Why Doesn’t the Heart Get Tired Like Other Muscles?

Anywhere from 60 to 100 times every minute of every day of every year of your (hopefully) long life, your heart beats. Unlike the other muscles in your body, however, your heart almost never gets tired . . . until it stops for good. Here's why.

Three Kinds of Muscles

The human body is composed of three types of muscles, skeletal, smooth and cardiac.

Skeletal Muscle

Skeletal muscles are striated (banded), and are what most of us think of when we envision a muscle. Attached to the bones and tendons, the skeletal muscles control pretty much all voluntary, and some involuntary (like the diaphragm that works automatically), body movement. Voluntary movement is stimulated by:

Nerve impulses (action potentials) traveling down the motor neurons of the sensory somatic branch of the nervous system [to] cause the skeletal muscle fibers at which they terminate to contract.

Like cardiac muscle, skeletal muscle derives its energy from mitochondria within its cells – "the more mitochondria, the greater the available energy for the muscle":

Because it has not been necessary in the course of evolution for humans to be able to flex our skeletal muscles for prolonged periods of time, the total volume of skeletal muscle contains an average of only 1 to 2% mitochondria. This is an entirely sufficient energy source for such intermittent muscular tasks as walking or running.

Supplementing its mitochondria reserves, skeletal muscle can also use glycogen (stored energy) to produce ATP, the basic unit that transports and releases energy across cells.

Smooth Muscle

Smooth muscle is exactly as described – smooth with no striations. Found in your hollow internal organs (except the heart), smooth muscles work automatically, helping you digest food, dilate your pupils and go pee.

Cardiac Muscle

Like skeletal muscle, cardiac muscle is striated. Uniquely, the cells of this kind of muscle are joined strongly together at adherens junctions that "enable the heart to contract forcefully without ripping the fibers apart."

The stimulus to make the heart pump comes from within and it "passes from fiber to fiber through gap junctions":

In a synchronous wave that sweeps from the atria down through the ventricles and pumps blood out of the heart. Anything that interferes with this synchronous wave . . . [like a] heart attack, may cause the fibers of the heart to beat at random – called fibrillation.

Although the heart pumps of its own volition, the nerves of the autonomic nervous system:

Do run to the heart, but their effect is simply to modulate – increase or decrease – the intrinsic rate and the strength of the heartbeat. Even if the nerves are destroyed (as they are in a transplanted heart), the heart continues to beat.

Cardiac muscle, like skeletal muscle, is also powered by mitochondria, but it has many, many more:

The total volume of the heart . . . is [comprised of] between 30 and 35% mitochondria. That massive amount of energy-generators means cardiac muscle, in a healthy state, need never rest: there is always some energy being transferred to the muscle at the same time that more energy is being derived from caloric intake.

However, this greater reliance on mitochondria means the heart also has a:

Greater dependence on cellular respiration for ATP . . . has little glycogen and gets little benefit from glycolysis when the supply of oxygen is limited. Thus anything that interrupts the flow of oxygenated blood to the heart leads quickly to damage – even death – of the affected part. This is what happens in heart attacks.

Broken Hearts

Although it seems indefatigable, the strength of the human heart is not without limits. Recent research has shown that after extremely strenuous use, even the healthiest hearts can suffer damage.

Heart Fatigue

In 2001, scientists studied cardiac fatigue in endurance athletes:

Cardiologist Euan Ashley . . . set up a mobile heart lab at the finishing line of the ultra-endurance race "Adrenaline Rush" in the Scottish Highlands. . . . . The winning team . . . collapsed across the finishing line after 90 continuous hours of biking, climbing, swimming, paddling and rope work with virtually no sleep . . . After testing their hearts . . . before and after the 400 km race . . . the scientists determined that the hearts of athletes who finished the competition pumped 10 percent less blood at the end of the race compared with the amount pumped at the beginning.

Ashley was quick to note that "the athletes' hearts that showed signs of cardiac fatigue did return to normal fairly quickly after the race and no permanent damage was done."

Permanent Damage

Although a single instance of endurance athletics may not cause permanent damage, more recent research indicates that a lifetime of extreme workouts may.

In a 2011 British study of:

Men who had been part of a British national or Olympic team in distance running or rowing, as well as [runners who] have completed at least a hundred marathons . . . [with] twelve age 50 or older . . . [and] another 17 . . . age 26 to 40 [was compared] to a group of 20 healthy men over 50, none of whom were endurance athletes. . . . . The different groups underwent a new type of magnetic resonance imaging of their hearts that identifies very early signs of fibrosis, or scarring, within the heart muscle . . . [a condition] which can contribute to irregular heart function, and, eventually, heart failure . . . . The results . . . were rather disquieting. None of the younger athletes or the older nonathletes had fibrosis in their hearts. But half of the older lifelong athletes showed some heart muscle scarring. The affected men were, in each case, those who'd trained the longest and hardest.

Nonetheless, even the scientists studying the affects of intense exercise on cardiac muscle agree that:

Too much exercise has not been a big problem in America. Most people just run to stay in shape, and for them, the evidence is quite strong that endurance exercise is good . . . There is no doubt that exercise in general is very good for heart health.

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Melissa writes for the wildly popular interesting fact website TodayIFoundOut.com. To subscribe to Today I Found Out's "Daily Knowledge" newsletter, click here or like them on Facebook here. You can also check 'em out on YouTube here.

This post has been republished with permission from TodayIFoundOut.com. Image byHoratiu Curutiu under Creative Commons license.