Biomechanics of Human MotionBiomechanics of Human Motion

Introduction to BiomechanicsIntroduction to Biomechanics

Background for the study of Background for the study of BiomechanicsBiomechanics

As a new discipline but early root back to As a new discipline but early root back to many centuriesmany centuries– AristotleAristotle– ArchimedesArchimedes– Leonardo De VinciLeonardo De Vinci– Alfonso BorelliAlfonso Borelli– Eadweard Muybridge: to settle of hour runs with Eadweard Muybridge: to settle of hour runs with

four legs off the group with pictures (fig.)four legs off the group with pictures (fig.)

Eadweard Muybridge’s studiesEadweard Muybridge’s studies

The Analysis of Human MotionThe Analysis of Human MotionBiomechanics is the Biomechanics is the science of physical science of physical principles applied to principles applied to biology systemsbiology systems– KinematicsKinematics

A description of the A description of the temporal and spatial temporal and spatial components of movementcomponents of movement

– KineticsKineticsA study of the forces A study of the forces acting on an objectacting on an object

Kinematic of Human MovementKinematic of Human Movement

Motion refers to an object changing its position in Motion refers to an object changing its position in space over a period of timespace over a period of timeIn most of activities, success depends on getting an In most of activities, success depends on getting an object to travel to a certain location as quickly as object to travel to a certain location as quickly as possiblepossible

Speed and VelocitySpeed and Velocity

Speed refers to how fast an object travelsSpeed refers to how fast an object travels

Velocity refers to how fast an object moves in Velocity refers to how fast an object moves in a a particular directionparticular direction

Speed = d / t; where d=distance, t=timeSpeed = d / t; where d=distance, t=time

Acceleration occurs when an object changes Acceleration occurs when an object changes its velocity over a particular time intervalits velocity over a particular time interval

Acceleration= Acceleration= VVff -V -Vii / Δt / Δt ; read as acceleration ; read as acceleration

change in velocity over a change in timechange in velocity over a change in time

Newton’s Law of MotionNewton’s Law of Motionare three physical laws that form the basis for are three physical laws that form the basis for classical mechanics. They describe the classical mechanics. They describe the relationship between the forces acting on a relationship between the forces acting on a body and its motion due to those forces.body and its motion due to those forces.

were first compiled by Sir Isaac Newton in his were first compiled by Sir Isaac Newton in his work Philosophiæ Naturalis Principia work Philosophiæ Naturalis Principia Mathematica, first published on July 5, 1687. Mathematica, first published on July 5, 1687. Newton used them to explain and investigate Newton used them to explain and investigate the motion of many physical objects and the motion of many physical objects and systems.systems.

Law of inertiaLaw of inertia: every object in a state of : every object in a state of uniform motion tends to remain in that state of uniform motion tends to remain in that state of motion unless an external force is applied to itmotion unless an external force is applied to it

Law of accelerationLaw of acceleration: the relationship between : the relationship between an object's mass m, its acceleration a, and the an object's mass m, its acceleration a, and the applied force F is F = ma. Acceleration and applied force F is F = ma. Acceleration and force are vectors; in this law the direction of force are vectors; in this law the direction of the force vector is the same as the direction of the force vector is the same as the direction of the acceleration vector. the acceleration vector.

Law of Action-ReactionLaw of Action-Reaction: For every action there : For every action there is an equal and opposite reactionis an equal and opposite reaction

Relationship of Force and MotionRelationship of Force and Motion

Using gravity, ground reaction forces, muscle Using gravity, ground reaction forces, muscle forces, friction and other forces, will clarify forces, friction and other forces, will clarify the concept of force and show how it is related the concept of force and show how it is related to the change in motion of an objectto the change in motion of an object

The impulse-Momentum relationshipThe impulse-Momentum relationship

Impulse = Ft = m(Impulse = Ft = m(VVff -V -Vii ); read the product of ); read the product of

force and the duration of time that the force is force and the duration of time that the force is appliedapplied

Momentum = mv ; represent a force applied to Momentum = mv ; represent a force applied to an object over a known amount of time causes a an object over a known amount of time causes a change in the momentum of that objectchange in the momentum of that object

The motion of a body represented by its The motion of a body represented by its momentum, is changed by the impulse applied to momentum, is changed by the impulse applied to the body, or force applied over time; Ft= the body, or force applied over time; Ft= ΔmvΔmv

GravityGravityIs a force that created as the attractive pull Is a force that created as the attractive pull between any two objectsbetween any two objects

The size of the pull is affected by both the The size of the pull is affected by both the mass of the two objects and the distance mass of the two objects and the distance between thembetween them

Center of gravity: the weigh concentrated at Center of gravity: the weigh concentrated at single pointsingle point

