ACS Distance Education UK
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:
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
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:
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:
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:
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).
Movements are described in respect to the anatomical position. Movements include:
The movement of adjacent body parts so that the angle between the body parts decreases. For example bending the knee or elbow,.
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.
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.
Moving the foot upwards, so the angle between the foot and the shin bone decreases and the toes become closer to the shin.
The movement of the head or trunk from side to side, such as when the left ear is brought down towards the left shoulder.
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.
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.
Rotation of a body part inwards, so that it’s anterior (front) surface turns in towards the midline.
Rotation of a body part externally, so that it’s anterior surface turns outwards, away from the midline.
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 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.
Most movements require skeletal muscles acting in groups rather than as individuals and are arranged into opposing pairs: