Effects of Task-Specific and Strength Training on Simulated Military Task Performance in Soldiers

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY 4.0 https://creativecommons.org/licenses/by/4.0/ Effects of Task-Specific and Strength Training on Simulated Military Task Performance in Soldiers © 2020 by the authors. Licensee MDPI, Basel, Switzerland. Published version Ojanen, Tommi; Häkkinen, Keijo; Hanhikoski, Jaakko; Kyröläinen, Heikki Ojanen, T., Häkkinen, K., Hanhikoski, J., & Kyröläinen, H. (2020). Effects of Task-Specific and Strength Training on Simulated Military Task Performance in Soldiers. International Journal of Environmental Research and Public Health, 17(21), Article 8000. https://doi.org/10.3390/ijerph17218000 2020 International Journal of Environmental Research and Public Health Article Effects of Task-Specific and Strength Training on Simulated Military Task Performance in Soldiers Tommi Ojanen 1,* , Keijo Häkkinen 2, Jaakko Hanhikoski 2and Heikki Kyröläinen 2,3 1Finnish Defence Research Agency, Finnish Defence Forces, P.O. Box 5, 04401 Järvenpää, Finland 2Biology of Physical Activity, Faculty of Sport and Health Sciences, University of Jyväskylä, P.O. Box 35, 40014 Jyväskylä, Finland; [email protected] (K.H.); [email protected] (J.H.); [email protected] (H.K.) 3National Defence University, Finnish Defence Forces, P.O. Box 7, 00861 Helsinki, Finland *Correspondence: [email protected] Received: 21 September 2020; Accepted: 29 October 2020; Published: 30 October 2020   Abstract: A soldier’s occupational physical task requirements are diverse and varied. However, the type of physical training that most effectively improves soldiers’ occupational task requirements has not been studied previously. The purpose of this study was to determine the important strength characteristics for soldiers during a repeated simulated military task course, and the type of training that may be effective to improve these abilities during a specialized military training period. Forty-two (n=42) soldiers participated in the study. They were divided into three training groups; a soldier task-specific training group (TSG, n=17), a strength training group (STG, n=15), and a control group (CON, n=10). Participants were measured before (PRE), middle (MID) and after (POST) the 12-week training intervention for strength performance and simulated military task test. Simulated military task performance improved significantly in TSG and STG between the PRE and MID measurements (from 9.4 to 15.7%). TSG and STG improved in various spilt times, especially in strength tasks; casualty drag (from 8.3 to 13.6%) and kettlebell carry (from 13.2 to 22.4%) between the PRE and MID measurements. The present study showed that both the training of TSG and STG were more effective than the training of CON (control group) in terms of improving the performance in the repeated simulated military task course. The present study showed that training of TSG was as effective as STG to improve repeated simulated military task course time. Therefore, an optimal training combination should include high-intensity simulated military task field training and strength training programmed with consideration of the military training phase and environmental possibilities. Keywords: neuromuscular performance; strength training; task-specific; occupational test; military; soldier 1. Introduction Common occupational physical tasks for soldiers can include patrolling long distances in different terrains, carrying and lifting objects with variable weight, and performing explosive movements in the battlefield [ 1 ]. All these requirements create complexity for constructing physical training programs for soldiers. In all, various factors including sex, age, physical training history, nutrition, recovery, and sleep, in addition to psychological, environmental, and social aspects can significantly influence physical training adaptations [ 2 ]. Moreover, optimizing a soldier’s performance in military environments is constantly challenged by external stress factors including carrying loads of various weights, sleep deprivation, extended physical activity, negative fluid and energy balance, and continuous readiness [ 3 – 5 ]. These stressors have been shown to cause disruptions in hormonal balance [ 6 – 9 ], leading to reduced physical