A lever in physical terms is an axis that moves a load about a fixed point. Joints, therefore, can be seen to act in levers as the fixed point. The axis/lever is the bone, while the load can be a body part or another object. The fixed point or joint in this case is termed a fulcrum. The other two key components in the lever are the weight or resistance and the effort.
For example, the elbow fulcrum can use the bicep effort to counteract an apple weight in the hand. Or, how about the neck fulcrum using the neck muscle effort to raise the head weight?
The arrangement of fulcrum/joint, effort and weight is not always the same. Sometimes the joint is in the middle, between the weight and the effort, and sometimes it’s at one end. Based on this, there are 3 types of lever: 1st order, 2nd order and 3rd order.
First order joints have the joint in the middle, like a balance, and the weight and effort move in the same direction.
For example, the head bobbing has the weight of the head falling downwards, as well as the effort to move it pulling downwards.
Second order and third order joints both have the joint on one side rather than in the middle. The difference is that second order joints have the weight in the middle, with the effort being exerted on the other end; while third order joints have the effort in the middle, with the weight being moved from the other end.
Most levers in the human body are third order