When you exercise
or compete in sports, you notice several things about your
body. You breathe heavier and faster, your heart beats faster,
your muscles hurt and you sweat. These are all normal
responses to exercise whether you work out regularly or only
once in a while or whether you are a "weekend warrior" or a
trained athlete. When you watch world-class athletes compete,
you see the same responses, only magnified.
The body has an incredibly complex set of processes to meet
the demands of working muscles. Every system in the body is
involved. In this edition of How Stuff
Works, we will look at how your body responds to
strenuous exercise -- how muscles, blood circulation,
breathing and body heat are affected. You will also see how
these responses can be enhanced by training!
Your Body's Response to Exercise Any type of
exercise uses your muscles. Running, swimming, weightlifting
-- any sport you can imagine -- uses different muscle groups
to generate motion. In running and swimming, your muscles are
working to accelerate your body and keep it moving. In
weightlifting, your muscles are working to move a weight.
Exercise means muscle activity!
As you use your muscles, they begin to make demands on the
rest of the body. In strenuous exercise, just about every
system in your body either focuses its efforts on helping the
muscles do their work, or it shuts down. For example, your
heart beats faster during strenuous exercise so that it can
pump more blood to the muscles, and your stomach shuts down
during strenuous exercise so that it does not waste energy
that the muscles can use.
When you exercise, your muscles act something like electric
motors. Your muscles take in a source of energy and they
use it to generate force. An electric motor uses electricity
to supply its energy. Your muscles are biochemical motors, and
they use a chemical called adenosine triphosphate (ATP) for
their energy source. During the process of "burning" ATP, your
muscles need three things:
They need oxygen, because chemical reactions require ATP
and oxygen is consumed to produce ATP.
They need to eliminate metabolic wastes (carbon dioxide,
lactic acid) that the chemical reactions generate.
They need to get rid of heat. Just like an electric
motor, a working muscle generates heat that it needs to get
In order to continue exercising, your
muscles must continuously make ATP. To make this happen, your
body must supply oxygen to the muscles and eliminate the waste
products and heat. The more strenuous the exercise, the
greater the demands of working muscle. If these needs are not
met, then exercise will cease -- that is, you become exhausted
and you won't be able to keep going.
To meet the needs of working muscle, the body
has an orchestrated response involving the heart, blood
vessels, nervous system, lungs,
liver and skin. It
really is an amazing system! Let's examine each need and how
it is met by the various systems of the body.
ATP is Energy! For your muscles -- in fact,
for every cell in your body -- the source of energy that keeps
everything going is called ATP. Adenosine triphosphate
(ATP) is the biochemical way to store and use energy.
The entire reaction that turns ATP into energy is a bit
complicated, but here is a good summary:
Chemically, ATP is an adenine nucleotide bound to three
There is a lot of energy stored in the bond between the
second and third phosphate groups that can be used to fuel
When a cell needs energy, it breaks this bond to form
adenosine diphosphate (ADP) and a free phosphate
In some instances, the second phosphate group can also
be broken to form adenosine monophosphate (AMP).
When the cell has excess energy, it stores this energy
by forming ATP from ADP and phosphate.
required for the biochemical reactions involved in any muscle
contraction. As the work of the
muscle increases, more and more ATP gets consumed and must be
replaced in order for the muscle to keep moving.
Because ATP is so important, the body has several different
systems to create ATP. These systems work together in phases.
The interesting thing is that different forms of exercise use
different systems, so a sprinter is getting ATP in a
completely different way from a marathon runner!
ATP comes from three different biochemical systems in the
muscle, in this order:
glycogen-lactic acid system
Let's look at each one in
Phosphagen System A muscle cell
has some amount of ATP floating around that it can use
immediately, but not very much -- only enough to last for
about three seconds. To replenish the ATP levels quickly,
muscle cells contain a high-energy phosphate compound called
creatine phosphate. The phosphate group is removed from
creatine phosphate by an enzyme
called creatine kinase, and is transferred to ADP to
form ATP. The cell turns ATP into ADP, and the phosphagen
rapidly turns the ADP back into ATP. As the muscle continues
to work, the creatine phosphate levels begin to decrease.
