Intense and prolonged physical exercise leads to changes in cardiovascular physiology known as ‘athlete's heart’. Chief among these alterations are increased cardiac size, volumes and mass and improved functional parameters, particularly diastolic function.1 Classically, two forms of cardiac remodeling have been described: concentric remodeling associated with static exercise, and eccentric remodeling associated with dynamic exercise.2 For example, marathon runners present eccentric remodeling due to volume overload resulting from increased cardiac output, while weightlifters show concentric remodeling due to pressure overload.3 It should be noted that this distinction is not absolute, since most sports involve both static and dynamic exercise, and hence lead to mixed remodeling.4

However, athletes are not the only individuals who undergo high-intensity training. Soldiers, especially in the special forces, also undergo physically demanding training that involves various forms of both static and dynamic exercise. Overall, the volume of exercise required of these soldiers can be compared to that of competitive athletes, and may often be even higher. However, military training is unique with regard to the methodologies used and the influence of other variables arising from military life.5

The manifestations of exercise-induced cardiac remodeling can overlap with those of pathological conditions, especially cardiomyopathy, and differential diagnosis is often challenging.6 In this context, pre-participation screening of athletes and interpretation of diagnostic exams in this population are of great importance. In the last decade, new imaging techniques have been developed that enable detailed myocardial assessment, such as analysis of myocardial mechanics by transthoracic echocardiography (TTE).7

The main aim of this study was to characterize and compare cardiac remodeling in professional basketball players over the course of a season and in soldiers on a special forces course.

This observational, longitudinal, case-control study assessed special forces soldiers and competitive athletes at the beginning and end of a military training course and a sports season, respectively. Male and female individuals between the ages of 18 and 35 years were included. The soldiers were selected from those who enrolled in a Portuguese Army commando course and were assessed between January and June 2016. The athletes belonged to two professional basketball teams, one men's and the other women's, competing in the national men's Division 1 and the national women's league, respectively, and were assessed in October 2015 and March/April 2016. Thus, both groups were assessed over a period of about six months. The group of soldiers consisted of the 17 out of 76 trainees (54 dropouts, approximately 70% of trainees) who successfully completed the course and met the inclusion criteria. All of these soldiers had previously participated in competitive sports. The athlete population was composed of 17 basketball players, eight male and nine female.

All of the study subjects were volunteers and gave their informed consent to participate in the study. The protocol was approved by the Ethics Committee of the Medical School of the University of Coimbra (reference protocol 087/2015).

Echocardiographic assessment

All transthoracic echocardiograms were performed by a cardiologist with a Vivid 7 ultrasound system (GE Healthcare®), and included the various echocardiographic views and techniques recommended by the European Society of Cardiology (two-dimensional; M-mode; color, pulsed wave, continuous wave and tissue Doppler; and myocardial strain assessed by speckle tracking).11

Wall thickness and left ventricular (LV) dimensions were measured via parasternal long-axis view. Relative wall thickness (RWT) was calculated using the formula (2×LV posterior wall thickness/LV end-diastolic diameter [LVEDD]) and LV mass was calculated using the Devereux formula.12 LV volume and left atrial (LA) volume were determined using the modified Simpson's rule, with images obtained in apical 4- and 2-chamber views. The LA volume index was obtained by indexing to body surface area. LV ejection fraction (LVEF) was determined using Simpson's method. Pulsed wave Doppler was acquired in apical 4-chamber view, and peak E and A wave velocities and E/A ratio were measured. Tissue Doppler images of the mitral and tricuspid annuli were obtained to measure the E and E′ waves, and to measure S′ wave velocities, respectively. Tricuspid annular plane systolic excursion (TAPSE) was determined by M-mode. Speckle tracking was used to calculate LV global longitudinal strain (GLS) using images from apical 4-, 2- and 3-chamber views and an 18-segment model. Cardiac cycles were acquired during the same respiratory phase (exhalation). Three consecutive cycles were recorded and mean sinus rhythm was calculated with a frame rate >60 frames per second. The quality of the exam was considered good when no more than two segments were excluded, and excellent when all segments were analyzed.

Figure 1 shows examples of TTE of a soldier and of an athlete after the course or season, respectively.

GLS) values; (B) transthoracic echocardiographic images of a basketball player after the sports season, with LV wall thickening near the upper normal limit and normal diastolic function parameters and GLS.'> GLS) values; (B) transthoracic echocardiographic images of a basketball player after the sports season, with LV wall thickening near the upper normal limit and normal diastolic function parameters and GLS.' title='(A) Transthoracic echocardiographic images of a soldier following the special forces course, showing no left ventricular (LV) wall thickening and LV end-diastolic diameter near the upper normal limit, above-normal diastolic function parameters and normal global longitudinal strain (GLS) values; (B) transthoracic echocardiographic images of a basketball player after the sports season, with LV wall thickening near the upper normal limit and normal diastolic function parameters and GLS.'/>

Figure 1.

(A) Transthoracic echocardiographic images of a soldier following the special forces course, showing no left ventricular (LV) wall thickening and LV end-diastolic diameter near the upper normal limit, above-normal diastolic function parameters and normal global longitudinal strain (GLS) values; (B) transthoracic echocardiographic images of a basketball player after the sports season, with LV wall thickening near the upper normal limit and normal diastolic function parameters and GLS.

