Bones, muscles, and ligaments form lever systems throughout the body that generate the power necessary for motion. A lever is a simple machine that operates by pivoting around a fixed point, or fulcrum. The fulcrum is the point a bar pivots around in a lever system; physiologically, this occurs in the synovial joints of the body. The other components of a lever system are the lever arm, which in the body is the movable bone; the load, which is the force of the object that is to be moved; and the effort, or force, that is generated by skeletal muscle. The effort acts on the muscle insertion point on the bone. The location of the insertion point of a muscle on the rotating bone determines the generated speed and force of the movement. The effort arm is the distance between the effort and the fulcrum: in other words, the distance between the point of muscle insertion and the joint. The resistance arm is the distance between the resistance (or load) a muscle is working against and the fulcrum, or the distance between the weight being moved and the joint. Levers have a mechanical advantage when the effort arm is longer than the resistance arm, allowing these muscles to use relatively little energy to produce a force great enough to move a large load.
There are three different types of levers, examples of which can be found in various movements in the body.
- In a first-class lever, the fulcrum is located between the effort and the load. The motion of nodding the head up and down is an example of a first-class lever. As muscles contract, creating the effort and pulling the back of the head down, the load moves in the opposite direction (face moves up). The fulcrum is the vertebrae when the head is pulled back and the chin is lifted.
- In a second-class lever, the load is between the effort and the fulcrum. An example of a second-class lever is when a person stands on the toes, raising the heel off the ground. In this case, the fulcrum is the ball of the foot, the load is the body weight, and the effort is applied by the leg muscles that insert on the heel. Second-class levers have a mechanical advantage so there is comparatively little energy required from the muscles to lift the entire weight of the body.
- In a third-class lever, the effort is applied between the load and the fulcrum. A physiological example of third-class lever is flexing the arm at the elbow. The effort comes from the biceps muscles, which are close to the joint, the fulcrum is the elbow, and the load is the force of the item carried in the hand and the weight of the arm itself. Third-class lever systems are the most common in the body. These levers have a low mechanical advantage but can maximize speed.