1. Chapter 13: Equilibrium and Human Movement
Basic Biomechanics, 4th edition
Susan J. Hall
Presentation Created by
TK Koesterer, Ph.D., ATC
Humboldt State University

2. Objectives
Define torque, quantify resultant torques, and identify the factors that affect resultant joint torques
Identify the mechanical advantages associated with the different classes of levers and explain the concept of leverage within the human body
Solve basic quantitative problems using the equations of static equilibrium
Define center of gravity and explain the significance of center of gravity location in the human body
Explain how mechanical factors affect the body/s stability

3. Equilibrium Torque Torque: T = Fd Moment arm:
In the body, moment arm of muscle is the perpendicular distance between muscle's line pull and joint center
Largest moment arm at an angle of pull ~900
Vector quantity, magnitude and direction
Fd & counterclockwise (+) & clockwise (-)

4. Resultant Joint Torques
Product of muscle tension and muscle moment arm produces a torque at the joint crossed by the muscle
Agonist and antagonist muscle groups
Net joint torque
Concentric and eccentric
Two joint muscles
Factors that affect net joint torques
Speed’s affect on net joint torques

5. Levers Lever: Fulcrum: First class lever: Second class lever:
Third class level:
Most levers within the body are third class

6. Lever Systems
Moment arm of applied force > moment arm of resistance
Resistance arm is longer than force arm
Mechanical advantage = Moment arm (force)
Moment arm (resistance)

7. Anatomical Levers
In the human body, most lever systems are third class
Arrangement promotes
Range of motion
Angular speed
Forces generated must be in excess of the resistance force
Two components of muscular force
rotary and parallel component

8. Equations of Static Equilibrium
Three conditions for equilibrium:
1. Fv = Fh = T = 0

9. Equations of Dynamic Equilibrium
Fx - māx = 0
Fy - māy = 0
TG - ī = 0

10. Center of Gravity (CG) Center of Mass
Center of Mass / Center of Gravity:
The CG of a symmetrical object of homogeneous density, is the exact center of the object
When mass in is not constant, CG shifts in the direction of greater mass.

11. Locating the Center of Gravity
For one-segment, balance point in three different planes
As projectile, the body’s CG follows a parabolic trajectory
Weight vector acts as the CG

12. Locating the Human Body Center of Gravity
Reaction board:
requires a scale, a platform & rigid board with sharp supports on either end.
Segmental method:
uses data for average locations of individual body segments CGs as related to a percentage of segment length

13. Stability and Balance Stability: Factors that affect:
Mass, friction, center of gravity & base of support
Balance:
Foot position affects standing balance

14. Summary
A muscle develops tension and produces torque at the joint that it crosses.
Muscle and bones function as levers.
The angle of muscle pull on a bone produces rotary and parallel components of force
When a body is motionless, it is in static equilibrium.
The behavior of a body is greatly influenced by location of center of gravity.
Stability is resistance to disruption og equilibrium