how do i move? chapter 8 · 2010-11-23 · 1 how do i move? chapter 8 the science of biomechanics...

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How Do I Move?Chapter 8

The Science of BiomechanicsThe Science of Biomechanics

Objectives:Identify the external forces acting on the human body.Identify the external forces acting on the human body.

Describe the resulting motion.Describe the resulting motion.

Describe the expected path and motion of any projectile.Describe the expected path and motion of any projectile.

Sport Books Publisher 2

Differentiate between throwDifferentiate between throw--like and pushlike and push--like skills.like skills.

Determine the degree of stability possessed by an athlete.Determine the degree of stability possessed by an athlete.

Understand the causes and effects of somersaulting.Understand the causes and effects of somersaulting.

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

Biomechanics

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

Aid i t h i l i dAid i t h i l i d


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

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

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Quantitative Analyses

For ExampleFor Example::

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


Pattern of forces using a force Pattern of forces using a force platformplatform

Sequence of muscle activity Sequence of muscle activity using electromyography (EMG)using electromyography (EMG)

ThreeThree--dimensional (3D) dimensional (3D) movements of each body movements of each body segment using high speed segment using high speed cinematographycinematography

golf swing

Qualitative Analyses

It requires:It requires:

A framework within which skilled A framework within which skilled

Involves obtaining visual or aural information to asses Involves obtaining visual or aural information to asses performanceperformance


performance can be observed performance can be observed

A set of principles with which A set of principles with which movement can be analyzed movement can be analyzed

A checklist to use when identifying A checklist to use when identifying errorserrors

Techniques to use for error detection Techniques to use for error detection and correctionand correction

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

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


displacement, velocity, and displacement, velocity, and acceleration.acceleration.

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

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Kinematics VariablesTimeTime:: temporal characteristics of a performance, either of the temporal characteristics of a performance, either of the total skill or its phasestotal skill or its phases

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

Angular DisplacementAngular Displacement:: direction of and smallest angulardirection of and smallest angular


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

VelocityVelocity:: displacement per unit of timedisplacement per unit of time

Angular VelocityAngular Velocity:: angular displacement per unit of timeangular displacement per unit of time

AccelerationAcceleration:: rate of change of velocityrate of change of velocity

Angular AccelerationAngular Acceleration:: angular velocity per unit of timeangular velocity per unit of time

Kinetics Study of Motion

Internal Forces:Internal Forces:Generated by muscles pulling via their Generated by muscles pulling via their tendons on bones, and to bonetendons on bones, and to bone--onon--

Focuses on the various forces that are associated with a Focuses on the various forces that are associated with a movementmovement


bone forces exerted across joint bone forces exerted across joint surfacessurfaces

External Forces:External Forces:Acting from without, such as the force Acting from without, such as the force of gravity or the force from any body of gravity or the force from any body contact with the ground, environment, contact with the ground, environment, sport equipment, or opponentsport equipment, or opponent

Human Body ModelsParticle Model:Particle Model:

Used when the object of interest (the human body Used when the object of interest (the human body or an object) is airborne after being thrown, struck, or an object) is airborne after being thrown, struck, or kickedor kicked


Stick Figure Model:Stick Figure Model:Used when the object is in contact with its Used when the object is in contact with its environmentenvironment

Rigid Segment Model:Rigid Segment Model:Used for more sophisticated quantitative analysesUsed for more sophisticated quantitative analyses

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Three models used to represent the diver

Particle Model


Stick Figure Model Rigid Segment Model

Particle Model

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

The body is isolated from its surroundingsThe body is isolated from its surroundings


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

Limited to bodies that are in flight Limited to bodies that are in flight (projectile (projectile motion)motion)

Preliminary Steps for Analyzing Human Motion

Step 1Step 1Identify the system to be studied, which is to separate the Identify the system to be studied, which is to separate the object of interest from its surroundings.object of interest from its surroundings.

Step 2Step 2Identify the frame of reference in which the movement Identify the frame of reference in which the movement


takes place.takes place.Step 3Step 3

Identify the type of motion that is occurring, the body Identify the type of motion that is occurring, the body planes in which movement takes place (sagittal, frontal, or planes in which movement takes place (sagittal, frontal, or transverse).transverse).Identify the axes of rotation about which rotational motion Identify the axes of rotation about which rotational motion occurs (sagittal, frontal, or vertical).occurs (sagittal, frontal, or vertical).

