Why Doesn't Your Heart Get Tired Like Other Muscles?
May 09, 2014
Story at-a-glance
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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
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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.
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.
© Copyright 1997-2014 Dr. Joseph Mercola. All Rights Reserved.
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