How Do I Move?
Chapter 9
The Science of Biomechanics
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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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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Quantitative Analyses

Involves measurement of variables that are thought to optimize
or maximize performance

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
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

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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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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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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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
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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

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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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).

Identify the axes of rotation about which rotational motion
occurs (sagittal, frontal, or vertical).
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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

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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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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