Together, the ATP levels and creatine phosphate levels are
called the phosphagen system. The phosphagen system can
supply the energy needs of working muscle at a high rate, but
only for 8 to 10 seconds.
System Muscles also have big reserves of a complex
called glycogen. Glycogen is a chain of glucose
molecules. A cell splits glycogen into glucose. Then the cell
uses anaerobic metabolism (anaerobic means "without
oxygen") to make ATP and a byproduct called lactic acid
from the glucose.
About 12 chemical reactions take place to make ATP under
this process, so it supplies ATP at a slower rate than the
phosphagen system. The system can still act rapidly and
produce enough ATP to last about 90 seconds. This system does
not need oxygen, which is handy because it takes the heart and
lungs some time to get their act together. It is also handy
because the rapidly contracting muscle squeezes off its own
blood vessels, depriving itself of oxygen-rich blood.
There is a definite limit to anerobic respiration because
of the lactic acid. The acid is what makes your muscles hurt.
Lactic acid builds up in the muscle tissue and causes the
fatigue and soreness you feel in your exercising muscles.
Aerobic Respiration By
two minutes of exercise, the body responds to supply working
muscles with oxygen. When oxygen is present, glucose can be
completely broken down into carbon dioxide and water in a
process called aerobic respiration. The glucose can
come from three different places:
remaining glycogen supplies in the muscles
breakdown of the liver's glycogen into glucose, which
gets to working muscle through the bloodstream
absorption of glucose from food in the intestine, which
gets to working muscle through the bloodstream
Aerobic respiration can also use fatty acids
from fat reserves in muscle and the body to produce ATP. In
extreme cases (like starvation), proteins can also be broken
down into amino acids
and used to make ATP. Aerobic respiration would use
carbohydrates first, then fats and finally proteins, if
necessary. Aerobic respiration takes even more chemical
reactions to produce ATP than either of the above systems.
Aerobic respiration produces ATP at the slowest rate of the
three systems, but it can continue to supply ATP for several
hours or longer, so long as the fuel supply lasts.
So imagine that you start running. Here's what happens:
The muscle cells burn off the ATP they have floating
around in about 3 seconds.
The phosphagen system kicks in and supplies energy for 8
to 10 seconds. This would be the major energy system used by
the muscles of a 100-meter sprinter or weight lifter, where
rapid acceleration, short-duration exercise occurs.
If exercise continues longer, then the glycogen-lactic
acid system kicks in. This would be true for short-distance
exercises such as a 200- or 400-meter dash or 100-meter
Finally, if exercise continues, then aerobic respiration
takes over. This would occur in endurance events such as
800-meter dash, marathon run, rowing, cross-country skiing
and distance skating.
When you start to look closely
at how the human body works, it is truly an amazing machine!
Getting Oxygen to the Cells If you are going
to be exercising for more than a couple of minutes, your body
needs to get oxygen to the muscles or the muscles will stop
working. Just how much oxygen your muscles will use depends on
two processes: getting blood to the muscles and extracting
oxygen from the blood into the muscle tissue. Your working
muscles can take oxygen out of the blood three times as well
as your resting muscles. Your body has several ways to
increase the flow of oxygen-rich blood to working muscle:
increased local blood flow to the working muscle
diversion of blood flow from nonessential organs to the
increased flow of blood from the heart
increased rate and depth of breathing
increased unloading of oxygen from hemoglobin in working
These mechanisms can increase the blood flow
to your working muscle by almost five times. That means that
the amount of oxygen available to the working muscle can be
increased by almost 15 times! Let's examine more closely how
blood flow to working muscle can be increased.
Making the Pipe Bigger As
you exercise, the blood vessels in your muscles dilate and the
blood flow is greater, just as more water flows through a fire
hose than through a garden hose. Your body has an interesting
way of making those vessels expand. As ATP gets used up in
working muscle, the muscle produces several metabolic
byproducts (such as adenosine, hydrogen ions and carbon
dioxide). These byproducts leave the muscle cells and cause
the capillaries (small, thin-walled blood vessels) within the
muscle to expand or dilate (vasodilation). The
increased blood flow delivers more oxygenated blood to the
As you begin to exercise, blood from organs is
diverted to the muscles.
