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Original Research

The Difference Between Exercise-Induced Autonomic and Fitness Changes Measured After 12 and 20 Weeks of Medium-to-High Intensity Military Training

Grant, Catharina C.; Mongwe, Lot; Janse van Rensburg, Dina C.; Fletcher, Lizelle; Wood, Paola S.; Terblanche, Etrisia; du Toit, Peet J.

Author Information
The Journal of Strength & Conditioning Research: September 2016 - Volume 30 - Issue 9 - p 2453-2459
doi: 10.1519/JSC.0b013e3182a1fe46

Abstract

Erratum

In the September 2016 issue of the in the article by Grant, Mongwe, Janse van Rensburg, Fletcher, Wood, Terblanche, and du Toit, “The Difference Between Exercise-Induced Autonomic and Fitness Changes Measured After 12 Weeks and 20 Weeks of Medium-to-High Intensity Military Training”, the affiliation for author Etrisia Terblanche should be listed as “South African National Defense Force, Pretoria, South Africa”, and the affiliation for author Peet J. Du Toit should be listed as “Physiology, University of Pretoria, Pretoria, South Africa”.

The Journal of Strength & Conditioning Research. 31(3):e67, March 2017.

Introduction

A primary aim of basic military training programs is to develop the physical fitness, stamina, and endurance of the soldiers to enable them to perform tasks of combat, peacekeeping, and relief missions during natural disasters. However, the time spent by different countries on basic training to achieve combat-specific physical fitness varies. The U.S. Army uses a 10-week basic training program to transform civilians to soldiers. The British Army fitness program is a 16-week program that has been developed by the Army Physical Training Corps. Basic military training in the Finnish Defense Force is 8 weeks long, including a gradual increase in physical fitness training, reaching a peak between 5 and 8 weeks (24).

In the South African National Defence Force (SANDF), 12 weeks of basic training was previously the standard, but it was recently (2010) changed to 20 weeks (28). In this study, the investigation, prompted by the SA military, focused on recruit fitness (as measured by 2max">VO2max), and autonomic function (measured by heart rate variability [HRV]), without considering other aspects such as muscle strength, bone and tendon adaptations, tactical performances, and others. The practical research question, specified by the SANDF, was to establish if after a medium-to-high intensity basic training program for 20 weeks produces a better conditioned recruit, specifically in terms of exercise ability, endurance, and autonomic function, than only 12 weeks of the same basic training. This is a critical issue for reasons such as manpower availability, cost of training, and availability of military personnel and infrastructure. Optimum recruit fitness levels are of paramount importance for survival, while a secondary benefit is the reduced risk of musculoskeletal injuries with increased fitness (20). On the other hand, the danger exists that 20 weeks of intensive basic training may be too harsh and signs of overreaching and overtraining may manifest.

Monitoring exercise ability and exercise-induced autonomic changes during a highly standardized exercise intervention, in a large homogeneous group of participants, may provide valuable information not only for the military but also for coaches and clinicians. Thus, the aim of the study was to evaluate the exercise ability, as measured by indirect 2max">VO2max, and the exercise-induced changes in supine and standing cardiac autonomic control, measured by heart rate (HR) and HRV, after 12 weeks and again after 20 weeks of intensive basic training in the SANDF. It was hypothesized that the 20 weeks of basic training would yield higher exercise ability (2max">VO2max), lower HR, lower relative sympathetic cardiac control, and higher parasympathetic (vagal) cardiac influence when compared with only 12 weeks of basic training.

Methods

Experimental Approach to the Problem

The study design was an experimental prospective study on a group of SANDF recruits aged between 18 and 22 years, where the participants acted as their own control. As stated above, the research questions asked by the SANDF focused around the training effects of 12 weeks vs. 20 weeks on (a) exercise ability/fitness, (b) autonomic function, and (c) the risk of developing overtraining symptoms.

Measurement of Exercise Ability/Fitness

Because of the large number of participants, and the logistical, infrastructure, and time constraints, exercise capacity/fitness was determined with only 1 indicator of exercise ability/fitness, i.e., the indirect calculation of 2max">VO2max using the validated 2.4-km run test (6).