and cognitive performance [ 3 , 10 – 13 ], prolonged recovery times [ 14 ], Int. J. Environ. Res. Public Health 2020,17, 8000; doi:10.3390/ijerph17218000 www.mdpi.com/journal/ijerph Int. J. Environ. Res. Public Health 2020,17, 8000 2 of 17 and increased susceptibility to infections [ 15 ]. Therefore, training load combined with these external stress factors can lead to compromised training adaptations, overreaching or even overtraining, in addition to increased musculoskeletal injury rates [ 16 – 18 ]. All these factors should be carefully considered when planning and implementing optimal physical training programs for soldiers. To meet the occupational requirements, it is important to determine the best training programs and periodization models for a soldier. Decreased overall physical activity and fitness of recruits creates more demands for their initial training [ 19 ]. The increase in daily activity in garrisons can result in a sudden increase in recruits’ physical activity, which increases the risk of overtraining or injury [ 16 , 18 , 20 , 21 ]. Therefore, it is crucial to have a well-planned and periodized physical training program taking into account the initial physical performance level. In the past, most physical programs have concentrated on improving aerobic endurance of recruits using traditional periodization [ 2 ]. A more suitable approach may be the use of an undulating or a block periodization model with elements of aerobic and anaerobic conditioning and strength training [ 22 – 27 ]. In addition, due to strict timetables, physical training should be incorporated into the occupational training of a soldier, and the majority of training should be performed wearing a uniform and in field conditions [ 28 ]. This would help soldiers to train more occupational skills and to better adapt to different terrains: possibly decreasing injuries during training. Both the decline in physical fitness and the higher demands in occupational requirements of the recruits have led to a situation, where systematically designed, tailored, and supervised physical training that meets the occupational task requirements needs to be performed during the basic training [ 29 ]. While designing a training plan for soldiers, it is essential to evaluate the components emphasizing the occupational task requirements. The result of physical training programs relies on training frequency, intensity (velocity and load), and volume (repetitions and duration), which form the foundation of training [ 2 ]. To enhance and optimize training adaptations and to reduce overtraining and training-related injuries in training, it is crucial to plan and organize the training individually [ 30 ] (p. 205). Nevertheless, it is critical that these emphasized training factors are well related to individual needs, and the initial fitness level of the trainee, and the training plan is well periodized. Additionally, variation in training stimulus is one of the most crucial components when considering the development of explosive or maximal strength and maximal aerobic capacity [ 30 ] (pp. 260–263 and 299–300). Daily military training can be too monotonous and may not enhance a soldier’s physical performance by the best possible way. Professional soldiers should have access to training programs, which first helps them to reach and then to maintain the required level of physical performance [31]. In the past, there have been studies regarding specific occupational task tests for soldiers [ 1 , 32 , 33 ]. These studies have shown that a soldier needs both aerobic and anaerobic endurance and muscle strength to fulfill occupational requirements. Mala et al. [ 32 ] observed that strength and power were strongly related to high-intensity military tasks with and without heavy load carriage. Pihlainen et al. [ 33 ] showed that the maximal countermovement jump (CMJ), 3000 m running time, skeletal muscle mass, and repeated push-ups explained about 60% of military simulation test time. In addition, in a study by Sporis et al. [ 26 ] anaerobic endurance and strength were found to be important factors in the soldiers’ performance. According to the current literature, it seems that there is no “gold standard” to measure soldiers’ occupational physical performance. As a consequence, the most effective way to train to improve