fw275 biomechanics

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Chapter 6Biomechanics


Exploring Biomechanics

Biomechanics: bio means “life” Study of the actions of forces Term first used 1970’s Includes both internal forces produced by

muscles and external forces that act on the body Biomechanics human movement is one sub

discipline of exercise science


Exploring Biomechanics Biomechanics

Academic backgrounds: zoology, medicine, engineering, physical therapy

Common interest in biomechanical aspects of the structure and function of living things

Undergraduate course Usually requires prerequisite courses Focusing on forces acting on the human body Biomechanics commonly a quantitative field of study Strong math, computer applications and problem solving


History of Biomechanics

Aristotle (384-322 B.C.)

Archimedes (287-212 B.C.)

Galen (131-201 A.D.) Leonardo da Vinci

(1452-1519) Galileo (1564-1642)

Giovanni Alfonso Borelli (1608-1679)

Isaac Newton (1643-1727) Eadweard Muybridge

(1831-1904) Julius Wolff (1836-1902) Christian Wilhelm Braune

(1831-1892) & Otto Fischer (1861-1971)


Basic Biomechanics--Kinematics

Kinematics The form, pattern, or sequencing of movement with

respect to time Qualitative

Involving nonnumeric description of quality Quantitative

Involving the use of numbers Kinematic analysis often requires the use of both

visual description and quantitative measurement



Kinematic quantities: distance, displacement, speed, velocity, acceleration

Vector quantities have both size and direction Displacement, velocity, acceleration



Linear displacement Change in linear position



Angular displacement Change in the angular position or orientation of a

line segment


Basic Biomechanics--Kinematics Angular--most human movement requires

angular quantities Rotation around a central line or point Primarily rotational motion occurs at joints

Linear velocity Rate of change in linear position

Angular velocity Angular displacement divided by the time interval

which displacement occurs



Acceleration Angular acceleration

Does not take into account particular circumstances or rules when faced with a moral problem

With every moral situation, there is a blank slate and each case must be viewed independently



Biomechanics use quantitative kinematics to research differences between skilled performers and non-athletes. Examples: Runners stride length & frequency Swimmer’s stroke rate & length Track events velocity of takeoff High jumpers vertical displacement Basketball angle of projection, angle of release Throwing events aerodynamics of flight path Swinging (bat, racquet) timing, project speed and

angle



Skilled performers move body segments at high rate of angular velocity

Most biomechanical studies of human kinematics involve non-athletes: Infant through maternity developmental stages Adapted P.E. kinematic patterns with motor disorders Therapeutic uses to aid in recovery of

injuries/surgeries



Quantitative kinematic analyses often use: High-speed cinematography videography

Reflective markers Digitizing


Kinetics Study of the action of forces

Force The product of mass and acceleration Push or pull acting on a body

Vector quantity: magnitude, direction & point of application to a body

Free body diagram First step used when analyzing actions Body, body segment or object of interest

Basic Biomechanics--Kinetics



Force rarely acts in isolation Net force

Overall effect of many forces acting on a body Vector sum of all the acting forces

Net force zero = acting forces balanced in magnitude and direction

Net force present = body moves in the direction of the net force with acceleration



Torque The rotary effect of a force

Torque quantified The product of force (F) and the perpendicular

distance (d┴) from the force’s line of action to the axis of rotation T = F d┴

The greater the force, the greater the tendency for rotation



Inertia--Sir Isaac Newton Tendency of a body to resist a change in its

state of motion Resistance to acceleration Inertia has no units of measurement Amount of inertia a body possesses is

directly proportional to its mass More massive an object is, the more it tends

to maintain its current state of motion



1st law of inertia A Body will maintain a state of rest or

constant velocity unless acted on by an external force that changes the state

A motionless object will remain motionless unless there is a net force acting on it



Inertia is influenced by mass (m) and distribution of mass in the body

Mass distribution is characterized by the radius of gyration (k)


Basic Biomechanics--Kinetics Newton’s 2nd law--law of acceleration

A force applied to a body causes an acceleration of that body of a magnitude proportional to the force, in the direction of the force, and inversely proportional to the body’s mass

The greater the amount of force applied, the greater the speed

F=ma Angular motion, the law becomes:

When a torque acts on a body, the resulting angular acceleration is in the direction of the torque and of a magnitude inversely proportional to its moment of inertia



Newton’s 3rd law--law of reaction For every action, there is an equal and opposite

reaction

In terms of forces: When one body exerts a force on a second, the

second body exerts a reaction force that is equal in magnitude and opposite in direction on the first body


Basic Biomechanics--Kinetics Ground reaction forces

Are measured during gait, each contact the foot makes with floor is analyzed

Walking, running patterns throughout development stages using force platforms

Magnitude of the vertical component of the GRF during running is generally two to three times the runner’s body weight

Runner’s are classified as rearfoot, midfoot, or forefoot strikers



GRF is an external force acting on the human body

Magnitude and direction have implications for performance in many sporting events High jumper’s Baseball pitching Golf



Friction is a force that acts at the interface of surfaces in contact in the direction opposite the direction of motion or impending motion

Units of force (N) The size of the friction force is the product of

the coefficient of friction (µ) and the normal (perpendicular) reaction force (R).