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Types of Motion


Linear motionLinear motion Angular motionAngular motionGeneral motionGeneral motion

Curvilinear Motion

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

TranslationTranslation refers to movement of the body as a refers to movement of the body as a


unit without individual segment parts of the body unit without individual segment parts of the body moving relative to one anothermoving relative to one another

Rectilinear motionRectilinear motion occurs when movement occurs when movement follows a straight line follows a straight line

Curvilinear motionCurvilinear motion occurs when the movement occurs when the movement path is curvedpath is curved

Angular and General MotionAngular Motion (rotation)Angular Motion (rotation)

Occurs when a body moves along Occurs when a body moves along a circular path, through the same a circular path, through the same angle, in the same direction, and angle, in the same direction, and at the same timeat the same timeThe axis of rotationThe axis of rotation is the point is the point about which movement occursabout which movement occurs


All joint motions are angular All joint motions are angular motionsmotions

General MotionGeneral Motion

A combination of linear and A combination of linear and angular motionangular motion

Includes most athletic and many Includes most athletic and many everyday activitieseveryday activities

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Causes of motionThe only cause of motion of the human body is the The only cause of motion of the human body is the application of an external forceapplication of an external force

ForceForce is any action, a push or pull, which tends to cause an object to is any action, a push or pull, which tends to cause an object to change its state of motion by experiencing an accelerationchange its state of motion by experiencing an acceleration


g y p gg y p g

Constant VelocityConstant Velocity occurs when an object is not acceleratingoccurs when an object is not accelerating

Linear MotionLinear Motion is caused by forces which act through a body’s is caused by forces which act through a body’s center of masscenter of mass

Angular motionAngular motion is caused by forces that do not go through the is caused by forces that do not go through the center of masscenter of mass

Li ti lt h th f A l ti lt h th fLinear 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

Scalar and Vector Quantities

Scalar quantitiesScalar quantities have only magnitude (time)have only magnitude (time)

Vector quantitiesVector quantities have magnitude and direction (force)have magnitude and direction (force)


-- vectors are straightvectors are straight--line segments with one end defined as line segments with one end defined as the tail and the arrow tip defined as the head.the tail and the arrow tip defined as the head.

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Adding VectorsThe head of a vector points in the direction of the quantity the The head of a vector points in the direction of the quantity the vector representsvector representsVectors can be added together using Vectors can be added together using the headthe head--toto--tail methodtail method-- to add vector B to vector A, an identical vector is drawn (same to add vector B to vector A, an identical vector is drawn (same length and direction) as vector B beginning at the head of vectorlength and direction) as vector B beginning at the head of vector


length and direction) as vector B beginning at the head of vector length and direction) as vector B beginning at the head of vector AA-- the resultant vector is directed from the tail of vector A to the the resultant vector is directed from the tail of vector A to the head of vector Bhead of vector B

a

b+

a b

ResultantResultant=

LeversSimple machines that augment the amount of Simple machines that augment the amount of work done by an applied forcework done by an applied forceA rigid body (i.e., long bone) that rotates about a A rigid body (i.e., long bone) that rotates about a fixed point (i.e., joint) called a fulcrum (F)fixed point (i.e., joint) called a fulcrum (F)Acting on the lever is a resistive force (R, i.e., Acting on the lever is a resistive force (R, i.e.,

i ht f li b t) li d f (AFi ht f li b t) li d f (AF


weight of a limb segment) an applied force (AF, weight of a limb segment) an applied force (AF, i.e., muscle contraction).i.e., muscle contraction).

Three classes of levers:Three classes of levers:a.a. First classFirst class (e.g., teeter(e.g., teeter--totter)totter)b.b. Second classSecond class (e.g., wheelbarrow)(e.g., wheelbarrow)c.c. Third classThird class (e.g., screen door with spring closing)(e.g., screen door with spring closing)


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Factors affecting the moment of force

A. Balanced teeter-totterD D

C. Increasing the applied force by adding a friend

B. Increasing the moment arm by leaning backwards

Calculating net external force using free body diagrams

Calculating Moments of Force

Moment armMoment arm is the is the shortest (perpendicular) shortest (perpendicular) distance from the axis of distance from the axis of rotation to the line of rotation to the line of action of the forceaction of the force


Moment of forceMoment of force is is influenced by the influenced by the magnitude of moment arm magnitude of moment arm and the magnitude of the and the magnitude of the forceforce

Moment of Force = Moment Arm x ForceMoment of Force = Moment Arm x Force By grasping the wrench at the end (A) a greater torque is generated because

the moment arm is greater than in (B)

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Mass and Inertia Concepts

MassMass is the measure of how is the measure of how much matter an object hasmuch matter an object has

InertiaInertia is the reluctance of an is the reluctance of an object to change its state of object to change its state of motion from rest to moving tomotion from rest to moving to


motion from rest to moving, to motion from rest to moving, to moving faster, or to slowing moving faster, or to slowing down back to restdown back to rest

Moment of inertiaMoment of inertia is a is a function of the mass of a function of the mass of a rotating object and how its mass rotating object and how its mass is distributed about its axis of is distributed about its axis of rotationrotation

The mass distribution of an individual about the longitudinal axis (A) and about a transverse

axis (B)

Typical moment of inertia for selected body positions

Body position Moment of inertia in the… about the…Body position Moment of inertia in the… about the…


Layout 1 kg.mLayout 1 kg.m22 Horiz. Plane Vertical AxisHoriz. Plane Vertical AxisLayout 10 kg.mLayout 10 kg.m22 Sagittal Plane Sagittal AxisSagittal Plane Sagittal AxisLayout 12 kg.mLayout 12 kg.m22 Front. Plane Frontal Axis Front. Plane Frontal Axis Pike 5 kg.mPike 5 kg.m22 Sagittal Plane Sagittal AxisSagittal Plane Sagittal AxisTuck 3 kg.mTuck 3 kg.m22 Sagittal Plane Sagittal AxisSagittal Plane Sagittal Axis