The Fosbury Flop is a style used in the The Fosbury Flop is a style used in the athletics event of high jump. It was athletics event of high jump. It was popularized and perfected by American athlete popularized and perfected by American athlete Dick Fosbury, whose gold medal in the 1968 Dick Fosbury, whose gold medal in the 1968 Summer Olympics brought it to the world's Summer Olympics brought it to the world's attention. Over the next few years the flop attention. Over the next few years the flop became the dominant style of the event and became the dominant style of the event and remains so todayremains so today

Contact Force Contact Force

When two bodies physically touch each other, When two bodies physically touch each other, contact forces, contact force are created. contact forces, contact force are created. – Ground reaction force (GRF)Ground reaction force (GRF)

The force exerted on the performer by the groundThe force exerted on the performer by the ground

GRF affects the vertical, anteroposterior, and GRF affects the vertical, anteroposterior, and mediolateral motion of the objectmediolateral motion of the object

Important consideration in injury preventionImportant consideration in injury prevention

– Demonstrating ground reaction forceDemonstrating ground reaction force

Measuring Ground Reaction Force Measuring Ground Reaction Force (GRF)(GRF)

An instrument called a An instrument called a force platform is used to force platform is used to measure the magnitude measure the magnitude and direction of the and direction of the ground reactionground reaction

Typically this interfaced Typically this interfaced with a computer to with a computer to record the GRF and record the GRF and facilitate analysisfacilitate analysis

FrictionFrictionis the force resisting the relative motion of solid is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:against each other. There are several types of friction:

– Dry frictionDry friction resists relative lateral motion of two solid resists relative lateral motion of two solid surfaces in contact. Dry friction is subdivided into static surfaces in contact. Dry friction is subdivided into static friction between non-moving surfaces, and kinetic friction friction between non-moving surfaces, and kinetic friction between moving surfaces. between moving surfaces.

– Fluid frictionFluid friction describes the friction between layers within describes the friction between layers within a viscous fluid that are moving relative to each other.a viscous fluid that are moving relative to each other.

– Lubricated frictionLubricated friction is a case of fluid friction where a fluid is a case of fluid friction where a fluid separates two solid surfaces.separates two solid surfaces.

– Skin frictionSkin friction is a component of drag, the force resisting the is a component of drag, the force resisting the motion of a solid body through a fluid. motion of a solid body through a fluid.

– Internal frictionInternal friction is the force resisting motion between the is the force resisting motion between the elements making up a solid material while it undergoes elements making up a solid material while it undergoes deformationdeformation

FrictionFriction

The relationship among The relationship among maximum limiting friction, maximum limiting friction, the nature of the surfaces in the nature of the surfaces in contact, and the normal contact, and the normal force force

Friction μN≦Friction μN≦WhereWhere

– μ (Greek letter “mu”); μ (Greek letter “mu”); coefficient of frictioncoefficient of friction

– N; normal forceN; normal force

Propelling forceFriction

Contact Surface as a Friction ElementContact Surface as a Friction ElementShoes’ sole match the Shoes’ sole match the conflicting needs for grip conflicting needs for grip (increased friction) and (increased friction) and slide (decreased friction)slide (decreased friction)The different patterns and The different patterns and materials used on this part materials used on this part of the soleof the sole– Rotation of a pivotRotation of a pivot

Hard and smooth in the ball Hard and smooth in the ball and metatarsal areaand metatarsal area

– Grip (turn, push off, stop)Grip (turn, push off, stop)Soft and ripple in the lateral Soft and ripple in the lateral side side

For a classical ski that kick wax will be applied to For a classical ski that kick wax will be applied to the grip zone.the grip zone.

No kick wax in the glide zones and no glide wax in No kick wax in the glide zones and no glide wax in the grip zonethe grip zone

The high-friction wax increased friction is The high-friction wax increased friction is necessary to develop a forward propelling push necessary to develop a forward propelling push during the kickduring the kick

Low-friction wax is applied at both tips of the skiLow-friction wax is applied at both tips of the ski

camber

KICK ZONE

Fluid ForceFluid ForceRefers to the forces imposed on an object Refers to the forces imposed on an object when it moves through a fluid such as air of when it moves through a fluid such as air of waterwaterIs created as the object disrupts the fluid when Is created as the object disrupts the fluid when passing through itpassing through itThe greater the disruption of the fluid the The greater the disruption of the fluid the greater the fluid forces developedgreater the fluid forces developedHow much the fluid gets disrupted depends on How much the fluid gets disrupted depends on factors related to both the fluid and the objectfactors related to both the fluid and the object

Fluid factorsFluid factors– Influence the size of the force exerted on an object Influence the size of the force exerted on an object

moving through it include moving through it include Density; the distribution of mass throughout a volume Density; the distribution of mass throughout a volume of spaceof space

Viscosity; a fluid’s resistance to flowViscosity; a fluid’s resistance to flow

– The more dense or more viscous a fluid is, the The more dense or more viscous a fluid is, the more it is disturbed as an object passes though itmore it is disturbed as an object passes though it

Fluid ResistanceFluid Resistance

Body factorsBody factors– Aerodynamic and hydrodynamic are used to Aerodynamic and hydrodynamic are used to

describe the features of the object that affect the describe the features of the object that affect the size and direction of the fluid resistancesize and direction of the fluid resistance