Taking Blood from the
Organs When you begin to exercise, a remarkable
diversion happens. Blood that would have gone to the stomach
or the kidneys goes instead to the muscles, and the way that
happens shows how the body's processes can sometimes override
one another. As your muscles begin to work, the sympathetic
nervous system, a part of the automatic or autonomic
nervous system (that is, the brainstem and spinal cord)
stimulates the nerves to the heart and blood vessels. This
nervous stimulation causes those blood vessels (arteries and
veins) to contract or constrict (vasoconstriction).
This vasoconstriction reduces blood flow to tissues. Your
muscles also get the command for vasoconstriction, but the
metabolic byproducts produced within the muscle override this
command and cause vasodilation, as we discussed above. Because
the rest of the body gets the message to constrict the blood
vessels and the muscles dilate their blood vessels, blood flow
from nonessential organs (for example, stomach, intestines and
kidney) is diverted to working muscle. This helps increase the
delivery of oxygenated blood to working muscle further.
Making the Heart Pump
Harder Your heart, also a muscle, gets a workout
during exercise, too, and its job is to get more blood out to
the body's hard-working muscles. The heart's blood flow
increases by about four or five times from that of its resting
state. Your body does this by increasing the rate of your
heartbeat and the amount of blood that comes through the heart
and goes out to the rest of the body. The rate of blood pumped
by the heart (cardiac output) is a product of the rate
at which the heart beats (heart rate) and the volume of
blood that the heart ejects with each beat (stroke
volume). In a resting heart, the cardiac output is about 5
liters a minute (0.07 L x 70 beats/min = 4.9 L/min). As you
begin to exercise, sympathetic nerves stimulate the heart to
beat with more force and faster; the heart rate can increase
about threefold. Also, the sympathetic nerve stimulation to
the veins causes them to constrict. This, along with more
blood being returned from the working muscles, increases the
amount of blood returned to the heart (venous return).
The increased venous return helps to increase the stroke
volume by about 30 to 40 percent. When the heart is pumping at
full force, the cardiac output is about 20-25 liters per
Breathing Faster and
Deeper So far, we have talked about getting more
blood to working muscle. Your lungs and the rest of your
respiratory system need to provide more oxygen for the blood,
too. The rate and depth of your breathing will increase
because of these events:
Sympathetic nerves stimulate the respiratory muscles to
increase the rate of breathing.
Metabolic byproducts from muscles (lactic acid, hydrogen
ions, carbon dioxide) in the blood stimulate the respiratory
centers in the brainstem, which, in turn, further stimulates
the respiratory muscles.
Slightly higher blood pressure, caused by the increased
force of each heartbeat and by the elevated cardiac output,
opens blood flow to more air sacs (alveoli) in the
lungs. This increases the ventilation and allows more oxygen
to enter the blood.
As the lungs absorb more oxygen
and the blood flow to the muscles increases, your muscles have
Getting More out of
Hemoglobin Your body has increased the flow of
oxygen-rich blood to your muscles, but your muscles still need
to get the oxygen out of the blood. An exchange of oxygen and
carbon dioxide is the key to this. A protein called
hemoglobin, which is found in red blood cells, carries
most of the oxygen in the blood. Hemoglobin can bind oxygen
and/or carbon dioxide; the amount of oxygen bound to
hemoglobin is determined by the oxygen concentration, carbon
dioxide concentration and pH. Normally, hemoglobin works like
Hemoglobin in red blood cells entering the lungs has
carbon dioxide bound to it.
In the lungs, oxygen concentration is high and carbon
dioxide concentration is low due to breathing.
Hemoglobin binds oxygen and releases carbon dioxide.
Hemoglobin gets transported through the heart and blood
vessels to the muscle.
In muscle, the carbon dioxide concentration is high and
the oxygen concentration is low due to metabolism.
Hemoglobin releases oxygen and binds carbon dioxide.
Hemoglobin gets transported back to the lungs and the
As you exercise, though, the
metabolic activity is high, more acids (hydrogen ions, lactic
acid) are produced and the local pH is lower than normal. The
low pH reduces the attraction between oxygen and hemoglobin
and causes the hemoglobin to release more oxygen than usual.
This increases the oxygen delivered to the muscle.
