Following endurance training, the weight, volume, and thickness of the left ventricular wall of the heart all increase. Cardiac muscle, like skeletal muscle, can hypertrophy with training. With endurance training, the left ventricle fills with a greater volume of blood, due to an increase in the blood volume, and the amount of blood returning to the heart. Over time, this causes the chamber of the left ventricle to increase in size allowing it to receive a greater volume of blood. In response, the cardiac muscle of the left ventricular wall increases in size so it can effectively deal with this greater volume of blood, by allowing a more forceful contraction of the ventricle wall during systole.
Comparison of the heart size in two 23 year olds – a non-athlete and a young professional cyclist - using echocardiography. Note the 10cm marker as a reference. The non-athlete’s heart is approximately 15 x 12cm vs 25 x 18cm for the athlete. Note also the increased resting heart rate (3 beats to 1) of the non-athlete compared to the athlete.