Here's another thought on this issue that I just ran across:

"In ‘The Science of Martial Arts Training’, Charles I. Staley, MSS, provides us a somewhat different benefit: “Intermittent exercise … accumulates a greater volume of stress on the blood pumping capacity of the heart. According to exercise physiologist Dr. Steven Seilor, the periodic elevations and decreases in intensity may create special loading stresses on the heart that are adaptive. Seilor suggests that during an interval, heart rate climbs high, then at the moment the interval stops, heart rate immediately starts to drop, but venous return remains high. These exposures to additional ventricular stretch may help trigger ventricular remodeling (increased heart ventricle volume).”