Measurement of Autonomic Function and Evaluation of Overtraining Syndrome Risk

Autonomic function can be evaluated with the noninvasive quantification of HRV, and it has been proposed (but not confirmed) as a tool for the diagnosis of overtraining syndrome (OTS) (9,12,16,27). Heart rate variability refers to the beat-to-beat alterations in HR because of the fluctuating influence of the sympathetic and parasympathetic pathway of the autonomic nervous system (ANS) cardiac control (2,10,18,19). Overtraining syndrome develops from a continuous imbalance between high levels of physical training and recovery time, which may manifest in hormonal fluctuations (17). This may lead to an autonomic imbalance that can be observed as a parasympathetic type of overtraining, also known as the Addison type, or the sympathetic type, also called the Basedow type (15). It has been proposed that during the initial stages of OTS, the activity of the sympathetic branch of the ANS is increased, triggering a change in the ANS balance (17). These changes may be observed in small increases in resting HR and the changes in the variability of between beat (RR) intervals.

Several studies suggested the use of HRV as indicator of overtraining (9,12,16,21,27), especially the autonomic balance indicator low frequency (LF)/high frequency (HF) (LF HRV power divided by HF HRV power). However, review articles by Bosquet et al. and Aubert et al. indicated limited value of HRV quantification during identification and prevention of overtraining. Still, both authors stressed the important influence (and the current lack) of standardized protocols and the small sample numbers in available studies. It was thus decided to use HR and HRV quantification, one of the most accessible physiological measures in sports medicine (5), to test the present study hypotheses. The dependent variables (2max">VO2max, supine and standing HR, RR intervals, and HRV indicators) were measured at baseline, 12 weeks, and 20 weeks of training.

Subjects

The study protocol was approved by the SANDF Ethics Committee and the Ethics Committee of the Faculty of Health Sciences, University of Pretoria. One hundred fifty-four (male, 89; female, 65) healthy recruits, aged between 18 and 22 years (mean = 20.91, SD = 1.29) with a body mass index (BMI) of 22.85 kg·m−2, SD = 2.78, participated in the 20 weeks medium-to-high intensity exercise intervention. None of the participants were professional athletes or high-level sport participants, during the training period July through November 2010 at the Lephalale Military Base in Southern Africa. All participants were subjected to the same strictly standardized 24-hour routine (exercise, diet, and sleep) for the duration of the 20 weeks of Basic Training program. All participants gave written informed consent to use their data before commencement of the basic training program. Recruits who did not agree to participate in the study were not included in the data sampling sessions. Exclusion criteria included a history of cardiovascular, hepatic, respiratory, or renal impairment, as well as pulmonary, metabolic, and orthopedic disease requiring medical attention; lung/respiratory tract infection in the previous 2 weeks; and the use of medication that could influence cardiovascular control.

Procedures

Data sampling included recruit's supine and standing HRV measurements, BMI, and their exercise capacity/fitness (2max">VO2max). During the HRV data sampling, special attention was given to standardization of temperature, luminosity, noise, and the consumption of caffeine and alcohol (5). The HRV data were sampled in the morning, between 0700 and 1100 hours, in a quiet environment at a temperature between 19 and 22° C. Participants did not exercise or drink any alcohol or caffeine during the preceding 24 hours. They fasted from 22:00 hours the previous night, had a good night's rest and were allowed to eat a low-protein breakfast (cereal with milk) in the morning of testing. Subjects remained calm and quiet before and during the test. Both supine and standing HRV measurements were recorded as it is theorized that posture change and an orthostatic challenge may highlight ANS changes better than measurements taken in only the resting supine position (10). The rested, fully hydrated recruits were tested in a supine position with spontaneous breathing for 10 minutes, followed by a standing period (backs leaning against the wall and their feet apart) of 10 minutes. Supine RR-intervals, from tachograms, obtained with Polar HR monitors (Polar Electro Oy, Kempele, Finland; sampling rate 500 Hz), measured during the 5 minutes before rising and 5 minutes of standing (starting 2 minutes after rising) were used for HRV analyses.