these occupational requirements has not been studied sufficiently. Thus, the purpose of this study was to determine which physical abilities are important for soldiers during a repeated simulated military task course and which type of training (task specific, strength, or traditional military fitness training) would be useful and contribute to gains in these abilities during a specialized military training period. Int. J. Environ. Res. Public Health 2020,17, 8000 3 of 17 2. Materials and Methods 2.1. Subjects A training group of forty-two (n=42) male soldiers (age ranging from 18 to 22 years) participated in the study and performed all the measurements during the 12-week study period. Their mean ( ± SD) age was 20 ( ± 1) years, height 180 ( ± 6) cm, body mass 72.9 ( ± 9.3) kg, and body mass index 22.6 ( ± 2.3) kg · m −2 (Table 1). The subjects were all from the same infantry company, and they were divided into three training groups by their platoon as follows: soldier task specific training group (TSG), strength training group (STG), and control group (CON). The present study was conducted according to the provisions of the Declaration of Helsinki and the ethical statement of the Ethical Committee, the University of Jyväskylä. The study was approved by the Finnish Defence Forces. All the recruits were informed of the experimental design, and the benefits and possible risks that could be associated with the study prior to signing an informed consent to voluntary participate in the study. Table 1. Mean (±SD) age, height, body mass, and body mass index (BMI) in the study groups. Variable TSG (n=17) STG (n=15) CON (n=10) Age (years.) 20 (±1) 20 (±1) 20 (±1) Height (cm) 180 (±7) 183 (±6) 177 (±4) Body mass (kg) 73.2 (±9.8) 73.8 (±7.8) 71.1 (±11.0) BMI (kg·m−2)22.7 (±2.2) 22.1 (±1.7) 22.7 (±3.7) TSG, task specific group; STG, strength training group; CON, control group; BMI, body mass index. 2.2. Procedures The study was implemented during the latter part of the Finnish conscript service, after the eight-week basic training period (Figure 1). Physical performance tests and simulated military task performance were repeated three times during the training study, in the ninth week (PRE), sixteenth week (MID), and twenty-second week (POST) of their conscript service. The testing began in the morning with body composition measurements. After breakfast, the subjects performed muscular power, strength, and strength endurance tests. Finally, after lunch they performed the simulated military task performance test. Each training group completed their tests during the same day. Figure 1. Timeline of the study. 2.2.1. Neuromuscular Performance Maximal isometric bilateral force of the extensor muscles of the lower (MVClower) and upper (MVCupper) extremities was measured in a sitting position. The measurements were conducted using an electromechanical dynamometer [ 34 ] manufactured by the University of Jyväskylä (Jyväskylä, Int. J. Environ. Res. Public Health 2020,17, 8000 4 of 17 Finland). In the upper extremity test, the pushing bar was adjusted to the height of the shoulders and the distance of the seat was set to maintain an elbow angle of 90 ◦ . In the lower extremity test, the distance of the seat was set to maintain knee and hip angles of 107 ◦ and 110 ◦ , respectively. A countermovement jump (CMJ) was performed on a contact mat (Newtest, Oulu, Finland). Flight time between contacts was used to calculate the jumping height for each jump [ 35 ]. A six-second cycle ergometer test (Wattbike Ltd., Nottingham, UK) was used to measure maximal power of the lower extremities. The six-second test has a seated stationary start with a dominant leg initiating the first down-stroke. The test started following a five-second countdown followed by verbal command. The completion of the test was also indicated with another verbal command [36]. 