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Muscle Fiber Types and Contraction
Muscles generate heat
and force for movement, help us breathe, and keep our
bodies upright. Skeletal muscle tissue is composed of
two fibers, actin (thin fibers) and myosin (thick fibers).
These two fibers give the muscle a striated appearance.
In order for muscle to contract it must first be stimulated
by nerves called motor neurons. A single motor neuron
and the muscle fibers stimulated by it is called a motor
unit. The recruiting motor units play a large part in
the force of the muscle during contraction. The more
motor units (muscle fibers) recruited, the stronger
the force of contraction.
Muscle fibers are classified as Type
I, Type IIa and Type IIb fibers. "Fast" and
"slow" twitch are also two other classifications
for muscle fibers. Type I fibers (slow twitch) fibers
are more resistant to fatigue than Type IIa or IIb fibers
and have a high capacity for aerobic metabolism, fatigue
faster and are mainly anaerobic.
Slow twitch fibers are mainly for endurance
while fast twitch are for speed and performance. A muscle
will generally have an equal amount of both fast and
slow twitch muscle fibers. In regards to hypertrophy
(muscle growth), fast twitch fibers grow faster and
larger than slow twitch. Within the fast twitch muscle
fibers, type IIa fibers are considered intermediate
between fast and slow twitch fibers in relation to speed
and contraction. For example, Type IIa fibers can become
more glycolytic or aerobic depending on the type of
training an athlete performs. If an endurance runner
were to stop running and start weight lifting, then
his or her Type IIa fibers would become more glycolytic
in order to handle the stress of the activity.
Muscle growth and endurance is an adaptation
to stress. For example, a sprinter will develop large
quadriceps and hamstrings in order to adapt to the stress,
while an endurance runner will develop more endurance
to efficiently handle the stress. Type I muscle fibers
respond to stress by becoming more efficient and stronger
with slight hypertrophy, rather than the extreme hypertrophy
seen with Type IIa and IIb muscle fibers. This is the
premise behind trainers recommending 6 reps for pure
strength/muscle gain and why 10-15 reps are recommended
to "tone" a muscle.
Finally, there are four different actions
a muscle can perform; isometric, eccentric, concentric,
and isotonic. An example of an isometric contraction
would be pushing against a wall. Lifting a dumbbell
during a bicep curl is considered the concentric portion
while lowering of the weight is called the eccentric
portion of the exercise. There are also called the positive
and negative portions respectively. And finally, isotonic
contractions are those that involve full body actions
such as skating or running.
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