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.