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Muscle: How it works, oxygen, nerves and fber types
INTRODUCTION
The human body is an amazing array of muscles each with a specific movement
function. Several muscles must work in perfect harmony to produce a specific series
of movements demanded by a sport skill.
We've discussed some aspects of muscle but now we are going to take a closer look
at how it works, and examine two important supporting structures that supply oxygen
and electrical stimulation – the capillaries and the nerves.
We will also briefly overview the three main categories of muscle – the slow twitch and
the two forms of fast twitch fibers. These three fiber types each have unique
characteristics making them useful for performing a variety of movement tasks. They
are recruited in a specific order. Part of the objective of training is to make sure the
athlete is able to recruit and train the correct muscle fiber type for optimal
performance.
When you have completed this module you will
• Understand how skeletal muscle works to produce the speed and force the
athlete needs to perform relevant movement skills – running long or running
fast, jumping high or far, throwing far, etc.
Quick review of muscle fiber structure
The muscle itself is made up thousands of muscle fibers. A single muscle fiber is
actually the muscle cell. A muscle cell can be very long, and is packed with contractile
units that are called sarcomeres. When a nerve stimulates a muscle fiber overlapping
filaments in the sarcomere slide past one another and the sarcomere shortens.
There's a lot of activity going on inside the sarcomere. This thick unit is called myosin
and contains myosin heads. The actin contains active spots where the myosin heads
bind. When a motor neuron stimulates a muscle fiber calcium ions are released from a
storage container surrounding the muscle. Calcium exposes the myosin binding site
on the actin. The myosin head is strongly attracted to these unexposed binding sites
and will bond to it forming a cross-bridge. ATP provides the energy to 'unbind' the
myosin head from the actin binding site. The myosin head detaches and goes back to
its resting state ready to bind to another binding site on the actin. All the myosin heads
act independently so they are not all binding and releasing at the same time.
The muscle's blood supply
The mitochondria require a steady and nearly instantaneous supply of oxygen. Every
minute 1 ml of mitochondria consumes about 5 mL of oxygen when the athlete is
working at VO2max. The blood supplying the muscle flows predominantly parallel past
each muscle cell in a network of capillaries. Capillary density around a muscle
depends on the mitochondria content of the fibers. A muscle producing ATP
aerobically will have a lot of mitochondria. For this reason endurance athletes have a
dense capillary network supplying oxygen to their working skeletal muscles.
However, the number of capillaries supplying a muscle is not directly proportional to
the number of mitochondria in the muscle. When you look at both the capillary network
and the number of erythrocytes (red blood cells) though, the two together are
proportional to mitochondrial volume. So, athletes not only have an increased network
of capillaries around their working muscles, their red blood cell count (hematocrit) is
also higher. It is more efficient for the body to split structural adaptations to higher
work output between the developing the capillary network and increasing red blood
cells. Capillaries are “expensive” to build in terms of energy and difficult to maintain,
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Muscle: How it works, oxygen, nerves and fber types
so the body will try to minimize structural building whenever possible. In male and
female athletes around 5-7 capillaries surround each muscle fiber. It is higher in
athletes than it is in the general population.
Red blood cell volume increases with training. A higher hematocrit (higher percentage
of red blood cells) increases blood viscosity making it more difficult for the heart to
pump the blood throughout the body. This is not normally a problem, though, because
an athlete’s plasma volume also increases in proportion to their red blood cell volume.
For this reason an athlete’s blood is less viscous than a sedentary individual’s blood
so long as the athlete did not artificially enhance red blood cell volume by using a
blood doping strategy.
The muscle's nerve supply
The brain is the control center that coordinates the activities of the body. The spinal
cord is the communication highway between the brain to the body. Together the spinal
cord and brain make up the central nervous system, or CNS.
Other nerves extend from the spinal cord to all parts of the body. These nerves make
up the peripheral nervous system, or PNS. Some nerves send information to the brain
or spinal cord providing information about the status of a muscle fiber and other
conditions in the body such as low oxygen levels, high carbon dioxide levels, low
glycogen levels, etc. Other nerves carry instructions from the brain and spinal cord to
all parts of the body. These are the instructions telling the muscles and other organs in
the body what to do.
Muscle will contract only when it receives an electrical signal from the brain.
Remember all that calcium entering into the muscle fiber from the storage container?
The signal from the brain (via the peripheral nerves) stimulates the calcium storage
container to release the calcium into the muscle fiber. The calcium, in turn, exposes
the hot spots on the actin so the myosin heads can attach to the actin and shorten the
sarcomere. ATP provides the energy needed to release the myosin head so it can
grab hold of another binding site on the actin. So long as calcium remains in the
muscle fiber to keep the hot spots on the actin exposed the muscle will keep
contracting. When the muscle has completed its action the brain stops sending its
signals and the calcium is pumped back into the storage container. The muscle
relaxes.
The nerves relaying information from the CNS to the muscle fibers are called efferent
fibers (think E for exit). The nerves sending information about what is going on in the
muscle to the CNS are called afferent fibers. Afferent fibers are connected to special
sensors in the muscle and its tendon that monitor the status of a muscle fiber, such as
its length and tension. We will talk more about afferent and efferent nerves when we
discuss the stretch-reflex mechanism of muscle.
