Post on 22-Dec-2015
Chapter 13: Equilibrium and
Human Movement
Basic Biomechanics, 4th edition
Susan J. Hall
Presentation Created by
TK Koesterer, Ph.D., ATC
Humboldt State University
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
EquilibriumTorque
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 (-)
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
Levers
Lever:
Fulcrum:
First class lever:
Second class lever:
Third class level:
Most levers within the body are third class
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)
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
Equations of Static Equilibrium
Equilibrium:• Three conditions for equilibrium:
1. Fv = 0 2. Fh = 0 3. T = 0
Equations of Dynamic Equilibrium
Dynamic equilibrium: Fx - māx = 0
Fy - māy = 0
TG - ī = 0
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.
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
Locating the Human BodyCenter 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
Stability and Balance
Stability:• Factors that affect:
– Mass, friction, center of gravity & base of support
Balance:• Foot position affects standing balance
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