Chapter 10 Module 3 Muscle Tension and Fiber Types

Chapter 10 Module 3 Muscle Tension and Fiber Types


This is Chapter 10, Module 3–
Types of Muscle Contractions and Types of Muscle Fibers. The learning objectives
of this module are one, distinguish between
the different types of muscle contractions, isometric
and isotonic, including concentric and eccentric, and
two, relate types of muscle fibers to muscular performance. When sarcomeres shorten
in a contraction, they shorten the muscle fiber. This shortening exerts tension
on the connective tissue fibers attached to the muscle fiber. The tension produced by an
individual muscle can vary. The amount of tension
produced by the muscle as a whole is the sum of
the tensions generated by individual fibers, since
they are all pulling together. For this reason, the
amount of tension produced by a skeletal muscle can
be controlled by the number of muscle fibers it stimulates. A typical skeletal
muscle contains thousands of muscle fibers. Some motor neurons control
a few muscle fibers, but most control
hundreds of them. All the muscle fibers controlled
by a single motor neuron constitute a motor unit. The size of a motor
unit is an indication of how fine the control
of the movement can be. In the muscles of the
eye, where precise control is extremely important,
a motor neuron may control four to
six muscle fibers. We have much less precise
control over our leg muscles, where a single motor neuron may
control 1,000 to 2,000 muscle fibers. In any skeletal muscle,
some motor units are always active, even when
the entire muscle is not contracting. Their contractions do not
produce enough tension to cause movement, but they
do tense and firm the muscle. This resting tension
in a skeletal muscle is called muscle tone. A muscle with little muscle
tone appears limp and flaccid, whereas one with moderate
muscle tone is firm and solid. Resting muscle tone
stabilizes the positions of bones and joints. For example, in order to
maintain body position, there must be enough motor
units stimulated to produce enough tension to do so. We classify muscle contractions
as isotonic or isometric. Isotonic contraction is
where the tension rises and the skeletal
muscles’ length changes. Lifting an object off the
desk, walking, and running involve isotonic contractions. Two types of
isotonic contractions exist– concentric
and eccentric. In a concentric contraction,
the muscle tension exceeds the load and
the muscle shortens. In an eccentric contraction,
the peak tension developed is less than the
load, and the muscle elongates due to the
contraction of another muscle or the pull of gravity. Think of a tug-of-war team
trying to stop a moving car. Although everyone pulls
as hard as they can, the rope slips
through their fingers. During physical
training, people commonly perform cycles of concentric
and eccentric contractions, as when you do bicep curls by
holding a weight in your hand and slowly flex and
extend your elbow. During flexion,
your biceps involve concentric contractions. During extension, your biceps
are still actively contracting, but now involve
eccentric contraction. In an isometric contraction,
the muscle as a whole does not change length and
the tension produced never exceeds the load. Examples of isometric
contractions include carrying
a bag of groceries and holding our heads up. When you perform an
isometric contraction, the contracting
muscle bulges, but not as much as it does during
an isotonic contraction. After a contraction,
a muscle fiber must return to its
original length. However, there are no active
mechanisms for muscle fibers to elongate. Therefore, the muscle fiber
returns to its original length through a combination of
elastic forces, opposing muscle contractions, and gravity. The tendons and the cell
fiber will gradually return the muscle’s fiber to its
original resting length because of its elasticity. The contraction of
opposing muscles can return a muscle
to its resting length more quickly than the
elastic factors can. For example, when the biceps
brachii muscle contracts, the triceps brachii
muscle extends the elbow and stretches the muscle fibers
of the biceps brachii muscle to the original length. And gravity may assist
opposing muscle groups in quickly returning a
muscle to its resting length after a contraction. For example, when the biceps
brachii muscle contracts, the elbow’s bent with
the forearm elevated. When the muscle relaxes, gravity
