Special Topics in BiomechanicsSpecial Topics in Biomechanics

IntroductionIntroduction

Biomechanics refers to the application of the Biomechanics refers to the application of the principles of physics to the study of human principles of physics to the study of human movement.movement.Human body is viewed as a machine or a Human body is viewed as a machine or a mechanical system subject to the laws of mechanical system subject to the laws of physicsphysicsThe tool commonly use in biomechanics have The tool commonly use in biomechanics have been employed to study a wide range of been employed to study a wide range of human activities, from daily tasks to those human activities, from daily tasks to those used at work to those use in athleticsused at work to those use in athletics

Gait AnalysisGait Analysis

is the systematic study of is the systematic study of animal locomotion animal locomotion (walking, running, (walking, running, hopping), more specific as a hopping), more specific as a study of human motion, study of human motion, using the eye and the brain using the eye and the brain of observers, augmented by of observers, augmented by instrumentation for instrumentation for measuring body measuring body movements, body movements, body mechanics, and the activity mechanics, and the activity of the musclesof the muscles

Reasons to analyze human gaitReasons to analyze human gait– To improve understanding of how the loco-motor To improve understanding of how the loco-motor

system operatessystem operates– To improve activity that is normal or above To improve activity that is normal or above

averageaverage– To bring abnormal or suboptimal performance To bring abnormal or suboptimal performance

closer to normal standardscloser to normal standards

A relationship exists among A relationship exists among stride length, stride stride length, stride frequency, and gait velocity frequency, and gait velocity equal to stride length times equal to stride length times stride frequency.stride frequency.

How to Throw a Curve BallHow to Throw a Curve Ball

The curveball is a type of pitch in baseball The curveball is a type of pitch in baseball thrown with a characteristic grip and hand thrown with a characteristic grip and hand movement that imparts forward spin to the ball movement that imparts forward spin to the ball causing it to dive in a downward path as it causing it to dive in a downward path as it approaches the plate. Its close relatives are the approaches the plate. Its close relatives are the slider and the slurve. slider and the slurve.

The "curve" of the ball varies from pitcher to The "curve" of the ball varies from pitcher to pitcher. pitcher.

The pitcher places the middle finger on and parallel The pitcher places the middle finger on and parallel to one of the long seams, and the thumb just behind to one of the long seams, and the thumb just behind the seam on the opposite side of the ball such that if the seam on the opposite side of the ball such that if looking from the top down, the hand should form a looking from the top down, the hand should form a "C shape" with the horseshoe pointing in towards "C shape" with the horseshoe pointing in towards the palm following the contour of the thumb. the palm following the contour of the thumb.

The index finger is placed alongside the middle The index finger is placed alongside the middle finger, and the other two extraneous fingers are finger, and the other two extraneous fingers are folded in towards the palm with the knuckle of the folded in towards the palm with the knuckle of the ring finger touching the leather.ring finger touching the leather.

The SeamThe Seam

Pitchers hold their index and middle fingers Pitchers hold their index and middle fingers close together along the seam of the ballclose together along the seam of the ballCan enhances the pitcher’s ability to throw a Can enhances the pitcher’s ability to throw a curve ball curve ball A pitched ball is attracted toward the earth’s A pitched ball is attracted toward the earth’s surface the instant it leaves the pitcher’s hand, surface the instant it leaves the pitcher’s hand, but the first portion of its flight, it may drop but the first portion of its flight, it may drop only a few inches, while in the second portion only a few inches, while in the second portion of its flight it can drop more than 2 feetof its flight it can drop more than 2 feet

The Boundary LayerThe Boundary Layer

When a ball spins, it creates an envelope of When a ball spins, it creates an envelope of thin layer of air around surface of the ball thin layer of air around surface of the ball called the boundary layer. called the boundary layer. This boundary layer moves with the ball This boundary layer moves with the ball whether it spins forward or backward or whether it spins forward or backward or sideways. sideways. The interaction of this boundary layer with the The interaction of this boundary layer with the surrounding air results in an outside force that surrounding air results in an outside force that changes the path of the baseball. This is the changes the path of the baseball. This is the Magnus effect.Magnus effect.

