The muscular system is the body’s network of tissues that controls movement both of the body and within it. Walking, running, jumping: all these actions propelling the body through space are possible only because of the contraction (shortening) and relaxation of muscles.
These major movements, however, are not the only ones directed by muscular activity. Muscles make it possible to stand, sit, speak, and blink. Even more, were it not for muscles, blood would not rush through blood vessels, air would not fill lungs, and food would not move through the digestive system. In short, muscles are the machines of the body, allowing it to work.

The muscles of the body are divided into three main types: skeletal, smooth, and cardiac.
There are just over 650 skeletal muscles in the whole human body. As their name implies, skeletal muscles are attached to the skeleton and move various parts of the body. They are composed of tissue fibres that are striated/striped.
The alternating bands of light and dark result from the pattern of the filaments (threadlike proteins) within each muscle cell. Skeletal muscles are called voluntary muscles because a person controls their use, such as in the flexing of an arm or the raising of a foot.
Smooth muscle is found in the stomach and intestinal walls, in artery and vein walls, and in various hollow organs. They are called involuntary muscles because a person generally cannot consciously control them. They are regulated by the autonomic nervous system (a division of the nervous system that affects internal organs such as the heart, lungs, stomach and liver). Unlike skeletal muscles, smooth muscles have no striations/stripes. In a vessel or organ, smooth muscles are arranged in sheets or layers. Often, there are two layers, one running circularly (around) and the other longitudinally (up and down). As the two layers alternately contract and relax, the shape of the vessel or organ changes and fluid or food is propelled along. Smooth muscles contract slowly and can remain contracted for a long period of time without tiring.

Cardiac muscle, called the myocardium, is found in only one place in the body: the heart. It is a unique type of muscle and like smooth muscle it is involuntary, controlled by the autonomic nervous system. The myocardium is composed of thick bundles of muscle that are twisted and whorled into ring-like arrangements. Forming the walls of the chambers of the heart, the myocardium contracts to pump blood throughout the body.

Muscle Shapes
The bundles of fibres within muscles will determine the shape of the muscle.

The commonest muscle fibre arrangements are:

• Parallel fibres – these muscles have fibres that run parallel to each other in length and can sometimes be called strap muscles. These muscles have great endurance but may not be that strong due to their length. An example would be the Sternocleidomastoid (SCM).
• Circular muscles – these muscles are usually circular in shape and an example would be the muscles surrounding the mouth and eye.
• Convergent – this is where the muscle fibres converge to an attachment to a bone. The fibres are arranged to allow maximum force and can sometimes cross joints which have a large range of movement such as the Pectoralis Major.
• Pennate – these are made up of short fibres so the pull is short but also strong, though the muscle tires easily.
• Fusiform – these are sometimes included within the parallel muscle group and are made up of spindle shaped fibres. A good example is the Biceps Brachii as the belly is wider than the origin and the insertion.

Muscle Movement

Muscles are only every able to contract or pull. This means they have to work in groups and even when carrying out an action, do not work alone. A joint, therefore has to have two or more muscles working together.
As a muscle contracts, the second muscle relaxes, and as this second muscle contracts, the first muscle relaxes. This is called Antagonistic action as  they are pulling in the opposite direction to each other but without working against each other. One end of the muscle needs to be fixed, which is known as the origin and as that muscle contracts, the other end of the muscle moves towards the origin. The name given to the end of the muscle that moves
towards the origin is called the insertion.

Muscle tone refers to the amount of tension or resistance to movement
in a muscle.

Muscle tone is what enables us to keep our bodies in a certain position
or posture. A change in muscle tone is what enables us to move. For
example, to bend your arm to brush your teeth, you must shorten
(increase the tone of) the bicep muscles on the front of your arm at the
same time you are lengthening (reducing the tone of) the tricep muscles
on the back of your arm. To complete a movement smoothly, the tone in
all muscle groups involved must be balanced. The brain must send
messages to each muscle group to actively change its resistance.
Muscle tissue has four main characteristics which allow it to carry out its
function. It is able to respond to stimuli (Excitability). It can contract
(Contractibility). It can extend without tearing (Extensibility) and it can
return to its normal shape (Elasticity)

Growth and Repair of the Muscles

Muscle hypertrophy is the term used for when a muscle cell grows in
size, and the commonest reason for this is due to exercise, where there
will be an increase in muscle fibre. When a muscle is damaged (torn) the
body has to repair it and will do this by using satellite cells which fuse
with the ends of the damaged fibre. If the damage is constant then the
process will repeat itself so that more satellite cells are used which will
create growth of the muscle.

The charts on the following pages show the main muscles that you will
need to have a good knowledge of.