2.2.2. Simulated Military Task Performance The simulated military task performance consisted of typical army soldier tasks and maneuvers, such as sprints, crawling, carrying objects, and casualty evacuation. It was performed inside on an artificial turf with soldiers wearing typical combat gear, including the helmet (total extra weight of 22-kg). From the starting (lying) supine position, the soldiers performed a 10 m sprint, followed by a 10 m low crawl. After the low crawl, the subjects lifted, carried, and lowered two 16-kg kettlebells (CompactFit Ltd., Helsinki, Finland) twice for a distance of 10 m taking a supine position when lowering the kettlebells. This was followed by a 75-kg sandbag drag (Rogue Sandbag, Rogue Fitness Europe Ltd., Pori, Finland) for 10 m, followed by a sprint of 10 m. The total length of the track was 60 m (Figure 2). Figure 2. Outline of the simulated military task performance course. Different tasks are numbered in the order of performance (1–5). The track was performed three times with a 60 s rest between the trials. Before the simulated military task performance, the subjects were individually familiarized with the track by a supervisor who also gave verbal instructions and encouraged the subjects during the test. The performance time was recorded by a stopwatch and video recorded for a later verification. The split times for different tasks were obtained offline from the video. Heart rate was recorded throughout the test by the Firstbeat team system (Firstbeat Technologies, Jyväskylä, Finland) and peak heart rate was determined from the data. Blood lactate was analyzed (Biosen c-line Sport, EKF Diagnostic, Madgeburg, Germany) from the fingertip (20 µL) five times (before, after each trial and 10 min after the final trial). The sensitivity for lactate analysis was 0.5 mmol/L and interassay coefficient of variation was 6.2%. Before and immediately after the simulated military task performance, the participants shot 10 rounds (Eko-Aims Ltd., Ylämylly, Finland) from a prone position with a similar army assault rifle replica (RK95, Finland) that they handle daily in their military training. The sum of ten shots was recorded for further analysis. Int. J. Environ. Res. Public Health 2020,17, 8000 5 of 17 2.3. Training Protocols Training programs were designed to be performed as a part of the recruits’ normal weekly physical training. During the 12-week training period, the total number of instructed physical training sessions performed by the recruits was 12 during the first six weeks and six sessions during the latter six weeks of the study. They also conducted their normal military training every day, which consisted of road marches, shooting exercises and other task specific exercises. Variation of the instructed physical training program was due to military field training (MFT), which was a part of their training program. During those weeks, the conscripts were not able to perform any extra physical training. All the groups participated in the instructed physical training, but the content of training varied between the groups. All of them performed 10 min active warm-up, which included body weight movements, active stretching, and running, before starting the actual exercise. The total length of one session was 60 min in all groups. The average daily RPE (Borg’s scale 6–20) varied during the training days as follows; TSG 10.7–13.4, STG 10.2–14.7, and CON 10.2–14.4 without significant differences between the groups. Training sessions of the TSG group included basic infantry-based exercise with full combat gear (27 kg), such as sprints, crawling, and casualty drag. Exercises were performed with anaerobic emphasis for 30–60 s. The STG group trained with the non-linear strength training program. The full-body program included squats, hamstring curls, pull and push exercises for the upper body, and different core exercises. The program began with low-load and high-repetition (40–60% of one-repetition maximum (1RM), 12–15 reps), continued to moderate-load and volume (70–85% of 1RM, 6–12 reps) culminating in high-load and low-volume training (85–100% of 1RM, 1–4 reps). CON performed normal Finnish Army military physical training only. A typical training session included circuit training with body weight, running with a constant pace, or playing different ball games. The training protocols for each group are presented in more detail in the previously published paper [37]. 