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Case Study: Athletes
Noah Mayeda
Liberty University
EXSC 510: Advanced Exercise Physiology
Dr. Kilian
September 18, 2022
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Our body adapts to the demands that are placed on it. By forming goals, creating a
training program, and progressive overload, an athlete can stimulate muscle growth and
adaptations for better performance. This is why athletes train exercises relating to their sport. For
example, a shot putter does not need endurance, so going for long runs will not benefit them and
could possibly lessen their progress. A shot putter would need to work on quick, explosive
motions to launch the shot put as far as possible. Along with muscle adaptions, other factors that
affect performance are energy systems, and genetics.
The first factor is looking at the types of muscle. There are generally three types of
skeletal muscle, type I, type IIa, and type IIx. Type I are considered slow twitch, while type IIa
and type IIx are considered fast twitch fibers. Type I fibers have a high volume of mitochondria,
lots of capillaries, and a high concentration of myoglobin. Those three aspects of type I fibers
create a large capacity for aerobic metabolism and high resistance to fatigue (Powers, 195). They
are considered slow twitch fibers because they have a lower contractile velocity and force. This
is due to less actin and myosin per cross sectional area, which are proteins that produce thin and
dense contractile filaments within muscle cells. Despite being less forceful, type I fibers are more
efficient due to greater mitochondria and a greater ability to utilize aerobic metabolism.
The next type of muscle fiber is type IIx. This fiber type is focused on quick, powerful
contractions. They have a lower number of mitochondria, limited ability for aerobic metabolism,
and less resistance to fatigue than slow fibers (Powers, 196). This muscle fiber type relies heavily
on anaerobic energy systems. Type IIx also has the highest myosin ATPase activity which results
in the highest maximum velocity of all muscle fibers, creating the highest amount of power
(Powers, 196). Although this type is powerful, it is also the least efficient. Since it has such great
power output it causes lots of energy to be expended.
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The third muscle fiber type is the greatest factor that influences muscle adaptations. The
type IIa fiber is also known as intermediate fibers. They lie somewhere between type I and type
IIa. Where the force produced by type IIa is similar to that of type IIx, but also has fatigue
characteristics between type IIx and type I fibers. This fiber type is important because it is the
most malleable to take on characteristics of the other two fiber types. Through proper training the
percentage of total fast or slow twitch muscle fibers can increase. The increase in type I fibers
result from an increase in mitochondrial density in fibers, greater ability to metabolize fat,
improved muscle antioxidant capacity, and increased capillary density (Powers, 320). This
training adaptation can also be applied to increasing the type IIx fibers, by forcing the type I and
type IIa fibers to take on characteristics of type IIx.
Apart from muscle adaptations, athletes also improve the energy system that their sport
requires. The energy systems Tommy and Jimmy use are the ATP-PC system and aerobic
processes. A shot putter executes their performance in just a few seconds, which would use
almost entirely the ATP-PC system. Which works with phosphocreatine stored in the muscles to
replace the third phosphate lost when ATP is used. Whereas a marathon runner will use almost
exclusively aerobic metabolic pathways, the ATP-PC system and glycolysis will still be active.
I believe the anaerobic glycolysis system is more important to the marathon runner. Even
though the marathon is an endurance event, the glycolysis system is still being used for the
entirety of the race. While the shot put will not always use glycolysis. The reason for this is that
the shot put only lasts a few seconds and the ATP-PC system lasts from 5-10 seconds, and
glycolysis is not used until after the ATP-PC system has been exhausted. Whereas the marathon
runner will use all three metabolic pathways, even though almost all their energy will be created
from aerobic metabolism.
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The third factor affecting their performance is genetics. Going back to muscle
adaptations, even though training can promote a shift in muscle fibers, it is not a complete shift.
Meaning, it is not a complete and permanent change. That our genetics determine how much we
have of each fiber type and “aerobic capacity possessed by elite male and female endurance
athletes is likely the genetic gift of a high percentage of slow muscle fibers” (Powers, 321).
-Noah Mayeda
