The cells that make up muscles contract and then relax back to original size. Tiny microscopic fibres in these cells compress by sliding in past each other like a sliding glass door being opened and then shut again. The cells of your muscles use chemical energy from the food you eat to do this. Without food, and particular kinds of nutrients, your muscles wouldn't be able to make the energy to contract! 

Some muscles are known as "voluntary" -- that is, they only work when you specifically tell them to. Do you want to say something? Or swing a bat? Or clap your hands? These are voluntary movements. Others, like the muscular contracting of your heart, the movement of your diaphragm so that you can breathe, or blinking your eyes, are automatic. They're called involuntary movements. These muscles move through signals from your nerves, and, in some cases, your brain, as well. 

How Skeletal Muscles Produce Movement

Skeletal muscles produce movement by exerting force on tendons, which in turn pull on bones or other structures.  Most muscles cross at least one joint and are attached to the articulating bones that form that joint.

Why do the two bones of a joint not move equally? When a muscle contracts, it draws one articulating bone towards the other, but the two bones do not move equally in response to the contraction.  One bone is held in position because other muscles contract to pull it in the opposite direction or because its structure makes it less movable.

How levers are used to produce muscular movement.
To produce movement the body needs the muscles and the bones to work effectively together. This occurs through a system based on levers. Muscles and bones work together to form levers which in turn to produce actual movement.

A lever is a rigid rod which turns about on a pivot point acting as a device to transmit force. This can be an item external to the body such as a golf club which pivots around the grip during a typical swing. However levers are also evident within the body where bones, joints and muscles work together to produce movement. 

There are four parts to the body’s lever system. These include:

  • The lever (for example the bone).
  • The pivot which is also called the fulcrum (for example the joint).
  • The force which initiates and carries out the movement and acts on the resistance being offered (eg the working muscle).
  • The load which is the resistance to the movement. This can be the weight of the actual body part being moved or an external resistance such as a hand weight or ball that is waiting to be lifted or kicked.

There are three types of levers that can occur in the body. The classification of these vary according to the relationship between the fulcrum, resistance and force. The different types of levers include:

  • First class levers
  • Second class levers
  • Third class levers

First class levers
In first class levers the fulcrum (pivot) is positioned between the force and resistance.

This lever is structured much like a see saw that you may see in a children’s playground. These types of levers are not as common in the human body. However one example of this is the ability to nod your head. Here the following components of first class levers can be identified in this movement:

  • The lever is the skull
  • The pivot is the point where the skull and spine meets.
  • The force is the muscle at the back of the skull which provides the force to move.
  • The resistance or load is the weight of the head.

Second class levers
In second class levers the resistance (load) is located between the fulcrum (pivot) and the force.

An example of a second class lever is movement to tippey – toe position. Here the following components of second class levers can be identified in this movement:

  • The lever is the foot
  • The pivot is the joints of the toes.
  • The force is the muscles of the calf.
  • The resistance is the weight of the body.

Third class levers
These are the most common levers that the body uses to create movement. In third class levers the effort is located between the fulcrum and the resistance.

An example of a third class lever is the ability to bend your arm in order to lift a hand weight. Here the following components of third class levers can be identified in this movement:

  • The lever is the arm.
  • The pivot is the joint between the elbow and forearm.
  • The force is the biceps.
  • The resistance is the hand weight.

Origin and Insertion
The attachment of a muscle to the more stationary bone is called its origin. The attachment of the muscle to the more movable bone is called its insertion. Generally, the origin is closer to the midline of the body, and the insertion is the distal end of the muscle. The fleshy portion of the muscle between the origin and the insertion is called the belly (gaster).

Types of movement

Movements are described in respect to the anatomical position. Movements include:

Flexion
The movement of adjacent body parts so that the angle between the body parts decreases. For example bending the knee or elbow,.

Extension
The opposite movement to flexion, extension is when the angle between two adjacent parts increases. Extension also describes any movement beyond neutral that is the opposite of flexion. For example straightening the knee or elbow from bent.

Plantar flexion
 Moving the foot downwards so the angle between the top of the foot and the shin bone increases, such as when the foot is pushed down onto the accelerator.

Dorsi flexion
Moving the foot upwards, so the angle between the foot and the shin bone decreases and the toes become closer to the shin.

Lateral flexion
The movement of the head or trunk from side to side, such as when the left ear is brought down towards the left shoulder.

Abduction
The movement of a body part away from the midline of a body, such as a star jump when arms and legs are moved out from the body to form a “star” shape. 

Adduction
The movement of a body part towards the midline of the body, such as the opposing action of a star jump, when arms and legs are brought from the star shape back to neutral, with legs together and arms resting alongside body.

Medial rotation
Rotation of a body part inwards, so that it’s anterior (front) surface turns in towards the midline.

Lateral rotation
Rotation of a body part externally, so that it’s anterior surface turns outwards, away from the midline.

Supination
A term used to describe movement of the hands and the feet. In anatomical position, supination of the hands is when the palms of the hands face forwards (anteriorly). For the feet, supination is when the foot turns so the sole of the foot rotates medially.

Pronation
Pronation is when the hand is rotated so the palm faces backwards (posteriorly) when standing in the anatomical position. For the foot, pronation is when the sole of the foot faces laterally.

Group Actions
Most movements require skeletal muscles acting in groups rather than as individuals and are arranged into opposing pairs:

  • Flexors vs Extensors
  • Supinators vs Pronators
  • Abductors vs Adductors
  • Elevators vs Depressors
  • Medial vs Lateral rotators
  • Protractors vs Retractors