Getting Rid of Waste Your
exercising body is using energy and producing waste, such as
lactic acid, carbon dioxide, adenosine and hydrogen ions. Your
muscles need to dump these metabolic wastes to continue
exercise. All that extra blood that is flowing to the muscles
and bringing more oxygen can also take the wastes away. The
hemoglobin in the blood will carry away the carbon dioxide,
Heating Up Your body heats up when you
exercise, and it will show on your skin. Your skin feels
hotter to the touch and may look flushed, and you sweat.
Although those things let you know how much heat your body is
giving off, they are actually the ways that the body cools
Working muscle produces heat in two ways:
The chemical energy used in muscles contracting is not
efficiently turned into mechanical energy. (It is about 20
to 25 percent efficient.) The excess energy is lost as heat.
The various metabolic reactions (anaerobic, aerobic)
also produce heat.
Your body needs to remove this
excess heat. The heat produced by exercising muscle causes
blood vessels in the skin to dilate, which increases the blood
flow to the skin. This elevated blood flow to the skin and the
large surface area of the skin allows the excess heat to be
lost to the surrounding air.
Also, receptors carry the message of excess heat to your
body's thermostat, the hypothalamus in the brain. Nerve
impulses from the hypothalamus stimulate sweat glands in the
skin to produce sweat. The fluid for the sweat also comes from
the increased skin blood flow. The sweat evaporates from the
skin, removing heat and cooling the body. Evaporation of sweat
removes fluid from the body, so it is important to maintain
fluids for blood flow and sweat production by drinking water
and/or sport drinks. Sports drinks also replace ions (sodium,
potassium) that are lost in the sweat, and provide additional
glucose to fuel anaerobic and aerobic respiration.
Evaporation of sweat is an important cooling system that
can efficiently remove heat. However, if exercise is done in a
hot, humid environment, then sweat does not evaporate. This
reduces the efficiency of this system and the person is
subject to heat stroke. Heat stroke is a
life-threatening condition. Here are its symptoms:
The core body temperature rises above 104 degrees F (40
Heart rate increases
Confusion, dizziness, nausea and headache occur
Heat stroke can cause a person to collapse, lose
consciousness and die. Emergency medical help involves these
two steps: lowering the body temperature (removing clothing,
spraying the person with cool mist, putting on ice packs,
immersing the person in ice water) and replacing fluids, if
You can avoid getting heat stroke by wearing shorts and
other loose clothing, drinking plenty of water or sports drink
and exercising in cool weather (below 82 degrees F or 28
Training Your Body Your body can get more
out of exercise and can exercise more easily with training.
Athletes spend a great deal of time training. It allows the
body to adapt its basic response to exercise and to improve
athletic performance. Training can:
make your muscles perform better
match what you eat with what your body will use in
improve the efficiency of oxygen delivery to working
get you used to the competition environment
Getting the Most from Muscles If you
exercise regularly or if you are an athlete in training, you
are trying to make your muscles work better. You want to be
stronger if you are a weightlifter, you want to be able to
throw a blistering fast ball if you are a baseball pitcher or
you want to be able to finish strong at the end of a 26-mile
race if you are a marathon runner. Those three activities
illustrate three major factors in muscle performance:
Muscle strength is the maximal
force that a muscle can develop. Strength is directly related
to the size (that is, the cross-sectional area) of the muscle.
Muscle fibers are capable of developing a maximal force of 3
to 4 kg/cm2 (average = 3.5
kg/cm2) of muscle area. So,
let's say that you have increased your muscle size from 100 to
150 cm2, then the maximal
resistance that you could lift could be increased from 350 kg
(770 lb) to 525 kg (1,155 lb).
The power of muscle contraction is how fast the
muscle can develop its maximum strength. Muscle power depends
on strength and speed [power = (force x distance)/time]. A
person can have extreme power from muscles (7,000 kg-m/min)
for a short period of time (about 10 seconds) and then power
reduces by 75 percent within 30 minutes; this aspect is
important for sprinters because it gives them great
acceleration. Muscle endurance is the capacity to
generate or sustain maximal force repeatedly.
But even if you train hard every day, you still might not
be able to make your muscles perform as well as another
person's. Athletes are not just made; they are born, too.
Strength, power and endurance may be due in part to the
distribution of fiber types within an individual's muscles.
