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Why Doesn't Your Heart Get Tired Like Other Muscles?

Exercise for Heart Health

Story at-a-glance -

  • Mitochondria make up, on average, about 1-2 percent of your skeletal muscle by volume, but this is generally enough to provide the needed energy for your daily movements
  • Whereas your skeletal muscle contains only 1-2 percent mitochondria, your cardiac muscle may contain up to 35 percent
  • This large volume of mitochondria supplies a steady source of energy right to your heart, and explains why your heart rarely needs to “rest” like your skeletal muscles do
  • Extreme endurance exercise significantly increases cardiac output, which may put excessive strain on your heart depending on the duration and intensity of your activity
  • Over-exercising, such as marathon running, may not make your heart feel tired like your other muscles, but it can cause scarring, inflammation, and other heart damage

By Dr. Mercola

After a vigorous workout, the muscles in your legs and arms may be fatigued and possibly sore. But have you ever wondered why your heart, which is also a muscle, doesn't feel similarly "tired"? The reason has to do with the three different types of muscle – skeletal, smooth, and cardiac -- and how they derive energy from your body.

Three Types of Muscles and How They Work

The muscles you probably think of when you hear the word "muscle" are skeletal muscles. These striated (banded) muscles attach to your bones and tendons and control most of your body's voluntary movements (and even some involuntary movement, like your diaphragm).

Your skeletal muscle derives its energy from your mitochondria -- the energy storehouse of your cells, responsible for the utilization of energy for all metabolic functions. Mitochondria make up, on average, about 1-2 percent of your skeletal muscle by volume, but this is generally enough to provide the needed energy for your daily movements.

As an alternative, your skeletal muscle can also use glycogen (stored sugar) to produce adenosine triphosphate (ATP), which is the molecule that provides cellular energy. (If you're fasting, it typically takes about eight to twelve hours for your body to metabolize your glycogen stores, and after that you start to shift to burning fat, which is a very beneficial metabolic state).

Unlike skeletal muscles, your smooth muscle has no striations and work automatically, helping your body to digest food, urinate, and much more. Your cardiac muscles, on the other hand, are similar to your skeletal muscle in that they are striated and use mitochondria for energy, but there is an important difference.

Your Cardiac Muscle Contains a Much Larger Volume of Mitochondria

Whereas your skeletal muscle contains only 1-2 percent mitochondria, your cardiac muscle may contain up to 35 percent. This large volume of mitochondria supplies a steady source of energy right to your heart, and explains why your heart rarely needs to "rest" like your skeletal muscles do.

Your mitochondria are little powerhouse structures—found in the cytoplasm, or inside your cells—inside of which energy is formed. This energy is called ATP, and without ATP, the cell dies.

Not only do your cells require oxygen in order to produce ATP, but coenzyme Q10 (CoQ10) and its reduced form, ubiquinol, are also an essential component to the mitochondria in facilitating generation of ATP. As explained by Dr. Robert Barry:

"…In the mitochondria, there is a thing called the electron transport system. What happens in the transfer of electrons, in that electron transport chain is fundamental to ATP production in every mitochondria in every cell in our body. Ubiquinol is an essential component in the electron transport chain and if ubiquinol is not there… you don't get the ATP production.

…You do produce it in your body… but that [natural production] diminishes as you age. Importantly, [if] the conversion of oxidized coenzyme Q10 to reduced ubiquinol in your body [is] not efficient, then you'll have problems."

In fact, one of the primary mechanisms of harm from statin cholesterol-lowering drugs in general appears to be related to the reduction in the liver's ability to produce CoQ10 , which can actually weaken your heart and eventually lead to heart failure. However, it's not only those taking statins who are at risk of depletion, as your body's natural production declines as you age.

This is why many people benefit from taking ubiquinol (the more bioavailable form of CoQ10 to replenish natural cellular energy that is used up through physical activity and/or stress), and counteract the natural drop in production levels that come with age.

While most healthy people need a maintenance dose of just 100 milligrams (mg) a day, world-class athletes, who require  extra ATP turnover, may need 300-600 mg/day and even typical athletes may need upwards of 300 mg/day to support their heart health.

