A 2011 paper uses MRI technique to challenge a hypothesis developed in 1975. The study found that endurance training increases left ventricular mass by increasing ventricular wall thickness and volume. Resistance training produced no affect on the heart.
The body adapts to exercise by changing the physiological parameters of the heart. Changes are observed in the left ventricle, the chamber responsible for pumping blood throughout the body (as opposed to the right ventricle, which pumps blood through the pulmonary circuit). The left ventricular end-diastolic volume is the volume of the left ventricle just before contraction (systole).
In 1975 the Morganroth Hypothesis was proposed (Comparative Left Ventricular Dimensions in Trained Athletes, 1975, Joel Morganroth, et al.). Morganroth and team studied the hearts of varsity college athletes in the sports of wrestling, swimming and endurance running as well as world-class track and field athletes. The hearts were studied echocardiographically, a revolutionary, non-invasive technique at the time, but primitive compared to today's MRI instruments. They found that the athletes involved in isotonic exercise (swimming and running) had a greater left ventricular mass due to a greater left ventricular end-diastolic volume. However, their ventricular wall thickness was no different than non-athletes. The athletes involved in isometric exercise (shot put and wrestling) were found to have a greater left-ventricular wall thickness than non-athletes. However, no difference in left ventricular end-diastole volume was observed. The increase in ventricular volume in isotonic athletes and ventricular wall thickness in ventricular wall thickness in isometric athletes independently resulted in an increase in left ventricular mass as the figure below demonstrates.
The Morganroth hypothesis was challenged by a paper released in 2011 (A prospective randomised longitudinal MRI study of left ventricular adaptation to endurance and resistance exercise training in humans, 2011, Spence AL, et al.). Study participants were put through a 24-week training program. The training program came in two flavors: endurance or resistance. The longitudinal study measured heart parameters using MRI at baseline, at the end of the training program and after a detraining period. The study found no significant difference in the heart physiology in the resistance trained group. However, the endurance trained group showed an increase in both left ventricle volume and ventricular wall thickness. As the figure below shows, after the detraining period the left ventricle wall thickness decreased, but the left ventricular volume did not change in the endurance trained participants.
In the endurance trained group left ventricular mass increased an average of 8%. Some other interesting findings from the study include the fact that fitness was retained in the resistance group and endurance group as measured by bench press, push-ups and squats after detraining. Leanness was conserved in the resistance group, but not the endurance group, after detraining.
In summary, advances in MRI imaging of the heart allow more precise measurements of changes in the heart. Using MRI, researchers found that left ventricular mass increased via volume and thickness in endurance trained athletes. However, no physiological changes in the heart were seen in the resistance trained group.