Muscles have a mixture of two basic types of fibers, fast
twitch and slow twitch. Fast-twitch fibers are
capable of developing greater forces and contacting faster and
have greater anaerobic capacity. In contrast, slow-twitch
fibers develop force slowly, can maintain contractions longer
and have higher aerobic capacity. Your genes largely determine
whether you have more of one kind of muscle fiber or another.
Sprinters tend to have more fast twitch fibers. Marathon
runners tend to have more slow twitch fibers. And the rest of
us tend to have an equal distribution of both fiber types. It
is not clear whether training can change the distribution of
fiber types within an individual.
The training to improve strength, power and endurance of
muscle performance is called resistance training (for
example, free weights, jump-training and isometric training).
Resistance training mostly increases the size of muscle fibers
(hypertrophy). It is not clear whether training can
increase the number of muscle fibers (hyperplasia).
Muscle fibers get bigger by having more muscle protein
content, and that is achieved by making new protein and
decreasing the rate at which existing proteins are broken
down. These proteins include contractile proteins as well as
the enzymes that
are involved in various metabolic reactions. By increasing the
strength of muscles, resistance training can also increase the
power of muscles. Increases in strength, diet and improved
cardiovascular performance can increase muscle endurance.
Finding the Right Diet You can help your
body to exercise better by eating the right foods. You know
that muscle metabolism involves the phosphagen system,
glycogen-lactic acid system and aerobic respiration. The major
fuels used are glucose and glycogen. So, if you want to do
well, whether you are competing or just exercising for
well-being, you should try to increase the stores of glycogen
in your liver and your muscles. Athletes eat solid,
high-carbohydrate diets (breads, pasta) the night before
competition, and liquid, high-glucose diets in the morning
before competition. Sports drinks containing glucose are good
to drink during competition to replace fluid and help to
maintain blood glucose levels.
Getting More Oxygen Quickly To become a
world-class athlete or to get the most out of your exercise,
you want your muscles to get the oxygen they need most
efficiently. To do that you need to increase:
the amount of oxygen carried by the blood
can do this by resistance training, possibly in combination
with cross-training, training for more than one sport
at a time or for multiple fitness components (strength,
endurance and flexibility) at the same time.
The main effects of training on the cardiac output appear
to be an increase in stroke volume (that is, a larger heart)
and a decrease in the resting heart rate. The increased stroke
volume allows the heart to pump more blood with each beat.
Because there is a limit to the maximum heart rate (180-190
beats/min), then a slower resting heart rate (50-60 beats/min
in the trained athlete vs. the normal 70-80 beats/min) allows
the heart to have a greater increase in heart rate during
exercise. The greater increase in heart rate during exercise
along with the larger volume increases cardiac output and
blood flow to working muscle.
Training can help the respiratory system by decreasing the
resting rate of breathing, increasing the respiration rate
during exercise and increasing the volume of air exchanged
with each breath (tidal volume). These changes allow
the lungs to take in more air during exercise. Training can
also boost the amount of oxygen that the working muscles take
from the blood, which probably reflects the increases in
You have probably heard about runners or cyclists who train
in the mountains. This kind of training can actually increases
the amount of oxygen carried by the blood forcing the body to
develop more hemoglobin in the blood. Because there is less
oxygen in high altitudes, the body responds by producing a
hormone called erythropoietin
(EPO), which causes the bone marrow to produce more red blood
cells and more hemoglobin. Some athletes try to take a
shortcut by injecting EPO directly into the bloodstream, but
this is a dangerous practice. The International Olympic
Committee has banned the use of EPO because it increases the
thickness of the blood, which can lead to circulatory problems
Train Where You'll Exercise If you are an
athlete and you'll be competing in a place at a high altitude,
as during the 1968 Olympics in Mexico City, then training at
high altitudes would be helpful. If the competition is in a
hot climate, then gradual periods of training in hot weather
can allow the body to increase its efficiency in eliminating
heat (increasing sweat production in the most-exposed areas of
The body's response to exercise is a carefully orchestrated
response of various systems (muscle, heart, blood vessels,
lungs, nervous system and skin) designed to meet the needs of
working muscles. These systems can be improved by training,
thereby improving athletic performance.