Chapter 13: Equilibrium and Human Movement

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Chapter 13 Equilibrium andHuman Movement

• Basic Biomechanics, 4th edition
• Susan J. Hall
• Presentation Created by
• TK Koesterer, Ph.D., ATC
• Humboldt State University

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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 bodys
  stability

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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 (-)

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6-13
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13-2
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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
• Speeds effect on net joint torques

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13-7
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13-8
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Levers

• Lever
• Fulcrum
• First class lever
• Second class lever
• Third class level
• Most levers within the body are third class

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Lever

• a simple machine consisting of a relatively rigid
  bar-like body that may be made to rotate about an
  axis

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Fulcrum

• the point of support, or axis, about which a
  lever may be made to rotate

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First Class Lever

• lever positioned with the applied force and the
  resistance on opposite sides of the axis of
  rotation

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Second Class Lever

• lever positioned with the resistance between the
  applied force and the fulcrum

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Third Class Lever

• lever positioned with the applied force between
  the fulcrum and resistance

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13-10
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13-11
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Lever Systems

• Moment arm of applied force gt moment arm of
  resistance
• Resistance arm is longer than force arm
• Mechanical advantage Moment arm (force)
• Moment arm (resistance)

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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

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6-20
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6-21
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13-14
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Factors Affecting Muscular Force Generation

• Force-Velocity Relationship
• Length-Tension Relationship
• Electromechanical Delay
• Stretch-Shortening Cycle

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Force-Velocity Relationship

• Maximal force developed by muscle governed by
  velocity of muscles shortening or lengthening.
• Holds true for all muscle types
• Does not imply
• Its impossible to move heavy resistance at a
  fast speed.
• Its impossible to move light loads at low speeds

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6-17
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Force-Velocity Relationship

• Maximum isometric tension
• Eccentric conditions
• Volitionally
• Represents contribution of the elastic components
  of muscle
• Eccentric Strength Training
• More effective than concentric training in
  increasing muscle size and strength.

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Length-Tension Relationship

• In human body, force generation increases when
  muscle is slightly stretched.
• Parallel fibers at max just over resting length
• Pennate fibers at max with 120-130 resting
  length.
• Due to contribution of elastic components of
  muscle (primarily the SEC)

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Electromechanical Delay

• The time between arrival of neural stimulus and
  tension development by the muscle
• Varies among human muscles (20-100 msec)
• Short EMDs produced by muscles with high
  percentage of FT fibers
• Not affected by muscle length, contraction type,
  contraction velocity, or fatigue

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Stretch-Shortening Cycle

• Pattern of eccentric contraction followed
  immediately by concentric contraction
• Elastic Recoil
• Stretch Reflex Activation
• Muscle can perform more work with active stretch
  prior to shortening contraction
• Eccentric training increases ability of
  musculotendinous unit to store and produce more
  elastic energy.

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Muscular Strength, Power, and Endurance

• Muscular Strength
• Muscular Power
• Muscular Endurance
• Muscular Fatigue
• Effect of Muscle Temperature

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Muscular Strength

• The ability of a given muscle group to generate
  torque at a particular joint.
• Derived from
• amount of tension the muscles can generate
• moment arms of contributing muscles with respect
  to joint center.

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Muscular Strength

• Tension-generating capability of a muscle
  affected by
• Cross-sectional area
• Training state
• Moment arm of a muscle affected by
• Distance between the muscles anatomical
  attachment to bone and the axis of rotation at
  the joint center
• Angle of muscles attachment to bone.

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Muscular Power

• The product of muscular force and the velocity of
  muscular shortening.
• The rate of torque production at a joint
• Max. power occurs at
• approx. 1/3 max. velocity, and
• approx. 1/3 max concentric force
• Affected by muscular strength and movement speed

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6-22
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Muscular Endurance

• The ability to exert tension over a period of
  time.
• Constant gymnast in iron cross
• Varying rowing, running, cycling
• Length of time dramatically affected by force and
  speed requirements of activity.
• Training involves many repetitions with light
  resistance.

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Resistance Devices used in Strength Training

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Free Weights

• Gravity dependent
• Resistance pattern constant or variable
• Concentric Eccentric action of same muscles
• Antagonistic muscles not utilized
• Momentum may be a factor in resistance pattern

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Gravity Dependent Machines

• Universal Gym
• Resistance moves upward
• Round pulleys changes direction of resistance
• Constant resistance

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Variable Resistance Machines

• Nautilus
• Cam design creates variable resistance
• Designed to mimic the strength curve

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Isokinetic Devices

• Biodex, Cybex, Orthotron, and hydraulic equipment
• Accommodating resistance
• Constant velocity

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Other Devices

• The body pushups, sit-ups, pull-ups
• Pushup variations
• Sit-ups, curl-ups - changing resistance
• Pull-ups pronated vs. supinated grip

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Equations of Static Equilibrium

• Equilibrium
• Three conditions for equilibrium
• 1. ?Fv 0 2. ?Fh 0 3. ?T 0

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Equations of Dynamic Equilibrium

• Dynamic equilibrium
• ?Fx - max 0
• ?Fy - may 0
• ?TG - i? 0

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Center of Gravity (CG)Center of Mass

• The point around which the mass and weight of a
  body are balanced in all directions
• The CG of a symmetrical object of homogeneous
  density is the exact center of the object
• When mass within an object is not constant, CG
  shifts in the direction of greater mass

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13-17
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Locating the Center of Gravity

• For one-segment, balance point in three different
  planes
• As projectile, the bodys CG follows a parabolic
  trajectory
• Weight vector acts through the CG (line of
  gravity)

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13-20
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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

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Stability and Balance

• Stability resistance to disruption of
  equilibrium
• Factors that affect stability
• Mass, friction, horizontal position and height of
  center of gravity with respect to the base of
  support
• Balance ability to control equilibrium
• Foot position affects standing balance

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13-22
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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
  the location of the center of gravity.
• Stability is the resistance to disruption of
  equilibrium

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The End