2.4. Statistical Analysis The data for the present study was analyzed using commercial statistical software (IBM SPSS 24.0 Chicago, IL, USA). All data were checked for normality when calculating descriptive values for all variables. Conventional statistics were used to calculate means, 95% confidence intervals, and standard deviations ( ± SD). Effects sizes were calculated according to Cohen [ 38 ] (0.2 =small; 0.5 =medium; and 0.8 =large effect). Probability adjusted ttests were used for pairwise comparisons when appropriate. A general linear model, with repeated measures ANOVA with group as a fixed factor, was used to analyze the time × group interaction and the differences between the different measuring points. Associations between physical performance and military task test were performed using a Pearson correlation coefficient. Statistical significance for this investigation was set at p≤0.05 (two-tailed). 3. Results 3.1. Neuromuscular Tests Maximal isometric strength produced during the bilateral leg press increased significantly between the PRE and MID and PRE and POST measurements in TSG, but not in STG and CON. Maximal isometric strength during the bilateral bench press increased significantly in STG between the PRE and MID and PRE and POST measurements. In TSG, a significant increase occurred between the MID and POST measurements. No significant differences between the groups were observed in these maximal isometric forces. In all groups significant increases occurred in 6 s maximal anaerobic power in the cycle ergometer test between PRE and POST (Table 2). TSG and STG also showed significant increases in power between the PRE and MID measurements, but not CON. In CMJ, a significant increase occurred in jump height between PRE and POST in TSG and between MID and POST in TSG. No significant differences were observed between the groups in the changes in CMJ. Int. J. Environ. Res. Public Health 2020,17, 8000 6 of 17 Table 2. Mean ±SD, 95% confidence interval (CI), and effect size values in the maximal neuromuscular tests in the PRE, MID, and POST measurement points. PRE MID POST |Effect Size| Group Mean (±SD) 95% CI Mean (±SD) 95% CI Mean (±SD) 95% CI 1 vs. 2 2 vs. 3 1 vs. 3 CMJ (cm) TSG 29 ±6 26; 32 29 ±5 26; 31 31 ±4 *,# 28; 33 0.00 0.43 0.38 STG 33 ±6 29; 36 33 ±5 30; 36 35 ±4 32; 37 0.00 0.43 0.38 CON 32 ±6 28; 36 31 ±7 26; 35 31 ±6 27; 35 0.15 0.00 0.16 POWER (w) TSG 1036 ±140 961; 1110 1078 ±138 * 1004; 1151 1094 ± 125 ** 1027; 1160 0.30 0.12 0.43 STG 1097 ±98 1043; 1152 1140 ±92 * 1090; 1192 1141 ±103 * 1084; 1198 0.44 0.01 0.43 CON 974 ±122 886; 1061 1007 ±144 904; 1110 1027 ±147 * 922; 1132 0.24 0.13 0.38 UPPER BODY STRENGTH (kg) TSG 78 ±14 71; 86 77 ±13 70; 84 80 ±12 ## 74; 87 0.07 0.23 0.15 STG 79 ±14 72; 87 83 ±14 * 75; 90 84 ±14 ** 77; 92 0.28 0.06 0.35 CON 75 ±14 65; 85 75 ±15 65; 86 77 ±16 65; 88 0.00 0.17 0.18 LOWER BODY STRENGTH (kg) TSG 236 ±40 214; 257 252 ±39 * 231; 273 255 ±50 ** 239; 284 0.40 0.07 0.41 STG 229 ±49 202; 256 242 ±39 221; 264 244 ±39 223; 266 0.29 0.05 0.33 CON 224 ±50 189; 260 234 ±57 193; 275 236 ±56 195; 276 0.18 0.03 0.22 CMJ, countermovement jump; POWER, maximal 6 s power cycling; UPPER BODY STRENGTH, maximal isometric bilateral extension; LOWER BODY STRENGTH, maximal bilateral extension; TSG, task specific group; STG, strength training group; CON, control group. * =compared to PRE values * =p<0.05, ** =p<0.01; # =compared to MID values, # =p<0.05, ## p<0.01. Int. J. Environ. Res. Public Health 2020,17, 8000 7 of 17 3.2. Simulated Military Task Performance 3.2.1. Total Time The total time of the first trial in the simulated military task performance improved in TSG and STG between the PRE and MID measurements by 11.1% (ES =0.64) and 9.4% (ES =0.91) and between the PRE and POST measurements by 11.4% (ES =0.74) and 8.8% (ES =0.72), respectively. No significant changes were observed in CON between the measurements in the first trial. In the second trial, the total time improved significantly in TSG and STG between the PRE and MID measurements by 14.6% and 13.1%. All groups showed significant improvements between the PRE and POST measurements (TSG: 17.0%, STG: 14.1%, and CON: 12.0%, ES =1.35, 1.12, and 0.76, respectively). There was also a significant improvement in CON between the MID and POST measurements (6.3%, ES =0.36). A similar trend was also observed in the third trial. Significant improvement was found between the PRE and MID measurements in TSG (14.6%, ES =0.78) and STG (15.7%, ES =1.05), but not in CON (2.1%, ES =0.08 ). Between the PRE and POST measurements, a significant improvement occurred in all groups (TSG 19.2%, ES =1.01, STG 18.2%, ES =1.23, and CON 12.1%, ES =0.77). TSG and CON showed a significant improvement between the MID and POST measurements (TSG 5.3%, ES =0.32, and CON 9.6%, ES =0.52; Figure 3). Figure 3. Total time (s) in the simulated military task course (first, second, and third trial). STG =strength training group ; TSG =soldier task specific group; CON =control group; *=p<0.05 , ** =p<0.01, *** =p<0.001. 