How Do I Move? The Science of Biomechanics

How Do I Move? Chapter 8

The Science of Biomechanics

Objectives: 

Identify the external forces acting on the human body.

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Describe the resulting motion.

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Describe the expected path and motion of any projectile.

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Differentiate between throw-like and push-like skills.

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Determine the degree of stability possessed by an athlete.

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Understand the causes and effects of somersaulting.

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Qualitatively analyze simple sport skills that involve throwing, striking, or hitting an object.

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Biomechanics 

Examines the internal and external forces acting on the human body and the effects produced by these forces.

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Aids in technique analysis and the development of innovative equipment designs.

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Draws on knowledge from sports medicine, physical therapy, kinesiology, and biomechanical engineering.

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Quantitative Analyses 

Involves measurement of variables that are thought to optimize or maximize performance

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For Example:

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Pattern of forces using a force platform

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Sequence of muscle activity using electromyography (EMG)

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Three-dimensional (3D) movements of each body segment using high speed cinematography golf swing

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Qualitative Analyses 

Involves obtaining visual or aural information to asses performance

It requires: 

A framework within which skilled performance can be observed

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A set of principles with which movement can be analyzed

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A checklist to use when identifying errors

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Techniques to use for error detection and correction

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Kinematics Study of Motion 

The study of time and space factors of a body in motion.

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The variables used to describe motion are time, displacement, velocity, and acceleration.

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These variables are used to describe both linear and angular motion (angular displacement, angular velocity, and angular acceleration). Sport Books Publisher

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Kinematics Variables 

Time: temporal characteristics of a performance, either of the total skill or its phases

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Displacement: length and direction of the path an athlete takes from start to finish

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Angular Displacement: direction of, and smallest angular change between, the rotating body’s initial and final position

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Velocity: displacement per unit of time

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Angular Velocity: angular displacement per unit of time

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Acceleration: rate of change of velocity

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Angular Acceleration: angular velocity per unit of time

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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-onbone 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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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 Sport Books Publisher

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Three models used to represent the diver

Particle Model

Stick Figure Model

Rigid Segment Model

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Particle Model 

The drawing consists of a single point, representing the body’s center of mass

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The body is isolated from its surroundings

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The only external forces acting on the body are gravity and air resistance (when a large velocity is involved)

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Limited to bodies that are in flight (projectile motion) Sport Books Publisher

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Preliminary Steps for Analyzing Human Motion Step 1 

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).

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Identify the axes of rotation about which rotational motion occurs (sagittal, frontal, or vertical). Sport Books Publisher

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Types of Motion

Linear motion

General motion

Angular motion

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Curvilinear Motion 

When all parts of the body move the same distance, in the same direction, at the same time

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Translation refers to movement of the body as a unit without individual segment parts of the body moving relative to one another

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Rectilinear motion occurs when movement follows a straight line

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Curvilinear motion occurs when the movement path is curved Sport Books Publisher

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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

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Includes most athletic and many everyday activities

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