately 15% of new conscripts interrupt the compulsory military service within 1 month. Consequently, many of the conscripts have a low fitness level. However, the same physical training program has to be performed by all conscripts in the military environment. In the present study, improvements in physical performance were observed during the first 4 weeks of BT, such as an increase in maximal aerobic capacity and decreased submaximal exercise-induced stress. Although general diagnostic parameters to diagnose OR or OTS have not yet been found, several changes in biomarkers and physical performance may indicate that the training load during the second half of the 8-week training period was not well tolerated by all the conscripts. Furthermore, both emotional and somatic symptoms of OR increased.
Because 33% of the subjects were classified as overreached, the end of the BT period has to be taken under careful consideration. If demanding training is continued after BT, OTS may develop. Thus, the weeks after BT should involve recovery-type training. Furthermore, if the amount of sick absence is set as a criterion for the tolerance to exercise training, these results suggest that to enhance physical fitness and to avoid OTS, conscripts could be divided into different training groups according to their aerobic fitness level. Initial serum basal SHBG and testosterone, and testosterone/cortisol levels, would give additional information, but in practice, collecting the samples from a large group of subjects is challenging. However, our results confirm that serum cortisol and the testosterone/cortisol and maximal La/RPE ratios could be useful tools for monitoring whether training is too stressful.
This study was supported by grants from the Finnish Ministry of Education, Finnish Cultural Foundation, Polar Electro Oy, and the Scientific Advisory Board for Defence.
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