3.2.2. Five Meters Run from the Start No significant changes were found in running time over the first 5 m during the first trial. (Table 3) During the second trial, TSG and CON significantly improved between the PRE and POST measurements. (Table 4) In the third trial, TSG and STG significantly improved between PRE and MID. CON improved significantly only between the PRE and POST measurements (Table 5). Int. J. Environ. Res. Public Health 2020,17, 8000 8 of 17 Table 3. Simulated military task performance times for different tasks (first 5 m run, crawl, kettlebell carry (KB), casualty drag, and final 10 m run) from the first trial in the PRE, MID, and POST measurements points. First Trial PRE MID POST |Effect Size| Performance Measure Group Mean (±SD) 95% CI Mean (±SD) 95% CI Mean (±SD) 95% CI 1 vs. 2 2 vs. 3 1 vs. 3 5 m run (s) TSG 2.5 ±0.3 2.4; 2.7 2.5 ±0.4 2.3; 2.7 2.4 ±0.3 2.3; 2.6 0.23 0.13 0.41 STG 2.5 ±0.2 2.4; 2.6 2.4 ±0.2 2.3; 2.5 2.4 ±0.2 2.3; 2.5 0.32 0.10 0.39 CON 2.6 ±0.4 2.3; 2.9 2.5 ±0.3 2.3; 2.8 2.5 ±0.3 2.3; 2.7 0.22 0.03 0.26 Crawl (s) TSG 8.0 ±2.0 7.0; 9.1 6.7 ±1.8 *** 5.8; 7.7 6.4 ±1.3 *** 5.7; 7.1 0.69 0.22 0.96 STG 7.5 ±1.4 6.8; 8.3 6.5 ±1.2 ** 5.8; 7.2 6.4 ±1.7 ** 5.4; 7.4 0.81 0.05 0.74 CON 7.3 ±1.0 6.5; 8.0 6.6 ±1.8 5.4; 7.9 6.6 ±1.8 5.3; 7.8 0.47 0.05 0.53 KB carry (s) TSG 12.0 ±1.9 11.0; 12.9 11.0 ±1.7 *** 10.1; 11.8 11.1 ±1.8 * 10.2; 12.0 0.58 0.08 0.50 STG 11.5 ±1.2 10.8; 12.1 10.8 ±1.1 * 10.2; 11.4 10.9 ±1.4 10.1; 11.7 0.61 0.07 0.47 CON 11.5 ±1.3 10.6; 12.4 11.2 ±1.4 10.2; 12.2 11.2 ±1.3 10.2; 12.1 0.22 0.04 0.26 Drag (s) TSG 11.7 ±3.7 9.8; 13.6 9.8 ±2.4 *** 8.6; 11.1 9.7 ±2.3 *** 8.5; 10.9 0.61 0.04 0.65 STG 10.6 ±2.4 9.3; 12.0 9.2 ±1.6 ** 8.4; 10.1 9.6 ±1.9 * 8.5; 10.7 0.72 0.11 0.49 CON 10.7 ±2.0 9.3; 12.1 10.6 ±3.2 8.3; 12.9 10.5 ±2.9 8.4; 12.5 0.04 0.05 0.11 10 m run (s) TSG 3.1 ±0.4 2.9; 3.4 3.0 ±0.4 * 2.8; 3.1 2.9 ±0.3 * 2.8; 3.1 0.54 0.03 0.64 STG 3.1 ±0.4 2.9; 3.4 2.9 ±0.3 *** 2.7; 3.0 2.9 ±0.3 * 2.7; 3.0 0.74 0.18 0.72 CON 3.1 ±0.5 2.7; 3.4 3.0 ±0.5 2.6; 3.4 3.0 ±0.6 2.6; 3.4 0.14 0.04 0.10 TSG =soldier task specific group; STG =strength training group; CON =control group; * =compared to PRE values * =p<0.05, ** =p<0.01, *** =p<0.001. Table 4. Simulated military task performance times for different tasks (first 5 m run, crawl, kettlebell carry (KB), casualty drag, and final 10 m run) from the second trial in the PRE, MID, and POST measurement points. Second Trial PRE MID POST |Effect Size| Performance Measure Group Mean (±SD) 95% CI Mean (±SD) 95% CI Mean (±SD) 95% CI 1 vs. 2 2 vs. 3 1 vs. 3 5 m run (s) TSG 2.6 ±0.4 2.4; 2.8 2.5 ±0.3 2.3; 2.6 2.5 ±0.3 * 2.3; 2.6 0.24 0.21 0.21 STG 2.6 ±0.2 2.4; 2.7 2.5 ±0.2 2.4; 2.7 2.4 ±0.2 2.3; 2.6 0.10 0.43 0.55 CON 2.8 ±0.2 2.7; 3.0 2.6 ±0.5 2.3; 3.0 2.6 ±0.4 * 2.4; 2.9 0.60 0.05 0.68 Crawl (s) TSG 9.7 ±2.9 8.2; 11.2 7.7 ±2.0 *** 6.7; 8.7 7.1 ±1.5 ***,## 6.3; 7.9 0.81 0.37 1.12 STG 8.9 ±1.9 7.8; 9.9 7.7 ±1.5 * 6.9; 8.6 7.5 ±1.9 *** 6.4; 8.5 0.69 0.16 0.77 CON 8.4 ±1.7 7.2; 9.7 7.8 ±2.2 6.2; 9.4 7.1 ±1.8 **,# 5.9; 8.4 0.33 0.35 0.78 KB carry (s) TSG 14.1 ±3.4 12,4; 15.8 12.3 ±1.9 ** 11.4; 13.3 11.9 ±1.8 ***,# 11.0; 12.8 0.68 0.23 0.85 STG 13.0 ±1.5 12.2; 13.8 11.9 ±1.1 *** 11.2; 12.5 11.6 ±1.2 *** 10.9; 12.3 0.90 0.22 1.06 CON 13.0 ±2.0 11.6; 14.4 12.7 ±2.1 11.2; 14.2 11.7 ±1.6 **,### 10.6; 12.8 0.33 0.58 0.79 Int. J. Environ. Res. Public Health 2020,17, 8000 15 of 17 12. Nindl, B.C.; Leone, C.D.; Tharion, W.J.; Johnson, R.F.; Castellani, J.W.; Patton, J.F.; Montain, S.J. Physical performance responses during 72 h of military operational stress. Med. Sci. Sports Exerc. 2002 ,34, 1814–1822. [CrossRef] 13. Richmond, V.L.; Horner, F.E.; Wilkinson, D.M.; Rayson, M.P.; Wright, A.; Izard, R. Energy Balance and Physical Demands during an 8-Week Arduous Military Training Course. Mil. Med. 2014 ,179, 421–427. [CrossRef] 14. Henning, P.C.; Scofield, D.E.; Spiering, B.A.; Staab, J.S.; Matheny, R.W.; Smith, M.A.; Bhasin, S.; Nindl, B.C. Recovery of Endocrine and Inflammatory Mediators Following an Extended Energy Deficit. J. Clin. Endocrinol. Metab. 