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Better Exercising
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.
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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.
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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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