1. The Science of Biomechanics
How Do I Move? Chapter 8
The Science of Biomechanics

2. Objectives: Identify the external forces acting on the human body.
Describe the resulting motion.
Describe the expected path and motion of any projectile.
Differentiate between throw-like and push-like skills.
Determine the degree of stability possessed by an athlete.
Understand the causes and effects of somersaulting.
Qualitatively analyze simple sport skills that involve throwing, striking, or hitting an object.
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3. Biomechanics
Examines the internal and external forces acting on the human body and the effects produced by these forces.
Aids in technique analysis and the development of innovative equipment designs.
Draws on knowledge from sports medicine, physical therapy, kinesiology, and biomechanical engineering.
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4. Quantitative Analyses
Involves measurement of variables that are thought to optimize
or maximize performance
golf swing
For Example:
Pattern of forces using a force platform
Sequence of muscle activity using electromyography (EMG)
Three-dimensional (3D) movements of each body segment using high speed cinematography
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5. Qualitative Analyses
Involves obtaining visual or aural information to asses performance
It requires:
A framework within which skilled performance can be observed
A set of principles with which movement can be analyzed
A checklist to use when identifying errors
Techniques to use for error detection and correction
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6. Kinematics Study of Motion
The study of time and space factors of a body in motion.
The variables used to describe motion are time, displacement, velocity, and acceleration.
These variables are used to describe both linear and angular motion (angular displacement, angular velocity, and angular acceleration).
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7. Kinematics Variables
Time: temporal characteristics of a performance, either of the total skill or its phases
Displacement: length and direction of the path an athlete takes from start to finish
Angular Displacement: direction of, and smallest angular change between, the rotating body’s initial and final position
Velocity: displacement per unit of time
Angular Velocity: angular displacement per unit of time
Acceleration: rate of change of velocity
Angular Acceleration: angular velocity per unit of time
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8. Kinetics Study of Motion
Focuses on the various forces that are associated with a movement
Internal Forces:
Generated by muscles pulling via their tendons on bones, and to bone-on-bone forces exerted across joint surfaces
External Forces:
Acting from without, such as the force of gravity or the force from any body contact with the ground, environment, sport equipment, or opponent
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9. Human Body Models Particle Model:
Used when the object of interest (the human body or an object) is airborne after being thrown, struck, or kicked
Stick Figure Model:
Used when the object is in contact with its environment
Rigid Segment Model:
Used for more sophisticated quantitative analyses
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10. Three models used to represent the diver
Particle Model
Rigid Segment Model
Stick Figure Model
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11. Particle Model
The drawing consists of a single point, representing the body’s center of mass
The body is isolated from its surroundings
The only external forces acting on the body are gravity and air resistance (when a large velocity is involved)
Limited to bodies that are in flight (projectile motion)
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12. Preliminary Steps for Analyzing Human Motion
Identify the system to be studied, which is to separate the object of interest from its surroundings.
Step 2
Identify the frame of reference in which the movement takes place.
Step 3
Identify the type of motion that is occurring, the body planes in which movement takes place (sagittal, frontal, or transverse).
Identify the axes of rotation about which rotational motion occurs (sagittal, frontal, or vertical).
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13. Types of Motion Angular motion General motion Linear motion
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14. Curvilinear Motion
When all parts of the body move the same distance, in the same direction, at the same time
Translation refers to movement of the body as a unit without individual segment parts of the body moving relative to one another
Rectilinear motion occurs when movement follows a straight line
Curvilinear motion occurs when the movement path is curved
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15. Angular and General Motion
Angular Motion (rotation)
Occurs when a body moves along a circular path, through the same angle, in the same direction, and at the same time
The axis of rotation is the point about which movement occurs
All joint motions are angular motions
General Motion
A combination of linear and angular motion
Includes most athletic and many everyday activities
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16. Causes of motion
The only cause of motion of the human body is the application of an external force
Force is any action, a push or pull, which tends to cause an object to change its state of motion by experiencing an acceleration
Constant Velocity occurs when an object is not accelerating
Linear Motion is caused by forces which act through a body’s center of mass
Angular motion is caused by forces that do not go through the center of mass
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17. Linear motion results when the forces
are applied through the center of mass
Angular motion results when the forces are applied away from the center of mass
Center of Mass
Center of Mass

18. Scalar and Vector Quantities
Scalar quantities have only magnitude (time)
Vector quantities have magnitude and direction (force)
- vectors are straight-line segments with one end defined as the tail and the arrow tip defined as the head.
tail
head
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19. Adding Vectors a b + b a = Resultant Resultant
The head of a vector points in the direction of the quantity the vector represents
Vectors can be added together using the head-to-tail method
- to add vector B to vector A, an identical vector is drawn (same length and direction) as vector B beginning at the head of vector A
- the resultant vector is directed from the tail of vector A to the head of vector B a b + b a =
Resultant
Resultant
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20. Levers
Simple machines that augment the amount of work done by an applied force
A rigid body (i.e., long bone) that rotates about a fixed point (i.e., joint) called a fulcrum (F)
Acting on the lever is a resistive force (R, i.e., weight of a limb segment) an applied force (AF, i.e., muscle contraction).
Three classes of levers:
a. First class (e.g., teeter-totter)
b. Second class (e.g., wheelbarrow)
c. Third class (e.g., screen door with spring closing)
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21. Sport Books Publisher

22. Factors affecting the moment of forceD
A. Balanced teeter-totter
B. Increasing the moment arm by leaning backwards
C. Increasing the applied force by adding a friend