Cross-section AreaCross-section Area– A larger cross-sectional area increases the size of A larger cross-sectional area increases the size of

the fluid resistance forcethe fluid resistance force

Nature of the SurfaceNature of the Surface– A smooth surface produces less disruption in the A smooth surface produces less disruption in the

fluid as it flows over the body and reduces fluid as it flows over the body and reduces resistance, and roughened surface vice versaresistance, and roughened surface vice versa

VelocityVelocity– The faster velocity, the greater fluid resistanceThe faster velocity, the greater fluid resistance

– This factor related to fluid resistance magnitude is very This factor related to fluid resistance magnitude is very critical in the performance of many skillscritical in the performance of many skills

– Relative motion of the body and the fluidRelative motion of the body and the fluid

– With a tail wind, the relative motion of the air over the ball With a tail wind, the relative motion of the air over the ball is reduced because the wind is pushing the air forwardis reduced because the wind is pushing the air forward

The force on a moving object due to a fluid is:The force on a moving object due to a fluid is:

Biomechanics of Muscle ForceBiomechanics of Muscle Force

Muscle Force-VelocityMuscle Force-Velocity

Muscles produce more force at some velocity than at Muscles produce more force at some velocity than at othersothersThe speed at which a muscle changes length (usually The speed at which a muscle changes length (usually regulated by external forces, such as load or other regulated by external forces, such as load or other muscles) also affects the force it can generate. muscles) also affects the force it can generate. Force declines in a hyperbolic fashion relative to the Force declines in a hyperbolic fashion relative to the isometric force as the shortening velocity increases, isometric force as the shortening velocity increases, eventually reaching zero at some maximum velocity. eventually reaching zero at some maximum velocity. The reverse holds true for when the muscle is The reverse holds true for when the muscle is stretched – force increases above isometric stretched – force increases above isometric maximum, until finally reaching an absolute maximum, until finally reaching an absolute maximum. maximum.

Muscle contraction occurs when individual myosin heads attach themselves to the actin filament, drawing the actin and myosin filaments in opposite directions past one another (Sliding Filament Theory)

This has strong implications for the rate at This has strong implications for the rate at which muscles can perform mechanical work which muscles can perform mechanical work (power). Since power is equal to force times (power). Since power is equal to force times velocity, the muscle generates no power at velocity, the muscle generates no power at either isometric force (due to zero velocity) or either isometric force (due to zero velocity) or maximal velocity (due to zero force). Instead, maximal velocity (due to zero force). Instead, the optimal shortening velocity for the optimal shortening velocity for power power generation is approximately one-third of generation is approximately one-third of maximum shortening velocitymaximum shortening velocity

Force–velocity Force–velocity relationship: right of the relationship: right of the vertical axis concentric vertical axis concentric contractions (the muscle contractions (the muscle is shortening), left of the is shortening), left of the axis eccentric axis eccentric contractions (the muscle contractions (the muscle is lengthened under is lengthened under load); power developed load); power developed by the muscle in redby the muscle in red

Muscle produces the Muscle produces the greatest force when there is greatest force when there is optimal overlap thick and optimal overlap thick and thin filament in the thin filament in the sarcomere (b and c)sarcomere (b and c)

No muscle force is produced No muscle force is produced at very long (a) or very at very long (a) or very short (e) muscle lengths.short (e) muscle lengths.

Less force is produced at Less force is produced at shorter and longer sacomere shorter and longer sacomere lengths (d)lengths (d)

Muscle length versus Muscle length versus isometric force during isometric force during active and passive active and passive isometric contractionisometric contraction

Who Use BiomechanicsWho Use Biomechanics

Sport scientists Sport scientists – Improving wind resistance in skiing, cycling, and sailingImproving wind resistance in skiing, cycling, and sailing

Physical therapists Physical therapists – Analyze gait defectsAnalyze gait defects

– Muscle functionMuscle function

ErgonomistErgonomist– Assembly line injuryAssembly line injury

RehabilitationRehabilitation– Design new prosthetic devicesDesign new prosthetic devices

– Artificial heartsArtificial hearts

Summary Points Summary Points

DiscussionDiscussionWhat’s the simplistic way to explain baseball What’s the simplistic way to explain baseball pitcher’s kinetics and kinematics.pitcher’s kinetics and kinematics.

How could you devise a test to quantify long How could you devise a test to quantify long distance runners’ running economy (movement distance runners’ running economy (movement efficiency) that has been considered as such an efficiency) that has been considered as such an important factor in athletic performance. important factor in athletic performance.

How you scheme to apply a force plate to How you scheme to apply a force plate to investigate the effects of balance (equilibrium) investigate the effects of balance (equilibrium) on athletic sport.on athletic sport.

How you explain the pitcher’s forkball How you explain the pitcher’s forkball underlying Newton law of motion.underlying Newton law of motion.