Toning up, or improving muscle definition, is one of the most common
goals of individuals starting an exercise program. The term tone comes
from “tonus,” which is the technical term for a state of contraction of a
muscle fiber. If all of the fibers within a muscle contract, it creates
the shape commonly associated with a well-defined muscle.
The primary purpose of muscle fibers is to control physical forces
moving through the body. Muscle-shortening actions can generate a force
to move a resistance; for example, when moving from a seated to a
standing position, the quadriceps and gluteus maximus shorten to help
the body stand up against gravity. Muscle-lengthening actions can be
applied to control and decelerate a force; for example, the quadriceps
and glutes lengthen to control the motion of the body as it returns to a
seated position. If you are interested in designing exercise programs
that produce results, it is important to have a good understanding of
how muscle fibers work and respond to exercise.
To more effectively help your clients achieve their fitness goals,
keep in mind these important facts about muscle fibers and how they are
affected by exercise:
1. Muscle fibers adapt to the specific type of exercise stimulus
imposed during training. Mechanical stress refers to the physical
stresses applied during resistance training, which cause microtrauma to
muscle fibers. As fibers are damaged from exercise, they signal the
biochemical reaction to produce new satellite cells responsible for
repairing the mechanical structure of the muscle cell and for building
new muscle proteins.
2. Muscle fibers are activated by a motor neuron, which is the
connection between the central nervous system and the specific muscle
required to perform a particular activity. A muscle motor unit is the
motor neuron and the attached muscle fibers. Think of a motor unit as a
light switch for the muscle—as a muscle is required to generate a force,
the motor units will “light up” to stimulate the fibers to shorten in
order to produce that force. There are a number of different muscle unit
types, which are typically organized into three general categories:
type I, type IIA and type IIB
3. According to the all-or-none theory, a motor unit is either active
or inactive. When it is signaled to contract, it activates all of its
attached muscle fibers. Slow-twitch motor units have a low threshold for
activation and low conduction velocities, and are attached to type I
muscle fibers. Fast-twitch motor units have a higher activation
threshold, are attached to type II muscle fibers and are capable of
conducting signals at higher velocities, resulting in greater amounts of
muscle force.
4. Type I fibers are known as aerobic fibers. These fibers have a
higher density of mitochondria, which are efficient at aerobic
metabolism (the process of creating energy to fuel muscle activity with
oxygen). The mitochondria give the cell a darker color, which is why
these are known as red muscle fibers. Type I fibers use oxygen to create
energy for lower-intensity, long-term, endurance-oriented activities
like walking, running, swimming, cycling or standing for extended
periods of time.
5. Type IIB fibers are known as anaerobic muscle fibers. Type IIB
fibers store energy that is released for short, explosive, extremely
high-intensity activities. Type IIB fibers do not have mitochondria and
have a colorless appearance, which is why they are known as white
fibers. Because they have a limited supply of stored energy, and thus
fatigue quickly, Type IIB fibers are used for strength and power
activities requiring a high amount of force in a short period of time.
6. Type IIA fibers have mitochondria, so they can be involved in
aerobic activities; however, they can also be used to produce force
rapidly during activities requiring a high amount of strength or power.
Fast-twitch muscle fibers also have a greater diameter than type I
fibers and play a more significant role in hypertrophy (the technical
term for muscle growth). Recruiting and innervating type II muscle
fibers requires creating enough mechanical overload to fatigue the
involved muscle by the end of the set
7. Muscle fibers work according to the size principle. As a muscle
requires force, it will start by activating the smaller type I motor
units. When these type I motor units can’t provide the necessary force
(or fatigue), the larger type II motor units and muscle fibers are
recruited to perform the work. A muscle has a finite number of motor
units and the higher-threshold type II motor units are not “turned on”
unless a high level of force is needed. The most common way to increase
motor unit activation is to lift heavier weights, because an increased
load placed on a muscle will cause a greater number of motor units to
activate more fibers to generate the force necessary to overcome the
resistance. This is why your muscles shake when you try to lift a heavy
weight for the first time—muscle motor units not previously used are
being “woken up” and called into action.
8. Muscle fibers experience two specific types of hypertrophy.
Myofibrillar hypertrophy
refers to the increase in size or thickness of individual actin and
myosin protein filaments, which can improve the force-production
capacity of individual fibers. Myofibrillar hypertrophy does not lead to
larger muscles; rather, it results in thicker muscle fibers capable of generating more force.
Sarcoplasmic hypertrophy
is an increase in the volume of the semifluid interfibrillar substance
surrounding an individual muscle fiber. This fluid contains the proteins
used to promote tissue repair and growth. The muscle “pump” that
bodybuilders work to achieve is actually sarcoplasmic hypertrophy—the
cross-section of muscle fibers will increase, but most of the enhanced
muscle size is due to an increased volume of the sarcoplasm and
non-contractile proteins not directly involved with force production.
9. One of the long-term adaptations of muscle to
resistance training
is an increase in muscle fiber cross-width. As the cross-sectional area
increases in size, the fibers have more surface tension and become
capable of generating higher amounts of force. Muscles with a larger
cross-sectional area of individual muscle fibers are capable of
producing greater amounts of force. In addition to being responsible for
producing the force necessary for dynamic movements, type II muscle
fibers have a greater diameter (cross-width) than type I fibers and are
responsible for the hypertrophy, or increased size, of a particular
muscle.
10. Type IIA and IIB muscle fibers are responsible for generating
movement as well as muscle size and definition. Both classifications of
type II muscle fibers create higher levels of force to produce human
movement and are known as phasic muscles. Type I fibers are responsible
for maintaining postural and joint stability, and can be categorized as
tonic muscles.
If your client has a fitness goal related to improving muscle
definition or building strength, the only way to achieve this is by
activating the type II motor units and muscle fibers. There is a common
misconception that developing muscle tone (“toning up”) is best achieved
by using lighter weights for higher repetitions. While it is possible
to achieve definition with lighter weights, it requires doing a high
volume of repetitions. Using light weights for high repetitions can
improve the aerobic efficiency of type I fibers, but unless the exercise
is performed to fatigue which occurs only after recruiting the type II
fibers, it will not result in the desired goal of definition. However,
if time is an issue (as it is for many people), using a heavier weight
for fewer repetitions is an effective technique for stimulating growth
and definition from the type II fibers. It is important to note that
exercising with higher repetition ranges will result in muscle growth
from sarcoplasmic hypertrophy, while using heavier loads for fewer reps
induces myofibrillar hypertrophy, which increases strength without
necessarily inducing significant gains in muscle growth.
Source
We often get into a routine in our exercise and consequently plateau. When you change your workout you can reap benefits like losing weight and looking great.
