Only skeletal muscles work with bones and muscles keep bones in place and also play a role in the movement of bones.
To allow motion, different bones are connected by joints.
A skeletal muscle may attach a bone to another bone (often across a joint) or a bone to another structure, such as skin.
When the muscle contracts, one of the structures usually remains stationary, while the other moves.
When muscles contract, the pull or force, of muscle contraction is applied to levers, such as bones, resulting in movement of those levers.
A lever is a rigid shaft capable of turning about a hinge, or pivot point, called a fulcrum and transferring a force applied at one point along the lever to a weight (resistance) placed at another point along the lever.
The joints function as fulcrums , the bones function as levers , and the muscles provide the pull to move the levers.
Lever are classified as type I, II, or III.
In a class I lever system, the fulcrum is located between the pull and the weight. A child's seesaw is an example of this type of lever. The head is an example of this type of lever in the body.
In a class II lever system, the weight is located between the fulcrum and the pull.
An example is a wheelbarrow; the wheel is the fulcrum and the person lifting on the handles provides the pull. In the body, an example of a class II lever is lifting the heel.
In a class III lever system, the most common type in the body, the pull is located between the fulcrum and the weight.
An example is a person using a shovel. The hand placed on the part of the handle closest to the blade provides the pull to lift the weight and the hand placed near the end of the handle acts as the fulcrum.
In the body, the action of the biceps brachii muscle (force) pulling on the radius to flex the elbow and elevate the hand is an example.