Basic Biomechanics--Kinetics Factors influencing the value of friction µ

Relative roughness and hardness of the surfaces Type of molecular interaction between surfaces

Greater the molecular interaction, the greater is the value of µ



Normal reaction force (R) Sum of all acting vertical forces (usually body’s

weight)

Altered magnitude R Increased or decreased When friction is altered, how does this change

motion?


Basic Biomechanics--Kinetics Altering the coefficient between two

surfaces can change friction Use of gloves, thin wax…

Important daily influence to prevent slippage


Basic Biomechanics--Kinetics Momentum (M) is a vector quantity

The quantity of motion that an object possesses mechanical quantity important in collisions

Linear momentum Product of an object’s mass and its velocity Body with zero velocity has no momentum What should the momentum be during most

weight training exercises?



Impulse Product of a force and the time interval over which

the force acts Impulse = Ft Impulse can change momentum

Impulse and momentum Changes in momentum depend not only on the

magnitude of the acting external forces but also on the length of time over which each force acts

When impulse acts on system, the result is a change in the system’s total momentum


Basic Biomechanics--Kinetics Angular momentum (H)

Quantity of angular motion possessed by a body, measured as the product of moment of inertia and angular velocity

H product of angular inertial property (moment of inertia) and angular velocity

Linear momentum Product of the linear inertial property (mass) and liner velocity

Liner motion: M – mvAngular motion: H = Iw

or: H = mk2



Factors affect magnitude of a body’s angular momentum

Mass (m) Distribution of that mass with respect to axis of rotation Angular velocity of the body (w)

If body has no angular velocity; it has no angular momentum

Mass or angular velocity increases; angular momentum increases proportionally


Basic Biomechanics--Kinetics Factor that most dramatically influences angular

momentum Distribution of mass with respect to the axis of rotation Why? Angular momentum is proportional to the momentum

result from multiplying units of mass, units of length squared, and units of angular velocity

Kg ·m2/s


Basic Biomechanics--Kinetics Angular impulse

Change in angular momentum equal to the product of torque and time interval over which the torque acts

Angular impulse generates a change in angular momentum


Basic Biomechanics--Fluid Mechanics

Fluid (interchangeably with liquid) Is any substance that tends to flow or continuously

deform when acted on by a shear force Both gases & liquids are fluids

Archimedes Principle The magnitude of the buoyant force is equal to the

weight of the fluid displaced by the body If magnitude of weight is greater than buoyant force,

the body sinks, moving downward in the direction of the net force



Buoyancy is studied in relation to floatation of human body in water

Difference in floatability is function of body density

Density of bone and muscle is greater than the density of fat

What changes with floatation when a person holds inspired air in the lungs?



Orientation of the body as it floats in water is determined by the relative position of the totally body center of gravity relative to the total body center of volume

What is center of gravity?


Basic Biomechanics--Fluid Mechanics COG is the point around which the body’s

weight is balanced in all directions Center of volume and Center of gravity vary

depending on body shape and size


Basic Biomechanics--Fluid Mechanics Drag

Generally, a resistance force: A force that slows the motion of a body moving

through a fluid Theoretical square law

The magnitude of the drag force increases approx with the square of velocity Effect seen with high velocity sports: cycling, speed

skating, downhill skiing, bobsled, luge 3 types of drag

Skin friction/surface drag Form drag/profile drag Wave drag



Form drag



Wave drag



Force that acts perpendicular to fluid flow is lift

Lift can assume any direction (not just vertically upward) Determined by direction of fluid flow and

orientation of body Can be generated by foil

Shape capable of generating lift in the presence of a fluid flow

Human hand during swimming


Basic Biomechanics

The study of biomechanics covers many other topics that help us to better understand human movement in relation to forces acting on the body, joint movements and forms of motion


Professional Organizations

Advantageous for students to consider student memberships with professional organizations in their chosen field of sport science study

Biomechanics has many supported journals Memberships include discounts: journal

subscriptions, conference fees and opportunities for students to interact with professionals


Professional Organizations

American Society of Biomechanics Canadian Society of Biomechanics European Society of Biomechanics International Society of Biomechanics International Society of Biomechanics in Sport Mulit-disciplinary organizations

American College of Sports Medicine American Alliance for Health, Physical Education,

Recreation and Dance


Careers

What are some potential career opportunities in biomechanics?