In the lexicon of our times, cardio has come to define any exercise that raises the heart rate. Walking, running, elliptical trainers, and rowing machines are all considered “cardio.” While there are health benefits from any exercise, coaches and exercise scientists know that low-intensity exercise has no significant cardiac muscle training effect for anyone except those with low fitness levels. A study by Jan Helgerud, PhD, found that the best way to strengthen the weakened hearts of cardiac patients was to engage in high-intensity interval training. But the term cardio is misapplied when referring to the way endurance athletes spend the majority of their time training.

The heart muscle, like your skeletal muscles, responds to heavy use by becoming stronger. Did you ever wonder why your heart muscle never gets tired like your leg muscles do? It’s because the cardiac muscle is made up of 100 percent slow twitch (ST) fibers, plus the heart gets first dibs on the blood (and oxygen) that it pumps. As with your leg muscles, the best stimulus for increasing the strength of the heart muscle is to force it to work really hard. You do that by raising your heart rate to 90–95 percent of its maximum, which is how you create a strength training effect in ST muscle fibers. The ventricular wall of the cardiac muscle responds by becoming thicker, and the muscle can then pump more forcefully, ejecting more oxygenated blood with each stroke (increased cardiac muscle stroke-volume). The end result is that the skeletal muscles get more oxygen so they can sustain a higher output of work. Basically, you move faster.

The heart forms the core of what sport scientists call the “central” system, which delivers the oxygen-bearing blood to the muscles. The skeletal muscles, on the other hand, are where this oxygen is used. They comprise the “peripheral” system.

While the heart’s output does indeed place a limit on the maximum work that is sustainable, the limit to endurance is determined by the skeletal muscles’ ability to make the most use of the oxygen that gets delivered to them. This skeletal muscle oxygen utilization is known as metabolism. Read this article, “What Enables Endurance?,” for a more complete explanation. Metabolism is one of the most trainable physiological systems in the body.

Training for endurance events is focused primarily on improving your endurance through affecting the skeletal muscles’ metabolism. This is done, as you probably know, with frequent, properly timed, long-duration, low-to-moderate-intensity training sessions. This is correctly termed aerobic training because it triggers adaptations in the aerobic metabolic pathway in the muscles that do the endurance work. Since this work is mostly done at lower intensities (and lower heart rates), there is not a significant cardiac muscle training effect for the athlete.

So when you are out for your next 2-hour trail run, call it aerobic endurance training. Call it aerobic capacity building. But don’t call it cardio.

-by Scott Johnston