Center of Mass

Located at the balance point Located at the balance point of a body; a point found in of a body; a point found in or about a body where the or about a body where the mass could be concentratedmass could be concentrated

center of mass outside of body

Center of mass outside of body


Generally, 15 cm above the Generally, 15 cm above the crotch, or approximately crotch, or approximately 55% of standing height in 55% of standing height in females and 57% in malesfemales and 57% in males

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Weight versus MassMass Mass -- Measure of inertiaMeasure of inertia-- Measured in kilograms (kg)Measured in kilograms (kg)

WeightWeight


Weight Weight -- Measure of the force of gravity (g)Measure of the force of gravity (g)-- Measured in Newtons (N)Measured in Newtons (N)-- Varies directly with the magnitude of Varies directly with the magnitude of the acceleration due to gravity (9.8 the acceleration due to gravity (9.8 m/sm/s22))

W = m x gW = m x g

The Laws of Motion

Newton’s Three Laws of MotionNewton’s Three Laws of Motion::

Describe the relationship between all the external forces Describe the relationship between all the external forces acting on the h man bod at an time and the res ltingacting on the h man bod at an time and the res lting


acting on the human body at any time and the resulting acting on the human body at any time and the resulting motion of the total body.motion of the total body.

Sir Isaac NewtonSir Isaac Newton developed these laws to explain why developed these laws to explain why things move the way they do.things move the way they do.

Newton’s First Law: Inertia

An object will not change its state of motion (it will An object will not change its state of motion (it will continue to be at rest or moving with constant velocity), continue to be at rest or moving with constant velocity), unless acted upon by a net, external force.unless acted upon by a net, external force.


For example:For example: because of their large mass, football linemen because of their large mass, football linemen are difficult to move out of the wayare difficult to move out of the way

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Newton’s Second Law: Acceleration

For linear movementsFor linear movements,, the acceleration (a) a body the acceleration (a) a body experiences is proportional to the force (F) causing it, and experiences is proportional to the force (F) causing it, and takes place in the same direction as the force.takes place in the same direction as the force.


F = mF = m..a where m is the mass of the bodya where m is the mass of the body

For angular movementsFor angular movements,, the angular acceleration of a the angular acceleration of a body is proportional to the moment of force causing it, and body is proportional to the moment of force causing it, and takes place in the same direction as the moment of force.takes place in the same direction as the moment of force.

Force, mass, and acceleration


The greater the force applied to a soccer ball that has the same mass, the greater the ball’s acceleration.



As the soccer ball’s mass increases, it experiences less acceleration from a kick of the same force.

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As the mass of the soccer ball is increased, greater force must be generated if the ball is to have the same acceleration.

Impulse, Impact, and MomentumMomentumMomentum is the amount of motion a body possesses.is the amount of motion a body possesses.

ImpulseImpulse is the application of an external force over a short time is the application of an external force over a short time period.period.

ImpactImpact is the application of an external force.is the application of an external force.

Momentum is created by an impulse and is lost through impactMomentum is created by an impulse and is lost through impact


Momentum is created by an impulse and is lost through impact.Momentum is created by an impulse and is lost through impact.

Impulse and impact are both associated with bodies that are changing Impulse and impact are both associated with bodies that are changing their state of motion by experiencing large accelerations over relative their state of motion by experiencing large accelerations over relative short time periods.short time periods.

Collision or impact skills can sometimes manipulate the time of Collision or impact skills can sometimes manipulate the time of contact and contact and reduce the magnitude of the external forcereduce the magnitude of the external force..

To increase impulse, a sprinter must To increase impulse, a sprinter must increase the net external force increase the net external force per stepper step.

Newton’s Third Law: Action-Reaction

Every action has an equal and opposite Every action has an equal and opposite reactionreaction

The two acting forces are equal in The two acting forces are equal in it d b t it i di tiit d b t it i di ti


magnitude, but opposite in directionmagnitude, but opposite in direction

Example:Example:-- the sprinter exerts a force onto the blocks, and the sprinter exerts a force onto the blocks, and simultaneously the blocks exert an equal force simultaneously the blocks exert an equal force back onto the sprinterback onto the sprinter

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Projectile Motion

Any airborne object is a Any airborne object is a projectile, including the human projectile, including the human bodybody

The center of mass of aThe center of mass of a


The center of mass of a The center of mass of a projectile will follow a projectile will follow a parabolic pathparabolic path

The parabolic path followed is The parabolic path followed is determined only as a function determined only as a function of the projectile’s takeoff of the projectile’s takeoff velocityvelocity

Projectile MotionObjectives of the projectile motionObjectives of the projectile motion::-- maximum vertical distance or maximum vertical distance or heightheight-- maximum horizontal distance or maximum horizontal distance or rangerange-- accuracy accuracy


Maximizing Height and Range

To maximize vertical distance (height)To maximize vertical distance (height), one must one must maximize the takeoff velocity and take off verticallymaximize the takeoff velocity and take off vertically


To maximize horizontal distance (range)To maximize horizontal distance (range), one must one must maximize the takeoff velocity and take off at an angle of maximize the takeoff velocity and take off at an angle of 45 degrees to the horizontal 45 degrees to the horizontal

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Taking off and Landing at Different Heights

If the human body is the projectile, If the human body is the projectile, a higher center of mass can be a higher center of mass can be achieved by elongating the body at achieved by elongating the body at takeoff and by raising as many takeoff and by raising as many


body parts as possible.body parts as possible.