Heart rate variability analyses were performed with software obtained from the University of Kuopio, Finland. The time domain, frequency domain, and Poincaré plot analysis methods were used in the quantification of HRV from RR interval data sets (2,25,26). Time domain indicators were determined by direct statistical analysis of the time (milliseconds) between consecutive heartbeats. Calculated indicators included average SDNNs (the SD of normal-to-normal interbeat intervals, estimating overall HRV), RMSSD (the square root of the mean squared differences of successive RR intervals and an estimate of vagal influence or short-term HRV components), and pNN50 (percentage of successive RR interval differences larger than 50 ms computed over the entire recording and an indicator of vagal influence or short-term HRV). The power spectrums of the RR intervals were obtained with autoregressive spectral analysis. The 2 main frequency bands observed in this study were LF (between 0.05 and 0.15 Hz) and HF (from 0.15 to 0.4 Hz). Low frequency and HF power were calculated in square milliseconds and normalized units (percentage of LF + HF), and the ratio of the two, i.e., LF/HF. With power spectrum analysis of the tachograms, it is possible to distinguish between the intrinsic sources of HRV, as these rhythms occur at different frequencies. Low frequency power is not only an indicator of sympathetic influence but also includes a parasympathetic component. High-frequency power is an indicator of only parasympathetic influence, and LF/HF represents the balance between the sympathetic and parasympathetic branches of the ANS (1). The RR intervals of the tachograms were plotted as a function of the preceding intervals, to produce a Poincaré plot. From this graph, 2 HRV indicators, SD1 and SD2, were determined. SD1 is an indicator of the SD of the immediate or short term, RR variability because of parasympathetic efferent (vagal) influence on the sinoatrial node. SD2 is an indicator of the SD of the long-term or slow variability of the HR. It is accepted that this value is a representative of the global variation in HRV (26).

Training Intervention

Basic training in the SANDF must ensure a combat-ready recruit at the end of this period. During the 20-week period, the soldiers completed activities, including drill, regimental aspects, compliments and saluting, musketry, shooting, signal training, mine awareness, map reading, buddy aid, field craft, water orientation, parade rehearsal, and physical training. The aim of the latter is to increase basic fitness components such as cardiorespiratory and muscular endurance and consists of vigorous exercise with fitness standards being raised incrementally. The physical and basic training program followed by the SANDF recruits is outlined in Table 1 and is classified as a medium-to-high intensity exercise program because an average of 8,485 kJ·per day were consumed (28).

Table 1
Table 1:
The physical training program.

Statistical Analyses

The exploratory analysis ascertained that several of the variables were positively skewed. The Kolmogorov-Smirnov and Shapiro-Wilks tests were hence used to determine the indicators that deviated significantly from the normal distribution. Because of the extremely skewed nature of several of the variables, it was decided to use nonparametric tests to assess the hypotheses. The Friedman test, which is analogous to a repeated measure analysis of variance, is appropriate to evaluate the impact of the exercise intervention across the 3 periods (baseline, 12 weeks, and 20 weeks). In the case of significant results from the Friedman tests, the Wilcoxon signed rank test was used to determine exercise-induced changes; the latter is analogous to a paired samples T-test. Effect sizes were calculated for the significant results of the Wilcoxon signed rank tests. The conventional 5% level of significance was specified.

Results

Body mass index, the 2.4-km run time, and the calculated exercise capacity (2max">VO2max) on weeks 1, 12, and 20 are summarized in Table 2.

Table 2
Table 2:
Participant (N = 154) body mass index and exercise capacity (2max">VO2max) on weeks 1, 12, and 20.

Because the hypotheses were evaluated using nonparametric tests which are based on the ranks and medians of the indicators, as opposed to the means in parametric tests, the medians and quartiles of the HRV study values are summarized in Tables 3 and 4, indicating supine and standing HRV indicator values at baseline, after 12, and 20 weeks.

