the forefoot, and the muscles are lifting the heel off the surface working against the resistance of the body weight. Second-class levers are designed to produce power; they provide an advantage so that smaller forces can move larger weights.
THIRD-CLASS LEVERS
Third-class levers are the most common levers used by the human musculoskeletal system. Similar to second- class levers, the axis of rotation is at the end of the bone
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in third-class levers. However, in third-class levers, the internal force or muscle force is closer to the axis, and the external force has the longest moment arm. Third-class lever systems are designed to produce range of motion and speed rather than power. A small amount of muscle shortening is able to produce a large arc or range of motion. This type of lever system is evident in most open kinetic chain movements of the body’s extremities. In each of these types of lever system, the lever creates an ability to take advantage of either the force applied or gains in the amount of motion (Fig. 4.8).
Effort (A)
Load (B)
Resistance arm Axis Effort arm (fulcrum)
Effort Resistance
Effort moment arm
Resistance moment arm
Axis
Figure 4.7 In second-class levers, the effort arm is greater than the resistance arm. These lever systems allow effort forces to counter large resistance forces.