The range a projectile will travel The range a projectile will travel increases if the takeoff height is increases if the takeoff height is greater than the landing height, and greater than the landing height, and decrease if the takeoff height is decrease if the takeoff height is less than the landing height.less than the landing height.

Air Resistance

Another force acting on a projectileAnother force acting on a projectile

ill h h f i f hill h h f i f h


Will change the state of motion of the Will change the state of motion of the projectile and its pathprojectile and its path

Optimizing Range

Increasing the takeoff height compared to the landing Increasing the takeoff height compared to the landing height lowers the optimum takeoff angle of 45 degrees.height lowers the optimum takeoff angle of 45 degrees.


Skills which take advantage of air resistance in increasing Skills which take advantage of air resistance in increasing range also have a lower optimum takeoff angle.range also have a lower optimum takeoff angle.

In most instances, increasing the magnitude of the takeoff In most instances, increasing the magnitude of the takeoff velocity gains the greatest increase in horizontal distance.velocity gains the greatest increase in horizontal distance.

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Projectile Motion Equations

v = magnitude of the takeoff velocityv = magnitude of the takeoff velocityθθ = takeoff angle with respect to the horizontal = takeoff angle with respect to the horizontal g = acceleration due to the gravity = 9.8 m/sg = acceleration due to the gravity = 9.8 m/s22


Maximum heightMaximum height = (vsin= (vsinθθ))22 / 2g/ 2g

Maximum distanceMaximum distance = v= v22 sin (2sin (2θθ) / g ) / g

Fluid Dynamics

All athletic events take place in a fluid environment, All athletic events take place in a fluid environment, whether in water (swimming), in air (cycling), or in a whether in water (swimming), in air (cycling), or in a combination of both (water polo).combination of both (water polo).

Fl id f i l dFl id f i l d d fd f dd lift flift f


Fluid forces include: Fluid forces include: drag forcedrag force and and lift forcelift force.

Drag and lift forces are perpendicular to each other, Drag and lift forces are perpendicular to each other, produce different effects, and are affected by different produce different effects, and are affected by different factors.factors.

Flow velocityFlow velocity –– the motion of the fluid flowing past an the motion of the fluid flowing past an object or the motion of the object through the fluid. object or the motion of the object through the fluid.

Discus with direction of travel and relative flow velocity vectors superimposed

F b d di f di i di ti lift

Effects of air resistance on a discus


Free body diagram of a discus indicating lift, drag, and gravity vectors

Angle of attack (theta) defined for a discus with direction of travel and longitudinal axis drawn

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Fluid Drag Forces

Two types: Two types: skinskin--frictionfriction and and profile dragprofile drag


Both skinBoth skin--friction and profile drag are proportional to friction and profile drag are proportional to relative flow velocity, crossrelative flow velocity, cross--sectional area, shape of sectional area, shape of object, smoothness of surface, and density of liquid.object, smoothness of surface, and density of liquid.

Skin-Friction DragCaused by the fluid tending to rub (shear) along the surface of the bodyCaused by the fluid tending to rub (shear) along the surface of the body

Parallel to the flow velocityParallel to the flow velocity

The layer of fluid next to the skin sticks to the body; however, the next The layer of fluid next to the skin sticks to the body; however, the next layer is towed along and therefore slides relative to the innermost layerlayer is towed along and therefore slides relative to the innermost layer

The region of relative motion between adjacent layers of fluid particles is The region of relative motion between adjacent layers of fluid particles is


g j y pg j y pcalled the called the Boundary LayerBoundary Layer

Two types of flow can occur within the boundary layer: Two types of flow can occur within the boundary layer: laminar flowlaminar flow and and turbulent flowturbulent flow

Skin-Friction DragLaminar flow Laminar flow

The smooth, layered, flow pattern of a fluid around an The smooth, layered, flow pattern of a fluid around an object with no disturbanceobject with no disturbanceOccurs if an object is small, streamlined, smooth, and Occurs if an object is small, streamlined, smooth, and relatively slow movingrelatively slow moving


relatively slow movingrelatively slow moving

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Profile Drag (pressure or form drag)

The main form of drag in skiing, cycling, running , all The main form of drag in skiing, cycling, running , all projectiles events, and swimmingprojectiles events, and swimming

h i i d i h bj ( ’h i i d i h bj ( ’


Represents the resistive drag against the object (Newton’s Represents the resistive drag against the object (Newton’s third law of motion)third law of motion)

Characterized by turbulent flow in which the pressure on Characterized by turbulent flow in which the pressure on the leading surface of a body is greater than the pressure the leading surface of a body is greater than the pressure on the trailing surfaceon the trailing surface

Profile DragTurbulent flowTurbulent flow

Occurs when the velocity of air flow past the object is too fast for the air to Occurs when the velocity of air flow past the object is too fast for the air to follow the contour of the trailing side of the object, causing “back flow” at the follow the contour of the trailing side of the object, causing “back flow” at the surface of the object.surface of the object.