Table 3
Table 3:
Participant (N = 154) supine median HRV indicator values at baseline, after 12 and 20 weeks.*
Table 4
Table 4:
Participant (N = 154) standing median HRV indicator at baseline, after 12 and 20 weeks.*

Effect size calculations indicated that the minimum effect size for changes in the supine HRV indicator values after 12 weeks is r = 0.20 (for SD of the RR intervals [STDRR]) and the maximum is r = 0.65 (for HR); whereas for the changes between weeks 12 and 20, the minimum effect size is r = 0.18 (for STDRR) and the maximum is r = 0.22 (for RR). For standing HRV indicator values, the effect sizes after 12-week range from 0.21 (for HF) to 0.58 (for HR); the effect sizes for changes between weeks 12 and 20 are similar to those in the supine position and also range from 0.18 (for STDRR) to 0.22 (for RR). Based on the guidelines that r = 0.10 represents a small effect size, r = 0. 30 represents a medium effect size, and r = 0.50 represents a large effect (8), these effects range from moderately small to large.

Discussion

The aim of the study was to determine if 20 weeks of basic training in the SANDF produce a better conditioned recruit in terms of exercise ability, endurance, and autonomic function, than only 12 weeks of the same basic training, without development of OTS. Measurements of exercise ability (2max">VO2max) and cardiac autonomic control (HR and HRV) obtained after 12 weeks training were compared with values sampled after 20 weeks of training. Results indicated that the extended basic training program did not yield significant increased exercise capacity, measured by indirect 2max">VO2max, nor did it seem to induce OTS as measured by HR and the HRV indicator of autonomic balance (LF/HF).

The fitness components important to the military and also identified by sport scientists are cardiorespiratory endurance, muscular strength and endurance, flexibility, and body composition (13,14). The basic training period is used to prepare the recruits mentally and physically to be combat ready. A high premium is placed by the SANDF on cardiorespiratory endurance and exercise ability. The present study results showed that improvement in the 2max">VO2max of the recruits were mostly completed after 12 weeks of basic training. In contrast to the difference between week 1 and 12 (2max">VO2max: 49.54 vs. 54.14; p < 0.001), the participant's 2max">VO2max did not change significantly between weeks 12 and 20 (2max">VO2max: 54.14 vs. 54.15; p = 0.44). Results were confirmed by the effect size calculations. However, these results do not imply that other physical changes such as muscle strength, bone and tendon adaptations did not occur.

In the supine position, an HR decrease of 11.9% was observed after 12 weeks, and it declined another 3.7%, between weeks 12 and 20 measurements. RR intervals increased with 12.7% (supine) and 10.9% (standing) after 12 weeks followed by a further 4.3% supine and 2.2% standing. The SD of the RR intervals (STDRRs) increased with 11.7% (supine) and 18.8% (standing) after 12 weeks. This was followed by smaller, but still significant increases of 9.9% (supine) and 8.0% (standing) measured at 20 weeks. Exercise induced bradycardia in healthy and clinical populations is well known and described in the literature (22,23). Textbook bradycardia is characterized by a HR below 60 b·min−1, whereas normal resting rate is considered to be between 60 and 100 b·min−1 (3). The above results concur with expected positive exercise induced changes in HR and total variance (4,7,23). These changes continued during the 12–20 weeks period of the basic training.

Signs of overtraining in the HR of endurance athletes may include unexplained small increases in their otherwise low HR (15,17). However, because of the modest scale of these changes, Bosquet et al. (5) warned that signs of overreaching and OTS may be concealed within day-to-day variations in HR. In this study, analyses of the data showed that the participant's exercise-induced lowering of supine HR, continued after 12 weeks, manifesting in exercise-induced bradycardia (mean HR = 58.48 b·min−1) after 20 weeks of basic training. This may suggest continued post–12 weeks exercise-induced conditioning of cardiac function without obvious signs of overtraining.