will pull the forearm down and stretch the muscle. The demand for ATP in a
contracting muscle fiber is so high that it would
be impossible to have all the necessary
energy available as ATP before the contraction begins. Instead, a resting muscle
fiber contains only enough ATP to sustain a contraction
until additional ATP can be generated. Throughout the rest
of the contraction, the muscle fiber will generate
ATP at roughly the same rate as it is used. Muscle performance capabilities
depend on muscle fiber type and physical condition. A muscle’s performance
involves force, which is the maximum
amount of tension produced by a particular
muscle, and endurance, which is the amount of time
during which the individual can perform a particular activity. Several factors determine
the performance capabilities of any skeletal
muscle– the type of distribution and size of
muscle fibers in the muscle, as well as physical
conditioning and training. The human body has
three major types of skeletal muscle fibers–
fast fibers, slow fibers, and intermediate fibers. Most of the skeletal
muscle fibers in the body are called fast
fibers, because they can reach peak twitch
tension very quickly. Fast fibers are large in
diameter and contain densely packed myofibrils, large
reserves of glycogen, and few mitochondria. Muscles dominated by fast fibers
produce powerful contractions. But the fibers fatigue rapidly,
because their contractions use ATP in massive amounts. With so few
mitochondria, they are unable to generate ATP to
support prolonged activity. Slow fibers have only about
half the diameter of fast fibers and take three times as
long to reach peak tension after stimulation. They are able to
continue contracting longer than a fast
fiber, because of the extensive capillary
network that supplies oxygen to support the activity
of the mitochondria. Slow fibers also contain
the red pigment myoglobin, which is similar to hemoglobin. Myoglobin helps to
deliver oxygen molecules. Skeletal muscles
dominated by slow fibers are dark red,
because slow fibers have both an extensive capillary
supply and a high concentration of myoglobin. Intermediate fibers most
closely resemble fast fibers, for they contain
little myoglobin and are relatively pale. They do have an intermediate
capillary network and mitochondrial
supply around them and are more resistant to
fatigue than are fast fibers. Muscles dominated by
fast fibers appear pale and are often called white
muscles or white meat. Chicken breasts
contain white meat because chickens use their
wings only for brief intervals, as when fleeing a predator. Muscles dominated by
slow fibers are known as red muscles or red meat. Chickens walk around
all day, so their legs contain dark meat, which contain
the capillaries and myoglobin. Most human muscles contain
a mixture of fiber types, and so appear pink. Many back and calf muscles
are dominated by slow fibers, and these muscles contract
almost continuously to help us remain upright. Our genes determine
the percentage of fast versus slow
fibers in each muscle. Athletic training
can also increase the ratio of intermediate
fibers to fast fibers. As a result of repeated
extensive stimulation, muscle fibers develop more
mitochondria, a higher concentration of
glycolytic enzymes, which are enzymes that break
down glycogen into glucose, and larger glycogen reserves. The effect of this
stimulation is hypertrophy, or an enlargement
of the stimulated muscle. The number of muscle fibers
does not change significantly, but the muscle as a whole
enlarges because each muscle fiber increases in diameter. A skeletal muscle that is
not regularly stimulated by a motor neuron loses
muscle tone and mass. The muscle becomes flaccid
and the muscle fibers become smaller and weaker. This reduction in muscle
size, tone, and power is called atrophy. Individuals paralyzed by spinal
cord injuries or other damage to the nervous system gradually
lose muscle tone and size in the areas affected. Muscle atrophy is
reversible at first, but dying muscle fibers
are not replaced. That is why physical therapy
is crucial for people who are temporarily
unable to move normally. Direct electrical stimulation
by an external device can substitute for
nerve stimulation and prevent or reduce
muscle atrophy. This ends Chapter 10, Module
3, Types of Muscle Contractions and Types of Muscle Fibers.

2 Replies to “Chapter 10 Module 3 Muscle Tension and Fiber Types”

Leave a Reply

Your email address will not be published. Required fields are marked *