This causes a large, turbulent, lowThis causes a large, turbulent, low--pressure zone to form behind the object.pressure zone to form behind the object.

The region of low pressure increases the amount of work done on the object.The region of low pressure increases the amount of work done on the object.


The region of low pressure increases the amount of work done on the object.The region of low pressure increases the amount of work done on the object.

Profile drag can be reduced by streamlining.Profile drag can be reduced by streamlining.

Fluid Lift ForcesAlways directed perpendicular to the flow velocityAlways directed perpendicular to the flow velocity

Can be directed Can be directed upwardupward (javelin), (javelin), downwarddownward (racing (racing cars), cars), sidewayssideways (sailboats)(sailboats)

More air flows over the upper curved surface than the More air flows over the upper curved surface than the l fl t f h th t th diff i l itl fl t f h th t th diff i l it


lower flat surface, such that the difference in velocity lower flat surface, such that the difference in velocity across the surfaces results in a pressure difference between across the surfaces results in a pressure difference between the two sidesthe two sides

The external force resulting from the pressure difference is The external force resulting from the pressure difference is perpendicular to the direction of flow velocity, and can perpendicular to the direction of flow velocity, and can change the motion of the objectchange the motion of the object

Bernoulli’s principleBernoulli’s principle - the inverse relationship between he inverse relationship between flow velocity and pressure flow velocity and pressure

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Fluid Lift Forces


Aerodynamic lift force acting on an airfoil (wing)

Angle of Attack

Refers to the tilt of an object relative to the flow velocityRefers to the tilt of an object relative to the flow velocity

C i i h ff f i f il b h i hC i i h ff f i f il b h i h


Can mimic the effects of an airfoil by changing the Can mimic the effects of an airfoil by changing the pressure difference across the surfaces of the objectpressure difference across the surfaces of the object

A function of the shape of an object and the flow velocityA function of the shape of an object and the flow velocity

If the angle of attack increases too much, it approaches a If the angle of attack increases too much, it approaches a critical maximum angle critical maximum angle (stall angle)(stall angle), beyond which the lift , beyond which the lift force decreases as the drag force becomes dominantforce decreases as the drag force becomes dominant

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The Magnus Effect

The pressure difference across opposite sides of an object The pressure difference across opposite sides of an object (which spins about an axis that is not aligned with the flow (which spins about an axis that is not aligned with the flow velocity vector) can generate a change in its flight path velocity vector) can generate a change in its flight path through a type of lift force known as through a type of lift force known as Magnus forceMagnus force.

The changes in flight path are always perpendicular to the The changes in flight path are always perpendicular to the


g g p y p pg g p y p pflow velocity of the projectile.flow velocity of the projectile.

TopspinTopspin restricts the horizontal distance with no loss in restricts the horizontal distance with no loss in takeoff velocity.takeoff velocity.

UnderspinUnderspin will increase the horizontal distance, time in will increase the horizontal distance, time in the air, or accuracy for a given takeoff velocity.the air, or accuracy for a given takeoff velocity.

SidespinsSidespins will curve the ball toward the spinning side.will curve the ball toward the spinning side.

The Magnus EffectThe Magnus force is directed from high to low pressure

Why Does a Curveball Curve?

The ball’s 108 stitches carry a layer of air with them as they spin. That layer makes the air on the bottom of the ball flow faster than the air at the top of the ball which produces the curve ball i e the Magnus


produces the curve ball, i.e., the Magnus effect at work.

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Application of the Stick Figure ModelBody segments are represented by rigid bars (sticks) linked Body segments are represented by rigid bars (sticks) linked together by jointstogether by joints

Indicate approximate body segment positions as well as Indicate approximate body segment positions as well as external forces acting on the bodyexternal forces acting on the body

Used to represent gross motor skills (e g running)Used to represent gross motor skills (e g running)


Used to represent gross motor skills (e.g., running)Used to represent gross motor skills (e.g., running)

Are not used for longitudinal rotations (e.g., twisting)Are not used for longitudinal rotations (e.g., twisting)

Composite diagramsComposite diagrams

A sequence of stick figures representing either the total A sequence of stick figures representing either the total body or a portion of the body body or a portion of the body

Give a quick picture of the body actions involved in a skillGive a quick picture of the body actions involved in a skill

Throwing and Pushing ActivitiesThrowThrow--like Skillslike Skills

The object The object lags behind the proximal segments behind the proximal segments that have finished their backswing and are now that have finished their backswing and are now moving forward.moving forward.

Include throws (baseball), kicks (soccer), and Include throws (baseball), kicks (soccer), and


( ), ( ),( ), ( ),strikes (tennis).strikes (tennis).

For maximal horizontal or vertical distance, or For maximal horizontal or vertical distance, or for accuracy where speed is important, the for accuracy where speed is important, the takeoff velocity is maximal, the height of the takeoff velocity is maximal, the height of the release is greatest, and the takeoff angle is release is greatest, and the takeoff angle is optimal.optimal.

