Understanding Biomechanics: Lever Systems and Mechanical Advantage

1. The lever system Module 2- Principles of Biomechanics

2. Lever • Movement occurs when muscle pulls on bone, rotating it about a joint • Movement occurs when force acts on a lever about an axis (fulcrum) • Distance between axis and point of force application is the moment arm • Typically two forces acting on lever, for our sake they will be known as the force arm and the resistance arm • Two forces=two moment arms

3. Figure 4.2

4. Lever • Rotating about an axis, therefore producing rotational force • Rotational force is called torque • Torque is the product of the force and the moment arm • See Saw example

5. Mechanical Advantage • The ratio of the moment arm through which an applied force acts to that through which a resistive force acts. A mechanical advantage greater than 1.0 allows the applied (muscle) force to be less than the resistive force to produce an equal amount of torque. A mechanical advantage of less than 1.0 is a disadvantage in the common sense of the term. • Two ways to increase mechanical advantage • Increase Force Arm • Decrease Resistance Arm

6. Lever Classes • 1st Class • Force and resistance are on opposite sides of the fulcrum • See saw • 2nd Class • Force and resistance are on the same side of the fulcrum (resistance is between fulcrum and force) • Wheelbarrow • Force will always be less than resistance

7. Lever Classes • 3rd Class • Most common type in the body • Force and resistance are on same side of fulcrum (force is between axis and resistance) • Force will always have to be greater than resistance • Biceps example • Speed advantage • In the “real world” the force arm and resistance arm are not constant through a range of motion

8. Figure 4.9 Reprinted, by permission, from Gowitzke and Milner, 1988.

9. Figure 4.7

10. Figure 4.8

11. Figure 4.6 Reprinted, by permission, from Gowitzke and Milner, 1988.