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This study demonstrated the occurrence of anthropometric and cardiovascular remodeling in both soldiers and basketball players. These adaptations had different characteristics in the two groups. In anthropometric terms, the soldiers had a greater increase in body weight, with gains in MM and reductions in FM. Echocardiographic assessment revealed different patterns of remodeling, with the soldiers developing predominantly eccentric and the basketball players developing mainly concentric adaptations.

The alterations in anthropometric data and vital signs following the military course or sports season were more significant in the soldiers’ group, even when a sub-analysis was performed (data not shown) comparing the soldiers with the Caucasian male athletes. There was a marked transformation in the body composition of the former group, with mean MM gains of 7.5% and mean FM reductions of around 30%. The differences in adaptation in the two groups may be related to the different characteristics of exercise in military training and in basketball, particularly the longer duration and greater intensity of exercise in military training, as well as specific features of the methodologies used. These differences likely explain the more marked adaptations in the soldiers. It has been shown that high-intensity interval training, in this case applied to the soldiers, achieves better results and greater adaptation.13 The mechanisms underlying this adaptation are still unclear, but this type of training is thought to enhance capacity for aerobic and anaerobic metabolism and to increase oxidative enzyme activity.14 Although there were no individuals with hypertension in either group, intense physical exercise led to reductions in the soldiers of around 4% and 9% in SBP and DBP, respectively. These results are in agreement with the literature.15

Cardiac remodeling occurred in both groups, but had different characteristics. In the soldiers, eccentric remodeling occurred with an increase in left chamber size and a decrease in RWT. At the end of the course, the proportion of soldiers with eccentric ventricular hypertrophy had doubled, from 5.9% to 11.8%.16 In the basketball players predominantly concentric remodeling was observed, with increases in LV wall thickness, particularly of the IVS, and RWT, and a slight decrease in LVEDD. One possible explanation for these different types of cardiac remodeling is the intensity of the exercise performed. Moderate- to high-intensity exercise is initially associated with concentric hypertrophy due to the stimulus of pressure overload.17 Vigorous- to near maximal- or maximal-intensity exercise leads to eccentric remodeling, due to volume overload from increased cardiac output, resulting from higher HR and increased ejection volumes, with only a moderate increase in BP.17,18 This moderate increase in BP seems to be associated with acetylcholine-induced endothelium-dependent vasorelaxation, which plateaus with moderate to vigorous levels of exercise.19 Thus, when the level of exercise increases from vigorous-intensity to near maximal- or maximal-intensity, there is a greater increase in cardiac output that is not accompanied by a proportional increase in BP, and this may lead to dilatation of the heart chambers that is disproportionate to the observed wall hypertrophy.

Despite the extent of the adaptations observed over the course or season, the changes were still within the upper limit of normal (no athlete with IVS >13 mm and only one with LVEDD >60 mm [61.4 mm]). There were also no statistically significant changes in parameters of diastolic function during the study. However, it should be noted that both soldiers and athletes already had above-normal values, as described in similar populations.20,21

Exercise-induced changes in functional parameters were also different in the two groups. In the soldiers, there was a decrease in resting LVEF, while in the basketball players this parameter remained similar to baseline values. The literature reports that elite athletes who are subject to intense and prolonged exertion may have slightly lower resting LVEF, with mean values of 50-55%, which increases as required when prompted by effort.22,23 In the soldiers, no individual had an LVEF <50% on the baseline or final echocardiogram; however, 12 soldiers (70% of the population) had an LVEF of 50-55% on the final exam.

Both the soldiers and the basketball players had GLS values within normal limits,7 with a trend toward reduction after the exercise program. This decrease was not significant when the two groups were analyzed separately. However, when they were compared, GLS decreased significantly. This behavior is consistent with that described in the literature,7 and may represent an exercise-induced physiological adaptation and another characteristic that distinguishes ‘athlete's heart’ from heart disease, particularly hypertrophic cardiomyopathy.24,25 However, the current scarcity of data in this area indicates a need for further studies designed to study the effect of physical exercise on this parameter.

The main limitation of this study is the small sample size for both special forces soldiers and athletes. The heterogeneity of the populations with regard to gender and race also limits the results, mainly because no female soldiers participated in the special forces course. Another limitation is the inability to objectively quantify unscheduled physical exercise performed by the soldiers during the course. These individuals are constantly involved in activities and assessments, in which the physical component is central, even when they are not performing conventional or scheduled exercise. Finally, although all of the participants strongly denied using stimulants, the study protocol did not enable this denial to be confirmed.

In this study, anthropometric and cardiac remodeling occurred in both special forces soldiers and competitive basketball players following a military course or sports season. This remodeling had different characteristics in the two groups, being predominantly eccentric in the soldiers and concentric in the basketball players. New echocardiographic techniques, particularly myocardial strain analysis, may reveal patterns that are compatible with physiological adaptation to exercise, and may help to differentiate between ‘athlete's heart’ and heart disease.

The authors have no conflicts of interest to declare.