The magnitude of the takeoff velocity is most The magnitude of the takeoff velocity is most important for success and depends upon the important for success and depends upon the velocity of the end segment upon release.velocity of the end segment upon release.

Throwing and Pushing Activities

To maximize the magnitude of the takeoff velocityTo maximize the magnitude of the takeoff velocity

The movement begins at a fixed endThe movement begins at a fixed end..The movement uses sequential segmental rotationsThe movement uses sequential segmental rotations


The movement uses sequential segmental rotations The movement uses sequential segmental rotations beginning from the fixed end to the end segment.beginning from the fixed end to the end segment.Once segments have been accelerated to a maximum Once segments have been accelerated to a maximum angular velocity, they experience deceleration.angular velocity, they experience deceleration.This results in a maximal end segment velocity.This results in a maximal end segment velocity.The end segment and object move in a curvilinear path The end segment and object move in a curvilinear path prior to the velocity.prior to the velocity.

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PushPush--like Skillslike Skills

The object to be pushed is always in front of or behindThe object to be pushed is always in front of or behind


The object to be pushed is always in front of, or behind, The object to be pushed is always in front of, or behind, the active body segments.the active body segments.

Include jumps, throws requiring large forces Include jumps, throws requiring large forces (weightlifting), and throws requiring accuracy (darts).(weightlifting), and throws requiring accuracy (darts).

The objective is to generate maximum power to project The objective is to generate maximum power to project the body, to manipulate a resistance by overcoming large the body, to manipulate a resistance by overcoming large external forces, or to project an object for accuracy when external forces, or to project an object for accuracy when speed is not important.speed is not important.


The Objectives of PushThe Objectives of Push--like Skills are achieved whenlike Skills are achieved when::

The movement uses simultaneous segmental rotations to The movement uses simultaneous segmental rotations to maximize force productionmaximize force production


Movement begins at a free segmentMovement begins at a free segment

The object is moved in a rectilinear path, which increases The object is moved in a rectilinear path, which increases the length of the arc, and thus, time availablethe length of the arc, and thus, time available

Throw- and Push-like Skills Differences

Their objectives (high velocity versus accuracy or force)Their objectives (high velocity versus accuracy or force)

Body segment patterns (sequential versus simultaneous)Body segment patterns (sequential versus simultaneous)

Path of the object (small versus large)Path of the object (small versus large)

The size of the projected object (small versus large)The size of the projected object (small versus large)


The size of the projected object (small versus large)The size of the projected object (small versus large)

Ease of holding the object (easy versus difficult) Ease of holding the object (easy versus difficult)

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Body Balance and Stability Control

Balance is a very important factor in athletic performanceBalance is a very important factor in athletic performance


Generally depends upon the location of the center of mass Generally depends upon the location of the center of mass and how stable that center of mass isand how stable that center of mass is

The stick figure model is usefulThe stick figure model is useful

Equilibrium

Static equilibriumStatic equilibrium

System is at restSystem is at rest

The state of a system that is not experiencing any change in its The state of a system that is not experiencing any change in its direction or speeddirection or speed


System is at restSystem is at rest

Dynamic equilibriumDynamic equilibrium

System is moving System is moving with constant with constant velocityvelocity

Balance

Process whereby the body’s state of Process whereby the body’s state of equilibrium is controlled for a given equilibrium is controlled for a given purpose.purpose.


The base of support and location of The base of support and location of the center of mass is manipulated.the center of mass is manipulated.

If the line of gravity passes through If the line of gravity passes through some part of the body’s base of some part of the body’s base of support, the body will be balanced.support, the body will be balanced.

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Stability

A measure of the difficulty with which equilibrium can be A measure of the difficulty with which equilibrium can be disturbed.disturbed.


A net external force is required to overcome the static A net external force is required to overcome the static equilibrium of a sprint start.equilibrium of a sprint start.

There is a tradeThere is a trade--off between maximizing stability and off between maximizing stability and acquiring speed off a mark.acquiring speed off a mark.

To increase stability in static equilibrium

Increase the base of supportIncrease the base of support

Increase the inertia of the bodyIncrease the inertia of the body


Decrease the vertical distance between the center of mass Decrease the vertical distance between the center of mass and the base of supportand the base of support

Increase the distance between the point where a vertical Increase the distance between the point where a vertical line from the center of mass intersects the base of supportline from the center of mass intersects the base of support

To increase stability in dynamic equilibrium

Enlarge the body’s base of support in the direction of the Enlarge the body’s base of support in the direction of the external horizontal forces.external horizontal forces.Adopt a starting position in which the center of pressure is Adopt a starting position in which the center of pressure is close to the edge of the base of support whenever a quick close to the edge of the base of support whenever a quick

l ti i i t tl ti i i t t


acceleration is important.acceleration is important.When slowing down or reversing directions in running, When slowing down or reversing directions in running, lean backwards but keep normal frictional forces high to lean backwards but keep normal frictional forces high to prevent slipping.prevent slipping.When tripping or falling, reflex movements may help to When tripping or falling, reflex movements may help to regain the loss of balance or to create a new base of regain the loss of balance or to create a new base of support.support.