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It's a Myth That Your Heart Never Gets Tired

While your heart certainly doesn't get fatigued the way your skeletal muscles do, it isn't impervious to fatigue or excess stress, such as from long-distance, endurance-type cardio exercise. When you engage in this type of training, your heart doesn't have much say in the matter, as it simply responds to biochemical signals from your body to ramp up cardiac output in order to keep up with your level of exertion.

You can't "feel its pain" until very late in the game, and at that point, damage has already been done and it may be a life-threatening situation. Extreme endurance exercise causes your heart to massively increase cardiac output, which it may have to sustain for several hours, depending on the duration and intensity of your activity.

Your heart pumps about five quarts of blood per minute when you're sitting. But when you're running, it goes up to 25 to 30 quarts, and it wasn't designed to do this for hours on end, day after day.1 It enters a state of "volume overload" that stretches the walls of your heart muscle, literally breaking fibers apart.

The problem is, many endurance athletes don't allow their bodies to fully recover between sessions. They often live in a perpetual post-workout state, which basically resembles chronic oxidative stress.2 Repeated damage to your heart muscle increases inflammation, which leads to increased plaque formation, because plaque is your body's way of "bandaging" the lining of your inflamed and damaged arteries.

Over time, as more damage is inflicted, the heart enlarges (hypertrophy), and forms scars (cardiac fibrosis). MRIs of long-time marathoners reveal abundant scarring over their entire heart. Scientists have also measured elevated cardiac enzyme levels after extreme exercise—just like after a heart attack, which can only mean one thing: this type of exercise is damaging people's hearts.

Although researchers don't yet understand all of the factors in this process, they have theorized that high-endurance exercise leads to cardiac fatigue, then a flood of catecholamines and adrenaline, which then triggers arrhythmias (abnormal heart rhythms) and other problems. So while your heart may not feel tired like your other muscles do, rest assured that if you're over-exercising you're putting extreme stress on your heart.

The Right Kind of Exercise Will Increase Your Cellular Energy Production

One of the best ways to support your heart health and your overall health, including limiting cellular aging, is to exercise – not to excess, as described above, but at the proper "dose." Exercise triggers mitochondrial biogenesis,3 a decline of which is common in aging. This reverses significant age-associated declines in mitochondrial mass, and in effect, stops aging in its tracks.

This is not the first time researchers have linked exercise to mitochondrial changes. A 2011 review in Applied Physiology, Nutrition and Metabolism points out that exercise induces changes in mitochondrial enzyme content and activity, which can increase your cellular energy production and in so doing decrease your risk of chronic disease.4  

Aside from impacting your skeletal muscle and fat tissue, researchers noted that exercise induces mitochondrial changes that may also benefit your liver, brain, and kidneys. Increasing mitochondrial activity is extremely important because free radicals, which are toxic byproducts of metabolism as well as exposures to chemicals, pollutants, and other toxins, can overwhelm your body's defenses. This can lead to oxidative damage to cells and tissues that can destroy cellular proteins, lipids, and DNA; this process often leads directly to the loss of mitochondrial function. In the long-term, irreversible damage in the mitochondria can occur, leading to:

  • Impaired ability to utilize carbohydrates and fat for energy
  • Insulin resistance
  • Lower threshold for physical exercise
  • Excessive weight gain
  • Accelerated aging

Interestingly, the powerful antioxidant astaxanthin also appears to boost mitochondrial function by scavenging free radicals, an effect that may even help boost your endurance.

What's the Best Type of Exercise for Your Heart?

Previous research has shown that exercise alone can reduce your risk of cardiovascular disease by a factor of three.5 However, endurance-type exercise, such as marathon running, can actually damage your heart and increase your cardiovascular risk by a factor of seven... All in all, such findings are a powerful lesson that excessive cardio may actually be counterproductive while short bursts of intense activity are safer and more effective than even conventional cardio—for your heart, general health, weight loss, and overall fitness.

High-intensity interval training like Peak Fitness, which requires but a fraction of the time compared to conventional cardio, has been shown to be FAR more efficient, and more effective. This type of physical activity mimics the movements of our hunter-gatherer ancestors, which included short bursts of high-intensity activities, but not long-distance running. This, researchers say, is what your body is hard-wired for.

Basically, by exercising in short bursts, followed by periods of recovery, you recreate exactly what your body needs for optimum health. Peak Fitness combined with a comprehensive fitness program will benefit all of the muscles in your body, providing them with appropriate amounts of beneficial stress without leading to over-exertion and excess fatigue.