Muscle fiber type categories
There are two basic categories of muscle fibers. One category is the fast twitch fiber,
and the other is the slow twitch fiber.
Different muscle fibers vary in terms of how fast the contract and relax. Muscle A may
reach its peak tension quickly and it also relaxes very quickly. Muscle B may take a bit
longer to reach its peak tension and a little bit longer to relax than Muscle A did.
Muscle C takes longer still. There reason for this difference is that muscles contain
different populations of fibers. Some fibers in a muscle contract very slowly while other
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Muscle: How it works, oxygen, nerves and fber types
fibers in another muscle contract very fast.
The slow acting fibers are called slow-twitch fibers and all the others are called fast
twitch fibers. The velocity of muscle contraction depends on how quickly the myosin
heads bind and release from the actin. The fast twitch fibers are able to use ATP very
quickly allowing the sarcomere to shorten at a very fast rate.
They also vary in the amount of tension they can produce. Slow twitch fibers have a
lower rate of force production.
Characteristics of slow twitch versus fast twitch fibers
There are many other significant differences between the slow twitch and fast twitch
fiber.
Slow twitch fibers are generally thinner and have a denser capillary network
surrounding them because they have a high number of mitochondria. They appear red
because of a large amount of oxygen binding protein called myoglobin within their
cytoplasm. Their rich capillary network facilitates oxygen transport to the mitochondria
to produce ATP. Slow twitch fibers have a large number of mitochondria because this
is their dominate ATP production method. They have have a low glycogen content and
low glycolytic enzyme activity. In other words, they are not good at producing ATP
anaerobically, but are excellent at producing ATP aerobically because of their
mitochondrial content.
Fast twitch appear white and have high glycogen content. For this reason
they are called glycolytic fibers. They are really good at producing ATP anaerobically.
Both fast twitch and slow twitch fibers are found in all muscles of the body, but their
proportions vary among the different muscles. The proportions also vary within
athletes based on their genetics.
Types of fast twitch fibers
When it comes to fast twitch fibers things get a bit complicated because there are two
types of fast twitch fibers. It's important to distinguish between the two types of fast
twitch fibers because you train them differently depending on what you want to
accomplish – speed or endurance.
Some fast twitch fibers are fatigue resistant because of their high proportion of
mitochondria and ability to produce ATP aerobically. They also have high glycogen
content and are able to produce ATP anaerobically as well. For this reason they are
referred to as fast oxidative glycolytic fibers or FOG fibers. An important distinction
between FOG fibers and slow twitch fibers is the high glycogen content of the FOG
fiber.
The other cluster of fast twitch fibers don’t have many mitochondria, but have a lot of
glycogen. For this reason they are called fast glycolytic or FG fibers. The FG fibers
have relatively few enzymes for aerobic energy production. However, they are very
rich in glycolytic enzymes enabling them to produce a lot ATP anaerobically. The FG
fiber is is also very “elastic” allowing it to store energy. Slow twitch fibers are generally
less elastic and stiffer than fast twitch fibers.
When FOG or FG fibers are producing ATP at a high rate, lactic acid is produced as a
byproduct. This is one reason why glycolytic fibers fatigue more quickly than slow
oxidative fibers do. The slow oxidative fibers don't have a lot of glycogen and therefore
their ability to produce lactic acid is quite low.
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Muscle: How it works, oxygen, nerves and fber types
You can train FOG fibers to act like slow twitch fibers or you can train them to act like
an FG fiber. When you train them to act like slow twitch fibers you are stimulating the
growth of mitochondrial content. When you train them to act like FG fibers you are
stimulating the glycolytic enzyme production capacity of the fiber. You can do both, but
they will end up being average FOG fibers more useful to endurance athletes than to
power athletes.
The FG fibers are capable of high force and are generally the largest of all the three
muscle fiber types.
Recruitment
How does the brain know which type of muscle cells to use for different
sports? Well, according to our current knowledge, the brain will always call
upon the slow oxidative muscle fibers first and then, if more force is needed it
will recruit the fast oxidative glycolytic fibers and then if even more force is
needed it will recruit the fast glycolytic fibers. As you can see on this chart the
FG fibers provide the highest level of power, the FOG fibers are next in terms
of power development followed by slow oxidative fibers that provide the
athlete with the lowest level of power.
The major factor is the level of power or force demanded to perform the
specific skill. A movement unit requiring low force such as jogging, fast
walking, casual cycling, will, for the most part, only need the slow oxidative
muscle fibers.
The important point is that the slow twitch fibers are always recruited first
regardless of the force required. However, if the skill demands a high force the
fast twitch fibers are called into action. They are also called into action during
prolonged activity that leads to fatigue. The high mitochondrial content of FOG
fibers makes then particularly useful fibers to support the slow twitch fibers
when necessary.
FG fibers are very high threshold fibers and untrained individuals are unable to
fully activate them. They demand very specific, high intensity training and are
recruited if slow twitch fibers and FOG fibers cannot meet the demands of the
exercise intensity.
Repetitive high stimulation activates high threshold fibers and this enhances
the athlete’s ability to produce a maximal force. We'll come back to the
significance of this when we talk about maximal force production and the
stretch-reflex mechanism.
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