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Somersaulting – Angular Kinetics

Angular kinetics is concerned with the generation of Angular kinetics is concerned with the generation of rotations and the control of these rotations.rotations and the control of these rotations.


All objects posses inertia and do not wish to begin rotating.All objects posses inertia and do not wish to begin rotating.

If an external moment of force is applied to the object, it If an external moment of force is applied to the object, it will experience an angular acceleration.will experience an angular acceleration.

Off-center External Forces

Any external force that acts away from the body’s center Any external force that acts away from the body’s center of mass will create a moment of force acting on the total of mass will create a moment of force acting on the total bodybody

U ll th ffU ll th ff t f i ti f f tht f i ti f f th


Usually, the offUsually, the off--center force is a reaction force from the center force is a reaction force from the ground or equipment, resulting from the internal muscle ground or equipment, resulting from the internal muscle forces generated by the athleteforces generated by the athlete

If an athlete has forward momentum and a pivot point is If an athlete has forward momentum and a pivot point is created, then some or all of the linear motion can be created, then some or all of the linear motion can be transferred into angular motion transferred into angular motion

Transfer of Momentum Within the Body

Sometimes athletes accelerate specific body Sometimes athletes accelerate specific body parts while in contact with the ground.parts while in contact with the ground.

Upon leaving the ground, athletes lock these Upon leaving the ground, athletes lock these


segments to the rest of their bodies.segments to the rest of their bodies.

The joint angles these segments make with The joint angles these segments make with the trunk become fixed.the trunk become fixed.

The momentum created by these segments The momentum created by these segments becomes part of the momentum of the total becomes part of the momentum of the total body.body.

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Rotations While Airborne

Rotations cannot be generated when airborne.Rotations cannot be generated when airborne.

Once the body is airborne, only gravity acts on the body.Once the body is airborne, only gravity acts on the body.

The angular momentum (the amount of angular motion) is The angular momentum (the amount of angular motion) is conservedconserved


conserved.conserved.

Changes in body shape will decrease/increase the moment Changes in body shape will decrease/increase the moment of inertia about the somersaulting axis.of inertia about the somersaulting axis.

If the moment of inertia increases, the athlete will have If the moment of inertia increases, the athlete will have more reluctance to rotating and will have slower angular more reluctance to rotating and will have slower angular velocity.velocity.

The inverse relationship between moment of inertia (I) and angular velocity (ω)


Qualitative Analysis of Human Motion

The study of human motion, in the absence of measuring, by The study of human motion, in the absence of measuring, by observing a movement and applying biomechanical principles in observing a movement and applying biomechanical principles in assessing performance.assessing performance.

It is subjective and yet systematic evaluation of a movement orIt is subjective and yet systematic evaluation of a movement or


It is subjective and yet systematic evaluation of a movement or It is subjective and yet systematic evaluation of a movement or skill.skill.

It is based on a direct visual observation of a movement or its It is based on a direct visual observation of a movement or its video recording.video recording.

It is dependent on the constraints or limitations of an event or It is dependent on the constraints or limitations of an event or performer.performer.

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Integrative model of qualitative analysis


Skill Objectives

An objective can be described for all skillsAn objective can be described for all skills

Secondary objectives refer to the speed of the movement orSecondary objectives refer to the speed of the movement or


Secondary objectives refer to the speed of the movement or Secondary objectives refer to the speed of the movement or the accuracy requiredthe accuracy required

Skills with similar overall objectives are governed by Skills with similar overall objectives are governed by similar biomechanical principlessimilar biomechanical principles

Analyzing a Skill

Divide the skill into phasesDivide the skill into phases (preparation, execution, follow(preparation, execution, follow--through)through)

Determine the biomechanical criteria for each phaseDetermine the biomechanical criteria for each phase (in(in


Determine the biomechanical criteria for each phaseDetermine the biomechanical criteria for each phase (in (in the execution phase, shot putters must maximize the velocity the execution phase, shot putters must maximize the velocity of the shot prior to the takeoff)of the shot prior to the takeoff)

Identify the key body movements involved in each phaseIdentify the key body movements involved in each phase(in the preparatory phase of the soccer kick, the foot of the (in the preparatory phase of the soccer kick, the foot of the stance leg is placed beside the ball) stance leg is placed beside the ball)

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Observation of PerformanceBefore going to practice, identify the skill you wish to Before going to practice, identify the skill you wish to observe and why.observe and why.

Be specific regarding the phase, body movements, and Be specific regarding the phase, body movements, and related biomechanical criteriarelated biomechanical criteria


related biomechanical criteria.related biomechanical criteria.

It is important to keep the surroundings as uncluttered as It is important to keep the surroundings as uncluttered as possible and to minimize distractions.possible and to minimize distractions.

Estimate how many times you have to view the skill.Estimate how many times you have to view the skill.

Use all your senses to observe a performance.Use all your senses to observe a performance.

Ask the athlete questions to confirm your observations.Ask the athlete questions to confirm your observations.