Sandy Koufax was a Sandy Koufax was a master of the curveball. master of the curveball. He is now in the Hall of He is now in the Hall of Fame. Fame.

Magnus Effect

一個球是沿著旋轉軸一個球是沿著旋轉軸垂直方向的平移運動垂直方向的平移運動旋轉的誘導升力：旋轉的誘導升力：

F = F = 升力升力 lift force =lift force = 流流體的密度 體的密度 v = v = 球的速球的速度 度 A = A = 球的橫截面積 球的橫截面積 CL = CL = 升力係數 升力係數 升力係數升力係數 CLCL 可以從使可以從使用雷諾數和旋轉比率用雷諾數和旋轉比率的實驗數據圖表確定的實驗數據圖表確定。與光滑的球旋轉的。與光滑的球旋轉的比例為比例為 0.50.5 到到 4.54.5 ，典，典型的升力係數的範圍型的升力係數的範圍從從 0.20.2 至至 0.6.0.6.

Fooling with Gravity – The Fosbury Fooling with Gravity – The Fosbury FlopFlop

is a style used in the athletics event of high is a style used in the athletics event of high jump. jump.

It was popularized and perfected by American It was popularized and perfected by American athlete Dick Fosbury, whose gold medal in the athlete Dick Fosbury, whose gold medal in the 1968 Summer Olympics brought it to the 1968 Summer Olympics brought it to the world's attentionworld's attention

This technique allows the center of gravity to be This technique allows the center of gravity to be lowered even before knee flexion, giving a longer lowered even before knee flexion, giving a longer time period for the take-off thrust. time period for the take-off thrust.

Additionally, on take-off the sudden move from Additionally, on take-off the sudden move from inward lean outwards produces a rotation of the inward lean outwards produces a rotation of the jumper's body along the axis of the bar, aiding jumper's body along the axis of the bar, aiding clearance. clearance.

Combined with the rotation around the jumper's Combined with the rotation around the jumper's vertical axis produced by the drive leg (think of an ice vertical axis produced by the drive leg (think of an ice skater spinning round on the spot) the resulting body skater spinning round on the spot) the resulting body position on bar clearance is laid out supine with the position on bar clearance is laid out supine with the body at ninety degrees to the bar with the head and body at ninety degrees to the bar with the head and shoulders crossing the bar before the trunk and legs. shoulders crossing the bar before the trunk and legs.

This gives the Flop its characteristic This gives the Flop its characteristic "backwards over the bar" appearance, with the "backwards over the bar" appearance, with the athlete landing on the mat on their shoulders athlete landing on the mat on their shoulders and backand back

While in flight the athlete can progressively While in flight the athlete can progressively arch shoulders, back and legs in a rolling arch shoulders, back and legs in a rolling motion, keeping as much of the body as motion, keeping as much of the body as possible below the bar. possible below the bar.

It is possible for the athlete to clear the bar It is possible for the athlete to clear the bar while his or her body's center of mass remains while his or her body's center of mass remains as much as 20 cm below itas much as 20 cm below it

Fooling with Gravity – Fosbury FlopFooling with Gravity – Fosbury Flop

Straddle Technique

The Biomechanics of SwimmingThe Biomechanics of Swimming

Archimedes’ principleArchimedes’ principleForce act on body; Force act on body; buoyant forcebuoyant forceWoman has more float Woman has more float than manthan manHuman use limbs to Human use limbs to move in water but do move in water but do this much more this much more inefficiently than inefficiently than aquatic aminalsaquatic aminals

Top human swimming speed is about 2.3 m/s, and a barracuda can swim bursts of up to 12 m/s !

