Muscle defined is a tissue capable
of changing shape and length to cause movement.
Although most of us think of muscle as a tissue that
allows us to move, push, pull or lift weight, there
are several other roles for different types of muscle
that are critical in human function. Skeletal ddmuscles
primarily serve the body as contractile tissue to
allow bones to move around joints. Smooth muscle helps
move blood through our blood vessels and food through
our digestive tract. Perhaps one of the most important
muscles is cardiac, which is your heart.
Muscles typically work in pairs to support and balance
the body. The front thigh (quadriceps) muscle is the
opposite to the back thigh (hamstrings) muscle. The
chest (protractor) muscles are the opposite to the
back (retractor) muscles. These are also called antagonistic
muscle groups. Each contracts to move joints in a
certain direction or rotation to the angle of force
being created. When one contracts, the other must
relax. For example, your bicep must relax and lengthen
when your triceps contract and shorten. If this relationship
did not exist and if both muscle groups were to contract
at the same time, your arm would lock and there would
be no movement at the joint.
At both ends of every muscle, the fascia covering
the muscle tapers to form a strong, rope-like length
of connective tissue called a tendon, which is connected
directly to one of your bones. One end, which connects
to a relatively unmoving skeletal part, is the origin
of the muscle. The point where it's attached to a
moving bone is the insertion of the muscle.
Every muscle is actually a wrapped package, containing
other smaller wrapped packages of long, slender cells
known as muscle fibers. The outer wrapping, made of
connective tissue, is called the muscle fascia. The
smaller packages are called muscle fasciculi (fascicle),
and each one contains a bundle of up to 150 muscle
fibers. Endomysium envelopes all the muscle fibers
in a fascicle. Perimysium wraps all of the fasciculi
and the epimysium surrounds the entire muscle. Thus,
when a muscle fiber contracts, it pulls on the endomysium
which pulls on the perimysium which pulls on the epimysium
which in turn pulls on connective tissue fascia which
finally pulls on the tendon, and this causes the bone
to move. The bigger the muscle, the more force it
can generate on the bone.
Each muscle fiber shares a nerve ending with other
nearby fibers, making up a group of fibers known as
a motor unit. A motor neuron must fire from the spinal
cord to produce a signal telling the muscle to contract.
Every time the master motor nerve fires (sends an
impulse to a muscle), this motor unit contracts simultaneously.
This effect is called the "all-or-nothing" principle
of muscle contraction. When the fibers in a motor
unit contract in unison, the result is a muscle contraction.
Whatever form of exercise you're doing, from running
to swimming to bicycling, your movements depend on
the repeated, coordinated firing of the appropriate
motor units. The more you develop an athletic skill
the more efficient your motor units become at firing
for the desired result.