2014,99, 956–964. [CrossRef] 15. Diment, B.C.; Fortes, M.B.; Greeves, J.P.; Casey, A.; Costa, R.J.S.; Walters, R.; Walsh, N.P. Effect of daily mixed nutritional supplementation on immune indices in soldiers undertaking an 8-week arduous training programme. Graefe’s Arch. Clin. Exp. Ophthalmol. 2011,112, 1411–1418. [CrossRef] 16. Knapik, J.J.; Sharp, M.A.; Canham-Chervak, M.; Hauret, K.; Patton, J.F.; Jones, B.H. Risk factors for training-related injuries among men and women in basic combat training. Med. Sci. Sports Exerc. 2001 ,33, 946–954. [CrossRef] [PubMed] 17. Rosendal, L.; Langberg, H.; Flyvbjerg, A.; Frystyk, J.; Ørskov, H.; Kjaer, M. Physical capacity influences the response of insulin-like growth factor and its binding proteins to training. J. Appl. Physiol. 2002 ,93, 1669–1675. [CrossRef] [PubMed] 18. Tanskanen, M.M.; Kyröläinen, H.; Uusitalo, A.L.; Huovinen, J.; Nissilä, J.; Kinnunen, H.; Atalay, M.; Häkkinen, K. Serum Sex Hormone–Binding Globulin and Cortisol Concentrations are Associated with Overreaching during Strenuous Military Training. J. Strength Cond. Res. 2011,25, 787–797. [CrossRef] 19. Knapik, J.J.; E Redmond, J.; Grier, T.L.; A Sharp, M. Secular Trends in the Physical Fitness of United States Army Infantry Units and Infantry Soldiers, 1976–2015. Mil. Med. 2018,183, e414–e426. [CrossRef] 20. Ojanen, M.T.; Häkkinen, K.; Vasankari, T.; Kyröläinen, H. Changes in Physical Performance during 21 d of Military Field Training in Warfighters. Mil. Med. 2018,183, e174–e181. [CrossRef] 21. Santtila, M.; Pihlainen, K.; Viskari, J.; Kyröläinen, H. Optimal Physical Training during Military Basic Training Period. J. Strength Cond. Res. 2015,29, S154–S157. [CrossRef] [PubMed] 22. Abt, J.P.; Perlsweig, K.; Nagai, T.; Sell, T.C.; Wirt, C.M.D.; Lephart, S.M. Effects of Age and Military Service on Strength and Physiological Characteristics of U.S. Army Soldiers. Mil. Med. 2016 ,181, 173–179. [CrossRef] [PubMed] 23. Grant, C.C.; Mongwe, L.; Van Rensburg, D.C.J.; Fletcher, L.; Wood, P.; Terblanche, E.; Du Toit, P.J.; Grant, T.C. The Difference Between Exercise-Induced Autonomic and Fitness Changes Measured after 12 and 20 Weeks of Medium-to-High Intensity Military Training. J. Strength Cond. Res. 2016 ,30, 2453–2459, Erratum in J. Strength Cond. Res. 2017,31, e67. [CrossRef] [PubMed] 24. Lester, M.E.; Sharp, M.A.; Werling, W.C.; Walker, L.A.; Cohen, B.S.; Ruediger, T.M. Effect of Specific Short-Term Physical Training on Fitness Measures in Conditioned Men. J. Strength Cond. Res. 2014 ,28, 679–688. [CrossRef] 25. Solberg, P.A.; Paulsen, G.; Slaathaug, O.G.; Skare, M.; Wood, D.; Huls, S.; Raastad, T. Development and Implementation of a New Physical Training Concept in the Norwegian Navy Special Operations Command. J. Strength Cond. Res. 2015,29, S204–S210. [CrossRef] 26. Sporiš, G.; Harasin, D.; Bai´c, M.; Kristiˇcevi´c, T.; Krakan, I.; Milanovi´c, Z.; Cular, D.; Bagari´c-Krakan, L. Effects of two different 5 weeks training programs on the physical fitness of military recruits. Coll. Antropol. 2014 , 38, S157–S164. 27. Vantarakis, A.; Chatzinikolaou, A.; Avloniti, A.; Vezos, N.; Douroudos, I.I.; Draganidis, D.; Jamurtas, A.Z.; Kambas, A.; Kalligeros, S.; Fatouros, I.G. A 2-Month Linear Periodized Resistance Exercise Training Improved Musculoskeletal Fitness and Specific Conditioning of Navy Cadets. J. Strength Cond. Res. 2017 ,31, 1362–1370. [CrossRef] 28. Kilen, A.; Hjelvang, L.B.; Dall, N.; Kruse, N.L.; Nordsborg, N.B. Adaptations to Short, Frequent Sessions of Endurance and Strength Training Are Similar to Longer, Less Frequent Exercise Sessions When the Total Volume Is the Same. J. Strength Cond. Res. 2015,29, S46–S51. [CrossRef] Int. J. Environ. Res. Public Health 2020,17, 8000 16 of 17 29. Kraemer, W.J.; Szivak, T.K. Strength Training for the Warfighter. J. Strength Cond. Res. 2012 ,26, S107–S118. [CrossRef] 30. Bomba, T.; Buzzichelli, C. Periodization; Theory and Methodology of Training, 6th ed.; Human Kinetics: Champaign, IL, USA, 2019. 