It has been long recognized that athletes may have larger hearts and changes on the EKG that are difficult to distinguish from abnormal findings such as those seen in an abnormally enlarged and thickened left ventricle with hypertrophic muscles as is seen in long standanding hypertension.
In fact athletes have enlarged heart muscle as well and cardiologists have attempted to provided some wall thickness values above which it is thought to be too big to indicate the physiologic increases in muscle and chamber size of the athlete and represent pathological and maladaptive muscle growth. A distinction based on various EKG criteria have never been that reliable.
In addition, all athletic activity does not seem to bring out the same type of physiological adaptive changes. A distinction is typically made between endurance athletes and strength athletes, recognizing that some athletes do both endurance and strength training.
Generally endurance athletes increased cardiac output under conditions of reduced peripheral resistance while strength trained athletes increased cardiac output against increased peripheral resistance. It would not be unexpected that these difference would be reflected in the heart's adaption
The stylized facts are that the endurance athletes have increased wall thickness as do the strength folks but endurance athletes have larger chambers . e.g left atrial size and left ventricular diastolic diameters. The usual definition of the upper limit of normal for the left atrium is often said to be 35ml while one study of endurance athletes has shown value of 37.7 ml.The left ventricular wall is sometimes greater than 13 mm ( in one study of elite endurance athletes 13% were greater than 13 but none above 15 mm. About 1/3 of elite endurance athletes have LV cavity end diastolic greater than 60 ml. whereas upper normal is typically consisted 55 ml Typically endurance athletes are said to have eccentric hypertrophy while endurance athletes have concentric but that concept has been challenged.
At least one study reported that endurance athletes had indicators of better diastolic function that do strength trained athletes. (Vinereanu,D, Clin Sci 2002).However, overall the data are conflicting as to whether endurance exercise improves diastolic filling apart from heart rate changes and
as to whether any diastolic function improvement persists in senior athletes to significantly mitigate the seemingly universal age related increases in myocardial stiffness. ( Perhaps another example of the Woody Allen physiological maxim ,paraphrased when you get old everything that should be soft gets hard and everything that should be hard gets soft.)
In any event the athletes hearts seems to work quite well particularity when compared with the enlarged heart s that can results from hypertension both in terms of pumping the blood during systole but also in rapidly refilling the ventricular chamber in diastole preparatory for the next ejection
Athletes hearts pump more effectively blood during systole This is usually expressed as as EF, value ( ejection fraction) which is generally greater in endurance athletes than in normals but the range overlaps. .But also the athlete's ventricles fill faster in the relation phase of the left ventricle , i.e. diastole so their enhanced cardiac output results from more filling and more ejection giving them higher stroke volumes.
Using various indicators of diastolic function ( such as the early phase of filling versus the later phase of filling (an e/a ratio being one such measure) it has been clearly demonstrated that left ventricular filing is normal or supra normal in the endurance athletes with increased wall thickness while the opposite occur to varying degrees in hypertensive heart disease and other causes of LVH such as aortic stenosis and hypertrophic cardiomyopathy .
Newer echo techniques have provided even more evidence of a distinction between the enhanced cardiac function of the endurance trained athletes and those with hypertensive heart disease and to aid in the differential diagnosis of hypertrophic cardiomyopathy.
In the last ten years a technique referred to as deformation imaging ( which can be subdivided into 1)TDI and 2)speckle-tracing 2D strain imaging has provide a new way to elucidate cardiac physiology. The techniques can distinguish between active and passive myocardial segment motion.
In echo speak "strain" mean deformation, unlike the everyone day English language meaning of stretching and these techniques measure strain and strain rate.Strain is considered the fractional change in length of a myocardial segment and can be expressed as a percentage.Ultra sound images contain natural acoustic markers, called speckles, which can be tracking as muscle segments move during contraction and relaxation and actively contracting muscles segment can be distinguished from areas moving poorly.
Speckle tracing studies have shown that the ventricular hypertrophy as in hypertension is associated with decrease strain ,that is less deformation, which is functionally disadvantageous while the hypertrophy of endurance athletes does not differ appreciably from normal controls and is not associated with functional impairment.In other words this is an advantageous remodeling.
A similar distinction between pathological left ventricular hypertrophy and physiologic athletic hypertrophy has been made using MRIs of the heart. ( Peterson SE, 2005 , J Cardiovas Mag Reason 2005:7,(3) 551. Speckle tracing is much less expensive .
For the most part newer testing techniques have done away with concerns about exercise causing the heart to enlarge in a deleterious physiological way with the interesting possible exceptions of harmful changes or remodeling of the right ventricle and of the left atrium with a putative inceased risk of atrial fibrillation. Several authors have argued and provided some evidence that prolonged endurance type exercise can bring about a condition similar to an inherited disease called arrhythmogenic right ventricular hypertrophy (ARRV), a topic I wrote about before see here.
addendum: added 10/1/15 5:46 PM. Here is more on the differences between strength athletes and endurance athletes. Rowers were compared with Long distance runners and the authors fond that the runners had larger left ventricular volumes,lower and in the normal range for ventricular mass and a tendency to less thickened left ventricular walls. (Wasfy,M Endurance exercise-induced cardiac remodeling: Not all Sports are created equal,Journal of the American society of echocardiography, 2015,Sept 9
addendum 2/9/16. Minor wording changes made in last paragraph.