## Levers and Torque

The skeletal system provides the levers and axes of rotation about which the muscular system generates movement. A lever is a simple machine that magnifies the force or speed of movement, or both. The levers are primarily the long bones of the body, and the axes are the joints where the bones meet. Human skeletal levers can be one of three types (Fig. 4-1). Human levers serve many different functions, including movement, manipulation of objects, and weight bearing. If a joint is misaligned, the lever structure is altered, and mechanical stress to the joint caused by external and internal

 —>- I A k Fc ><

Class I Class II Class III

Figure 4-1 The three classes of lever. The lever arms are of lengths ^ and l2. Equilibrium is achieved if ^ x L = l2 x F. Mechanical advantage (M) = L/F = L^L2. Ma1 (class I lever ) can be greater or less than 1; Ma2 > 1, whereas Ma3 < 1. F, Applied force; Fc, fulcrum; L, load.

forces increases, all of which result in injury to the joint or soft tissue. From the perspective of physics, human levers are constructed with mechanical disadvantage. This means that the muscles have to produce 50 pounds (22.7 kg) of force to hold a 5-pound (2.3-kg) object. However, this mechanical disadvantage is compensated by the gain in speed of the lever movement.

When a force is applied to a lever, the lever rotates about the fulcrum. Torque is an expression for how a force changes the angular motion of a lever, which signifies the angular velocity. To calculate the torque M, the magnitude of the force F is multiplied by the distance l between the force and the rotating point. In Figure 4-1, the torque is calculated as

Equilibrium is achieved when the torque on the left equals that on the right:

Human skeletal levers and torques are discussed in the following section.