Class III levers
A third class lever is classified when the effort and resistance act on the same side of the
fulcrum, but the moment arm of the muscular force is much smaller than that of the resistance
moment arm, as shown in Figure 4. In this classification, a much greater force is needed by the
muscular contraction to move the resistant force. A good example of this is the biceps brachii.
Figure 4. Example of a Class III lever
The class III lever system is the most common used in the human body. It places the
most strain on the musculoskeletal system. It is clear that the human body is at a mechanical
disadvantage most of the time with this system.
The Phase of Locomotion
The Gait Cycle
The gate cycle is typically broken down into two phases: the swing and the stance phase.- Swing Phase: The swing phase is repositioning of the non-weight bearing limb
resulting in passive flexion of the hip, with some small bursts of muscular effort
to start the limb swinging, and then to stop the limb from swinging and to prepare for
weight bearing. - Stance Phase: Body weight is supported by the limb, the center of gravity passes
through the foot from back to front, and ends with a propulsive thrust. Stance phase
is commonly broken down into three separate components.
- Stance Phase - “Heel Strike”: The heel strikes the surface and the limb (with a
slightly bent knee), acts as a shock absorber. The forward momentum of