Lift in swimming is created Lift in swimming is created by the 3 dimensional by the 3 dimensional motion of the limbsmotion of the limbs

During the first part of the During the first part of the underwater arm stroke, the underwater arm stroke, the swimmer’s hand and swimmer’s hand and foreare are pressed foreare are pressed downward and slightly downward and slightly outwardoutward

Body is pushed forward by Body is pushed forward by the lift force createdthe lift force created

Four competitive swimming strokesFour competitive swimming strokes– Front crawl; most economic strokeFront crawl; most economic stroke

– Back crawlBack crawl

– Breast strokeBreast stroke

– ButterflyButterfly

Good swimmers use the front and back crawl, they Good swimmers use the front and back crawl, they rotate both sides of their body about 45° downward to rotate both sides of their body about 45° downward to the surface of the water with each stroke to maximizing the surface of the water with each stroke to maximizing shoulder muscle to generate leverage during the shoulder muscle to generate leverage during the propulsive phase of the strokepropulsive phase of the stroke

Moving large fin slowly is more efficient (e.g. dolphin)Moving large fin slowly is more efficient (e.g. dolphin)

CyclingCyclingBicycle consists of Bicycle consists of – A frame with a seatA frame with a seat– A steering mechanismA steering mechanism– Two wheels engaged to a chain and gearing Two wheels engaged to a chain and gearing

systems that transmit the rider’s effort from pedals systems that transmit the rider’s effort from pedals to the wheelsto the wheels

Human walking consumes 0.75 cal/kg/km that Human walking consumes 0.75 cal/kg/km that reduces to 0.15 cal/kg/km by cyclingreduces to 0.15 cal/kg/km by cyclingPedal transmit force through the crank and the Pedal transmit force through the crank and the pedal is located a distance from the crank axis pedal is located a distance from the crank axis of rotation; a torque results (force applied to of rotation; a torque results (force applied to pedal and length of crankshaft)pedal and length of crankshaft)

Mechanics of CyclingMechanics of Cycling

A greater torque applied over a large angular A greater torque applied over a large angular distance in a short time results in a high distance in a short time results in a high mechanical power; and chain and gearing mechanical power; and chain and gearing system transmit this mechanical power to the system transmit this mechanical power to the wheelwheel

The longer the pedal crank, the less optimal The longer the pedal crank, the less optimal position for legs generating force; pedal crank position for legs generating force; pedal crank lengths range from 16 to 17 cmlengths range from 16 to 17 cm

The effective components of the applied force The effective components of the applied force on a bicycle pedal is the force that on a bicycle pedal is the force that perpendicular to the crankperpendicular to the crank

Only the torque generated by the cyclist during Only the torque generated by the cyclist during the downward phase of the pedal stroke makes the downward phase of the pedal stroke makes wheel go around, wheel go around,

Recovery phase of the pedal stroke, the Recovery phase of the pedal stroke, the magnitude and direction of the force results in magnitude and direction of the force results in a small amount of torque productiona small amount of torque production

Varying pedal forces Varying pedal forces throughout a single throughout a single pedal cyclepedal cycle

The length of the arrows The length of the arrows indicate the amount of indicate the amount of force appliedforce applied

Pedaling CadencePedaling Cadence

Most recreational riders Most recreational riders pedals at 50 to 60 rpm in a pedals at 50 to 60 rpm in a high gearhigh gearRacing cyclists pedal about Racing cyclists pedal about 80 to 110 rpm in a middle 80 to 110 rpm in a middle geargearThe best pedaling rate= the The best pedaling rate= the most efficient most efficient cadence=produce the most cadence=produce the most work for the least amount of work for the least amount of efforteffortThe upright position results The upright position results in the greatest efficiency of in the greatest efficiency of the riderthe rider

Human-Powered vehicles Human-Powered vehicles

Human –powered Human –powered vehicles are forms of vehicles are forms of bicycle that have been bicycle that have been used to set distance and used to set distance and speed recordsspeed records

DiscussionDiscussionCan you try to explain what is the underlying Can you try to explain what is the underlying biomechanics of the “sinker” on baseball biomechanics of the “sinker” on baseball pitcher. pitcher.

From kinematic aspect, why the Fosbury flop From kinematic aspect, why the Fosbury flop dominates high jump competition instead of dominates high jump competition instead of the straddle technique.the straddle technique.

How do you manipulate a bicycle’s chain and How do you manipulate a bicycle’s chain and gearing system in a road racing period in order gearing system in a road racing period in order to transmit a maximal mechanical power to the to transmit a maximal mechanical power to the wheel. wheel.