Exercise-induced increases in supine and standing vagal cardiac control after 12 weeks were confirmed by indicators of vagal-induced variability (RMSSD, pNN50, SD1, and HF [squared milliseconds]). The highly significant increases in supine vagal cardiac control between weeks 1 and 12 (p < 0.001 for RMSSD, pNN50, SD1) continued between weeks 12 and 20 (RMSSD: p = 0.015; SD1: p = 0.013; HF [squared milliseconds]: p = 0.023), although not significant for pNN50 (p = 0.083). The same trend was seen in the standing vagal indicator values between weeks 1 and 12 (p < 0.001 for RMSSD, pNN50, SD1; and p = 0.009 for HF [squared milliseconds]) and weeks 12 to 20 (RMSSD: p = 0.021; pNN50: p = 0.011; SD1: p = 0.011; HF [squared milliseconds]: p = 0.011).

The protective influence of increased vagal control in response to exercise is well known, and exercise-based clinical interventions are widely recommended to reduce morbidity and all-cause mortality (22). In the present study, this health benefit can be inferred from the significant increases in vagal power of HR control obtained during the first 12 weeks and during the last 8 weeks of basic training. These results were confirmed after 12 weeks by the highly significant supine changes (p < 0.001) in autonomic balance toward increased vagal control, as shown by LF/HF, LFnu, and HFnu. However, these indicators of autonomic balance showed no change during the extra 8 weeks of intensive training in the supine and standing positions. It may be reasoned that the increased power in vagal controlled HRV, without a concurrent change In autonomic balance, points to a decrease in sympathetic cardiac influence. This observation is in line with a current theory that exercise interventions decrease sympathetic cardiac control while parasympathetic control is increased (11,22,23). Thus, HRV indicators of pure vagal influence and of autonomic balance suggested that healthy cardiac conditioning (increased vagal and decreased sympathetic influence) occurred during the 12- to 20-week period of basic training.

Results from HRV indicators of mixed origin (sympathetic and parasympathetic influence) were not conclusive. Between the supine 12 and 20 weeks testing, no significant changes in variability were observed for SD2 and LF (squared milliseconds). In the standing position, changes were observed from baseline to 12 weeks but not when comparing the 12-week results with the week 20 results. Exercise interventions are thought to decrease supine measured sympathetic cardiac control and decrease parasympathetic control (11). Because of the presence of these opposing forces, it is difficult to make conclusions based on the changes in indicator values influenced by both the sympathetic and parasympathetic branches of the ANS.

In terms of signs of overtraining, most supine HRV indicator values (RMSSD, pNN50, SD1, and HF [squared milliseconds]) steadily progressed toward higher vagal power, and it was confirmed by the autonomic balance HRV indicator (LF/HF). During the last 8 weeks, the LF/HF change were still in the same direction of increased vagal balance, it was, however, not statistically significant (p = 0.731). The initial stages of OTS are thought to increase the activity of the sympathetic branch of the ANS, thereby change the ANS balance (17), which may manifest in small increases in resting HR and a larger LF/HF value. Results from this study did not show obvious signs of the initial stages of OTS, such as a move toward increased sympathetic cardiac control of HR (17). This observation was confirmed by the changes observed in HR and all indicators of vagal influence and the indicators of autonomic balance.

Cardiorespiratory fitness (2max">VO2max) did not increase during the 12- to 20-week period, although HR and sympathetic cardiac control decreased with a concurrent increase in vagal cardiac control (Table 5). Other aspects of fitness such as muscle strength, bone, and tendon adaptations were not considered in the present study. It is recommended that these factors should be jointly investigated in future. No signs of overreaching or overtraining were visible in the HR or HRV results. However, as autonomic function was only tested with non–proven/validated indicators of OTS (HR and HRV), results should be interpreted with caution and preferably be confirmed with a battery of OTS tests.

Table 5
Table 5:
Summary of study findings.*

Practical Applications

We conclude that the extension of a medium-to-high intensity exercise program, from 12 to 20 weeks provide little benefit in terms of increased exercise capacity and aerobic fitness as measured by 2max">VO2max. For the SANDF and the sports coach, where exercise program length and seasonal planning is critical, a 20-week exercise program does not show any advantage compared with a 12-week exercise program in terms of 2max">VO2max. However, for the clinician, the continued cardiac conditioning during 12–20 weeks of a medium-to-high intensity rehabilitation program may be worth the training time and effort by medical staff and patients.

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Keywords:

2max">VO2max; heart rate variability; basic training

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