Special Topics in Biomechanics

1. Special Topics in Biomechanics

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

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

5. Reasons to analyze human gait • To improve understanding of how the loco-motor system operates • To improve activity that is normal or above average • To bring abnormal or suboptimal performance closer to normal standards

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

8. How to Throw a Curve Ball • The curveball is a type of pitch in baseball thrown with a characteristic grip and hand movement that imparts forward spin to the ball causing it to dive in a downward path as it approaches the plate. Its close relatives are the slider and the slurve. • The "curve" of the ball varies from pitcher to pitcher.

11. The pitcher places the middle finger on and parallel to one of the long seams, and the thumb just behind the seam on the opposite side of the ball such that if looking from the top down, the hand should form a "C shape" with the horseshoe pointing in towards the palm following the contour of the thumb. • The index finger is placed alongside the middle finger, and the other two extraneous fingers are folded in towards the palm with the knuckle of the ring finger touching the leather.

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

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

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

15. 一個球是沿著旋轉軸垂直方向的平移運動旋轉的誘導升力：一個球是沿著旋轉軸垂直方向的平移運動旋轉的誘導升力： • F = 升力lift force =流體的密度 v = 球的速度 A = 球的橫截面積 CL = 升力係數 • 升力係數CL可以從使用雷諾數和旋轉比率的實驗數據圖表確定。與光滑的球旋轉的比例為0.5到4.5，典型的升力係數的範圍從0.2至0.6.

16. Fooling with Gravity – The Fosbury Flop • is a style used in the athletics event of high jump. • It was popularized and perfected by American athlete Dick Fosbury, whose gold medal in the 1968 Summer Olympics brought it to the world's attention

17. This technique allows the center of gravity to be lowered even before knee flexion, giving a longer time period for the take-off thrust. • Additionally, on take-off the sudden move from inward lean outwards produces a rotation of the jumper's body along the axis of the bar, aiding clearance. • Combined with the rotation around the jumper's vertical axis produced by the drive leg (think of an ice skater spinning round on the spot) the resulting body position on bar clearance is laid out supine with the body at ninety degrees to the bar with the head and shoulders crossing the bar before the trunk and legs.

18. This gives the Flop its characteristic "backwards over the bar" appearance, with the athlete landing on the mat on their shoulders and back • While in flight the athlete can progressively arch shoulders, back and legs in a rolling motion, keeping as much of the body as possible below the bar. • It is possible for the athlete to clear the bar while his or her body's center of mass remains as much as 20 cm below it

19. Fooling with Gravity – Fosbury Flop Straddle Technique

20. The Biomechanics of Swimming • Archimedes’ principle • Force act on body; buoyant force • Woman has more float than man • Human use limbs to move in water but do this much more inefficiently than aquatic aminals

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

22. Lift in swimming is created by the 3 dimensional motion of the limbs • During the first part of the underwater arm stroke, the swimmer’s hand and foreare are pressed downward and slightly outward • Body is pushed forward by the lift force created

23. Four competitive swimming strokes • Front crawl; most economic stroke • Back crawl • Breast stroke • Butterfly • Good swimmers use the front and back crawl, they rotate both sides of their body about 45° downward to the surface of the water with each stroke to maximizing shoulder muscle to generate leverage during the propulsive phase of the stroke • Moving large fin slowly is more efficient (e.g. dolphin)

24. Cycling • Bicycle consists of • A frame with a seat • A steering mechanism • Two wheels engaged to a chain and gearing systems that transmit the rider’s effort from pedals to the wheels • Human walking consumes 0.75 cal/kg/km that reduces to 0.15 cal/kg/km by cycling • Pedal transmit force through the crank and the pedal is located a distance from the crank axis of rotation; a torque results (force applied to pedal and length of crankshaft)

25. Mechanics of Cycling • A greater torque applied over a large angular distance in a short time results in a high mechanical power; and chain and gearing system transmit this mechanical power to the wheel • The longer the pedal crank, the less optimal position for legs generating force; pedal crank lengths range from 16 to 17 cm

26. The effective components of the applied force on a bicycle pedal is the force that perpendicular to the crank • Only the torque generated by the cyclist during the downward phase of the pedal stroke makes wheel go around, • Recovery phase of the pedal stroke, the magnitude and direction of the force results in a small amount of torque production

27. Varying pedal forces throughout a single pedal cycle • The length of the arrows indicate the amount of force applied

28. Pedaling Cadence • Most recreational riders pedals at 50 to 60 rpm in a high gear • Racing cyclists pedal about 80 to 110 rpm in a middle gear • The best pedaling rate= the most efficient cadence=produce the most work for the least amount of effort • The upright position results in the greatest efficiency of the rider

29. Human-Powered vehicles • Human –powered vehicles are forms of bicycle that have been used to set distance and speed records

30. Discussion • Can you try to explain what is the underlying biomechanics of the “sinker” on baseball pitcher. • From kinematic aspect, why the Fosbury flop dominates high jump competition instead of the straddle technique. • How do you manipulate a bicycle’s chain and gearing system in a road racing period in order to transmit a maximal mechanical power to the wheel.