Joints and levers in the human body
Bones, ligaments, and muscles are the structures that form levers in the body to create human movement. In simple terms, a joint (where two or more bones join together) forms the axis (or fulcrum), and the muscles crossing the joint apply the force to move a weight or resistance. Levers are typically labeled as first class, second class, or third class. All three types are found in the body, but most levers in the human body are third class.
A first-class lever has the axis (fulcrum) located between the weight (resistance) and the force (figure 1.21a). An example of a first-class lever is a pair of pliers or scissors. First-class levers in the human body are rare. One example is the joint between the head and the first vertebra (the atlantooccipital joint) (figure 1.21b). The weight (resistance) is the head, the axis is the joint, and the muscular action (force) come from any of the posterior muscles attaching to the skull, such as the trapezius.
Muscles and bones act together to form levers. A lever is a rigid rod (usually a length of bone) that turns about a pivot (usually a joint). Levers can be used so that a small force can move a much bigger force. This is called mechanical advantage.
There are four parts to a lever – lever arm, pivot, effort and load. In our bodies:
bones act as lever arms
joints act as pivots
muscles provide the effort forces to move loads
load forces are often the weights of the body parts that are moved or forces needed to lift, push or pull things outside our bodies.
Levers can also be used to magnify movement, for example, when kicking a ball, small contractions of leg muscles produce a much larger movement at the end of the leg.
Levers are able to give us a strength advantage or a movement advantage but not both together.