Since cardiac muscle cells lose their ability to divide 3-6 months post-birth, they respond to stress by hypertrophy alone, not hyperplasia. New sarcomeres can be added in series, resulting in longer myofibrils and muscle fibers (myocytes), or they can be added in parallel, resulting in thicker muscle fibers.
The former pattern is seen in eccentric cardiomegaly, as induced by aerobic endurance exercise (swimming, running), and results in increased shortening velocity and capacity but no increase in strength.
The latter pattern is seen in concentric cardiomegaly, as induced by static exercise (weight lifting, wrestling), and results in increased force (tension) generation but no change in shortening velocity or capacity.
A resulting larger heart may have a larger end-diastolic volume (eccentric cardiomegaly) which could increase stroke volume, or instead thicker walls (concentric cardiomegaly) with which to generate increased tension in the face of elevated afterload.
Different modes of gene expression occur in eccentric versus concentric cardiac hypertrophy.
BACK TO INDEX