31. Friedl, K.E.; Knapik, J.J.; Häkkinen, K.; Baumgartner, N.; Groeller, H.; Taylor, N.A.; Duarte, A.F.; Kyröläinen, H.; Jones, B.H.; Kraemer, W.J.; et al. Perspectives on Aerobic and Strength Influences on Military Physical Readiness. J. Strength Cond. Res. 2015,29, S10–S23. [CrossRef] 32. Mala, J.; Szivak, T.K.; Flanagan, S.D.; Comstock, B.A.; Laferrier, J.Z.; Maresh, C.M.; Kraemer, W.J. The role of strength and power during performance of high intensity military tasks under heavy load carriage. U.S. Army Med. Dep. J. 2015 , 3–11. Available online: https://pubmed.ncbi.nlm.nih.gov/26101902/(accessed on 20 September 2020). 33. Pihlainen, K.; Santtila, M.; Häkkinen, K.; Kyröläinen, H. Associations of Physical Fitness and Body Composition Characteristics with Simulated Military Task Performance. J. Strength Cond. Res. 2018 ,32, 1089–1098. [CrossRef] [PubMed] 34. Häkkinen, K.; Kallinen, M.; Izquierdo, M.; Jokelainen, K.; Lassila, H.; Mälkiä, E.; Kraemer, W.J.; Newton, R.U.; Alen, M. Changes in agonist-antagonist EMG, muscle CSA, and force during strength training in middle-aged and older people. J. Appl. Physiol. 1998,84, 1341–1349. [CrossRef] [PubMed] 35. Bosco, C.; Luhtanen, P.; Komi, P.V. A simple method for measurement of mechanical power in jumping. Graefe’s Arch. Clin. Exp. Ophthalmol. 1983,50, 273–282. [CrossRef] 36. Herbert, P.; Sculthorpe, N.; Baker, J.S.; Grace, F.; Sculthorpe, N.F. Validation of a Six Second Cycle Test for the Determination of Peak Power Output. Res. Sports Med. 2015,23, 115–125. [CrossRef] 37. Ojanen, T.; Kyröläinen, H.; Kozharskaya, E.; Häkkinen, K. Changes in strength and power performance and serum hormone concentrations during 12 weeks of task-specific or strength training in conscripts. Physiol. Rep. 2020,8, e14422. [CrossRef] 38. Cohen, J. Statistical Power Analysis for the Behavioral Sciences, 2nd ed.; Erlbaum: Hillsdale, NJ, USA, 1988. 39. Conkright, W.R.; Barringer, N.D.; Lescure, P.B.; Feeney, K.A.; Smith, M.A.; Nindl, B.C. Differential recovery rates of fitness following U.S. Army Ranger training. J. Sci. Med. Sport 2020,23, 529–534. [CrossRef] 40. Hickson, R.C.; Rosenkoetter, M.A.; Brown, M.M. Strength training effects on aerobic power and short-term endurance. Med. Sci. Sports Exerc. 1980,12, 336–339. [CrossRef] [PubMed] 41. Wilson, J.M.; Marin, P.J.; Rhea, M.R.; Wilson, S.M.; Loenneke, J.P.; Anderson, J.C. Concurrent Training. J. Strength Cond. Res. 2012,26, 2293–2307. [CrossRef] 42. Coffey, V.G.; Hawley, J.A. Concurrent exercise training: Do opposites distract? J. Physiol. 2016 ,595, 2883–2896. [CrossRef] 43. Harman, E.A.; Gutekunst, D.J.; Frykman, P.N.; Nindl, B.C.; Alemany, J.A.; Mello, R.P.; Sharp, M.A. Effects of two different eight-week training programs on military physical performance. J. Strength Cond. Res. 2008 ,22, 524–534. [CrossRef] 44. Vaara, J.P.; Kokko, J.; Isoranta, M.; Kyröläinen, H. Effects of Added Resistance Training on Physical Fitness, Body Composition, and Serum Hormone Concentrations during Eight Weeks of Special Military Training Period. J. Strength Cond. Res. 2015,29, S168–S172. [CrossRef] 45. Santtila, M.; Häkkinen, K.; Nindl, B.C.; Kyröläinen, H. Cardiovascular and Neuromuscular Performance Responses Induced by 8 Weeks of Basic Training Followed by 8 Weeks of Specialized Military Training. J. Strength Cond. Res. 2012,26, 745–751. [CrossRef] 46. Gamble, P. Implications and Applications of Training Specificity for Coaches and Athletes. Strength Cond. J. 2006,28, 54–58. [CrossRef] 47. Gibala, M. Molecular responses to high-intensity interval exercise. Appl. Physiol. Nutr. Metab. 2009 ,34, 428–432. [CrossRef] [PubMed] 48. Knapik, J.J.; A Harman, E.; A Steelman, R.; Graham, B.S. A Systematic Review of the Effects of Physical Training on Load Carriage Performance. J. Strength Cond. Res. 2012,26, 585–597. [CrossRef] [PubMed] 49. Kraemer, W.J.; Ratamess, N.A. Fundamentals of Resistance Training: Progression and Exercise Prescription. Med. Sci. Sports Exerc. 2004,36, 674–688. [CrossRef] Int. J. Environ. Res. Public Health 2020,17, 8000 17 of 17 50. Harman, E.A.; Gutekunst, D.J.; Frykman, P.N.; Sharp, M.A.; Nindl, B.C.; Alemany, J.A.; Mello, R.P. Prediction of simulated battlefield physical performance from field-expedient tests. Mil. Med. 2008,173, 36–41. [CrossRef] 51. Treloar, A.K.L.; Billing, D.C. Effect of Load Carriage on Performance of an Explosive, Anaerobic Military Task. Mil. Med. 2011,176, 1027–1031. [CrossRef] Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. © 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).