Yes, there are different types of muscle fibers in the body,
which are classified based on how they produce energy. Yes, the
different muscle fibers can be trained using specific exercises
designed to focus on how they create energy or generate force.
While a variety of types of muscle fiber have been identified,
including type I, type IC, type IIC, type IIAC, type IIA, type IIA
and type IIX, they are generally classified as being either
slow-twitch or fast-twitch (see table).
Six things to know about slow-twitch, or type I, muscle
fibers:
- Slow-twitch fibers contain mitochondria, the organelles that
use oxygen to help create adenosine triphosphate (ATP), which is
the chemical that actually fuels muscle contractions, and are
considered aerobic.
- Slow-twitch fibers are also called red fibers because they
contain more blood-carrying myoglobin, which creates a darker
appearance.
- Because they can provide their own source of energy,
slow-twitch fibers can sustain force for an extended period of
time, but they are not able to generate a significant amount of
force.
- Slow-twitch fibers have a low activation threshold, meaning
they are the first recruited when a muscle contracts. If they can’t
generate the amount of force necessary for the specific activity,
the fast-twitch muscle fibers are engaged.
- The tonic muscles responsible for maintaining posture have a
higher density of slow-twitch fibers.
- Steady-state endurance training can help increase mitochondrial
density, which improves the efficiency of how the body uses oxygen
to produce ATP.
As you can see, slow-twitch fibers have specific characteristics
for how they function, which means they can be trained to be more
aerobically efficient with the proper exercise program.
Techniques for training slow-twitch fibers:
- Exercises that feature sustained isometric contractions with
little-to-no joint movement keep the slow-twitch muscle fibers
under contraction for an extended period of time. This can help
improve their ability to utilize oxygen to produce energy. Examples
include the front plank, the side plank and the single-leg
balance.
- Resistance-training exercises using lighter weights with slower
movement tempos for higher numbers of repetitions (i.e., more than
15) can engage the slow-twitch fibers to use aerobic metabolism to
fuel the activity.
- Circuit training, which involves alternating from one exercise
to the next with little-to-no rest while using lighter weights, can
be an effective way to challenge slow-twitch fibers.
- Body-weight exercises for higher numbers of repetitions can be
an effective way to challenge aerobic metabolism, which helps
improve the efficiency of slow-twitch fibers.
- When working with body-weight only or lighter amounts of
resistance, use shorter rest intervals of approximately 30 seconds
between sets to challenge the slow-twitch fibers to use aerobic
metabolism to fuel the workout.
Here are things to know about fast-twitch, or type II, muscle
fibers:
- Fast-twitch fibers can be further classified into (1)
fast-twitch IIa - fast oxidative glycolytic, because they use
oxygen to help convert glycogen to ATP, and (2) fast-twitch type
IIb - fast glycolytic, which rely on ATP stored in the muscle cell
to generate energy.
- Fast-twitch fibers have a high threshold and will be recruited
or activated only when the force demands are greater than the
slow-twitch fibers can meet.
- The larger fast-twitch fibers take a shorter time to reach peak
force and can generate higher amounts of force than slow-twitch
fibers.
- Fast-twitch fibers can generate more force, but are quicker to
fatigue when compared to slow-twitch fibers.
- The phasic muscles responsible for generating movement in the
body contain a higher density of fast-twitch fibers.
- Strength and power training can increase the number of
fast-twitch muscle fibers recruited for a specific movement.
- Fast-twitch fibers are responsible for the size and definition
of a particular muscle.
- Fast-twitch fibers are called “white fibers” because do not
contain much blood, which gives them a lighter appearance than
slow-twitch fibers.
As you can see, the characteristics of fast-twitch fibers are
more suited for explosive, strength-and power-based sports like
football. Therefore, when an announcer talks about how a training
program benefits a specific type of muscle fiber, they are being
accurate with the science.
If you want to engage more fast-twitch fibers to help you
increase strength levels or become more explosive, here are a few
specific techniques that work.
Techniques for engaging fast-twitch fibers:
- Resistance training with heavy weight stimulates muscle motor
units to activate more muscle fibers. The heavier the weight, the
greater the number of fast-twitch fibers will be recruited.
- Performing explosive, power-based movements, whether it is with
a barbell, kettlebell, medicine ball or simply your own body
weight, will recruit greater levels of fast-twitch fibers.
- Fast-twitch fibers will fatigue quickly, so focus on using
heavy weight or explosive movements for only a limited number of
repetitions (e.g., two to six) for maximum effectiveness.
- Because they deplete energy quickly, fast-twitch fibers require
longer rest periods to allow motor units to recover and to replace
spent ATP. Therefore, allow at least 60 to 90 seconds of rest after
each explosive or strength exercise.
Understanding how the physiology of the body adapts to exercise
can help you develop more effective exercise programs for your
specific needs. Genetics determines how much of each muscle-fiber
type you possess; however, identifying whether you are fast- or
slow-twitch dominant would require an invasive muscle biopsy.
Therefore, if you find that you tend to enjoy more endurance-based
activities and that they are relatively easy for you, you probably
have a greater number of slow-twitch fibers. Conversely, if you
really dislike going for long runs, but enjoy playing sports that
rely on short bursts of explosive movements, or if you like weight
training because it is relatively easy, you are probably
fast-twitch fiber dominant. An exercise program that applies the
right training strategies for your muscle fibers can help you to
maximize the efficiency and enjoyment of your workout time.