Good coaching includes askingAthletes if given instructions were understood correctly, particularlyUnder distracting environmental conditions, i.e., noise, distance, low


visibility, etc.

Error detection

Visual observation (use direct observation, video, Visual observation (use direct observation, video, or observe traces left by the athlete)or observe traces left by the athlete)

Observe the outcome or followObserve the outcome or follow--throughthrough


Observe the outcome or followObserve the outcome or follow throughthrough

Use a checklistUse a checklist

Ask athletes how they felt during the performanceAsk athletes how they felt during the performance

Avoid comparison with an ideal form or mental Avoid comparison with an ideal form or mental modelmodel

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Error CorrectionUsually done verbally, with visual feedback, or using Usually done verbally, with visual feedback, or using mental imagerymental imagery

Begin with the earliest occurring errorBegin with the earliest occurring error


Correct only one or two faults before reCorrect only one or two faults before re--observing the skillobserving the skill

Accept that error correction is an ongoing, multiAccept that error correction is an ongoing, multi--step step process process

Use positive and specific language appropriate to the age, Use positive and specific language appropriate to the age, developmental level, and competitive level of the athletedevelopmental level, and competitive level of the athlete

Ask for feedback Ask for feedback


In sports where there is a considerable distance between the coach and the athlete, a walkie talkie or other long-range communication device comes in very handy.

In this chapter you have learned:

How to generate Free Body Diagrams of the body using the particle How to generate Free Body Diagrams of the body using the particle and stick figure modelsand stick figure modelsThose factors affecting the height and range of a projectileThose factors affecting the height and range of a projectileHow the forces of air resistance, drag, and lift affect projectile pathsHow the forces of air resistance, drag, and lift affect projectile paths


The role of impulse and impact on many sporting activitiesThe role of impulse and impact on many sporting activitiesThe body actions affecting throwThe body actions affecting throw--like and pushlike and push--like skillslike skillsHow angular motion is generated when somersaultingHow angular motion is generated when somersaultingHow angular motion is manipulated while airborneHow angular motion is manipulated while airborneA technique for performing a biomechanical qualitative analysis of a A technique for performing a biomechanical qualitative analysis of a skillskillSome tips on performing error detection and correction Some tips on performing error detection and correction

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Discussion Questions:1. Define Biomechanics. Briefly discuss each of the components in the model 1. Define Biomechanics. Briefly discuss each of the components in the model

of Biomechanics presented.of Biomechanics presented.2. Discuss three ways in which Biomechanics would be helpful to a coach.2. Discuss three ways in which Biomechanics would be helpful to a coach.3. Apply each of Newton’s three laws of motion to a skill from your sport. Be 3. Apply each of Newton’s three laws of motion to a skill from your sport. Be

specific in identifying forces, masses, accelerations, etc.specific in identifying forces, masses, accelerations, etc.4. Briefly discuss how you would advise an athlete in maximizing impulse and 4. Briefly discuss how you would advise an athlete in maximizing impulse and

minimizing the harmful effects of impactminimizing the harmful effects of impact


minimizing the harmful effects of impact.minimizing the harmful effects of impact.5. List advantages and disadvantages of the particle and stick figure models.5. List advantages and disadvantages of the particle and stick figure models.6. What factors affect the horizontal distance of a projectile? Which factor is 6. What factors affect the horizontal distance of a projectile? Which factor is

most important?most important?7. Differentiate between throw7. Differentiate between throw--like and pushlike and push--like patterns of motion. Give like patterns of motion. Give

examples of each.examples of each.8. Select a sport skill in which a light object is thrown. Discuss the effects of 8. Select a sport skill in which a light object is thrown. Discuss the effects of

air resistance on the path of the object.air resistance on the path of the object.9. Identify three ways athletes can decrease drag forces acting on their bodies.9. Identify three ways athletes can decrease drag forces acting on their bodies.

Discussion Questions:10. Identify two sport skills in which lift forces have a large effect on the 10. Identify two sport skills in which lift forces have a large effect on the

resulting motion.resulting motion.11. Differentiate between equilibrium, stability, and balance.11. Differentiate between equilibrium, stability, and balance.12. For a skill from your sport, identify how one maintains or loses balance.12. For a skill from your sport, identify how one maintains or loses balance.13. Draw a free body diagram of a diver during the takeoff phase with the 13. Draw a free body diagram of a diver during the takeoff phase with the

b d Id tif ll ibl th di i th t fb d Id tif ll ibl th di i th t f


board. Identify all possible ways the diver may increase the amount of board. Identify all possible ways the diver may increase the amount of rotation.rotation.

14. Discuss the relationship between moment of inertia and angular velocity.14. Discuss the relationship between moment of inertia and angular velocity.15. Analyze a skill from your sport biomechanically.15. Analyze a skill from your sport biomechanically.16. Prepare an observation plan to observe the above skill from your sport.16. Prepare an observation plan to observe the above skill from your sport.17. Draw a series of free body diagrams to represent the human body during 17. Draw a series of free body diagrams to represent the human body during

each phase of the above skill.each phase of the above skill.

how do i move? chapter 8 · 2010-11-23 · 1 how do i move? chapter 8 the science of biomechanics...

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