Understanding Human Motion Biomechanics

1. Biomechanics of Human Motion Introduction to Biomechanics

2. Background for the study of Biomechanics • As a new discipline but early root back to many centuries • Aristotle • Archimedes • Leonardo De Vinci • Alfonso Borelli • Eadweard Muybridge: to settle of hour runs with four legs off the group with pictures (fig.)

3. Eadweard Muybridge’s studies

4. The Analysis of Human Motion • Biomechanics is the science of physical principles applied to biology systems • Kinematics • A description of the temporal and spatial components of movement • Kinetics • A study of the forces acting on an object

5. Kinematic of Human Movement • Motion refers to an object changing its position in space over a period of time • In most of activities, success depends on getting an object to travel to a certain location as quickly as possible

6. Speed and Velocity • Speed refers to how fast an object travels • Velocity refers to how fast an object moves in a particular direction • Speed = d / t; where d=distance, t=time • Acceleration occurs when an object changes its velocity over a particular time interval • Acceleration= Vf -Vi / Δt ; read as acceleration change in velocity over a change in time

7. Newton’s Law of Motion • are three physical laws that form the basis for classical mechanics. They describe the relationship between the forces acting on a body and its motion due to those forces. • were first compiled by Sir Isaac Newton in his work Philosophiæ Naturalis Principia Mathematica, first published on July 5, 1687. Newton used them to explain and investigate the motion of many physical objects and systems.

8. Law of inertia: every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it • Law of acceleration: the relationship between an object's mass m, its acceleration a, and the applied force F is F = ma. Acceleration and force are vectors; in this law the direction of the force vector is the same as the direction of the acceleration vector. • Law of Action-Reaction: For every action there is an equal and opposite reaction

9. Relationship of Force and Motion • Using gravity, ground reaction forces, muscle forces, friction and other forces, will clarify the concept of force and show how it is related to the change in motion of an object

10. The impulse-Momentum relationship • Impulse = Ft = m(Vf -Vi); read the product of force and the duration of time that the force is applied • Momentum = mv ; represent a force applied to an object over a known amount of time causes a change in the momentum of that object • The motion of a body represented by its momentum, is changed by the impulse applied to the body, or force applied over time; Ft= Δmv

13. Gravity • Is a force that created as the attractive pull between any two objects • The size of the pull is affected by both the mass of the two objects and the distance between them • Center of gravity: the weigh concentrated at single point

14. 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. Over the next few years the flop became the dominant style of the event and remains so today

16. Contact Force • When two bodies physically touch each other, contact forces, contact force are created. • Ground reaction force (GRF) • The force exerted on the performer by the ground • GRF affects the vertical, anteroposterior, and mediolateral motion of the object • Important consideration in injury prevention • Demonstrating ground reaction force

19. Measuring Ground Reaction Force (GRF) • An instrument called a force platform is used to measure the magnitude and direction of the ground reaction • Typically this interfaced with a computer to record the GRF and facilitate analysis

20. Friction • is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction: • Dry friction resists relative lateral motion of two solid surfaces in contact. Dry friction is subdivided into static friction between non-moving surfaces, and kinetic friction between moving surfaces. • Fluid friction describes the friction between layers within a viscous fluid that are moving relative to each other. • Lubricated friction is a case of fluid friction where a fluid separates two solid surfaces. • Skin friction is a component of drag, the force resisting the motion of a solid body through a fluid. • Internal friction is the force resisting motion between the elements making up a solid material while it undergoes deformation

21. Friction

22. The relationship among maximum limiting friction, the nature of the surfaces in contact, and the normal force • Friction ≦ μN • Where • μ (Greek letter “mu”); coefficient of friction • N; normal force Friction Propelling force

23. Contact Surface as a Friction Element • Shoes’ sole match the conflicting needs for grip (increased friction) and slide (decreased friction) • The different patterns and materials used on this part of the sole • Rotation of a pivot • Hard and smooth in the ball and metatarsal area • Grip (turn, push off, stop) • Soft and ripple in the lateral side

24. For a classical ski that kick wax will be applied to the grip zone. • No kick wax in the glide zones and no glide wax in the grip zone • The high-friction wax increased friction is necessary to develop a forward propelling push during the kick • Low-friction wax is applied at both tips of the ski camber KICK ZONE

25. Fluid Force • Refers to the forces imposed on an object when it moves through a fluid such as air of water • Is created as the object disrupts the fluid when passing through it • The greater the disruption of the fluid the greater the fluid forces developed • How much the fluid gets disrupted depends on factors related to both the fluid and the object

26. Fluid factors • Influence the size of the force exerted on an object moving through it include • Density; the distribution of mass throughout a volume of space • Viscosity; a fluid’s resistance to flow • The more dense or more viscous a fluid is, the more it is disturbed as an object passes though it

27. Fluid Resistance • Body factors • Aerodynamic and hydrodynamic are used to describe the features of the object that affect the size and direction of the fluid resistance

28. Cross-section Area • A larger cross-sectional area increases the size of the fluid resistance force • Nature of the Surface • A smooth surface produces less disruption in the fluid as it flows over the body and reduces resistance, and roughened surface vice versa

29. Velocity • The faster velocity, the greater fluid resistance • This factor related to fluid resistance magnitude is very critical in the performance of many skills • Relative motion of the body and the fluid • With a tail wind, the relative motion of the air over the ball is reduced because the wind is pushing the air forward

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

31. Biomechanics of Muscle Force

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

33. 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)

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

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

36. Muscle produces the greatest force when there is optimal overlap thick and thin filament in the sarcomere (b and c) • No muscle force is produced at very long (a) or very short (e) muscle lengths. • Less force is produced at shorter and longer sacomere lengths (d)

37. Muscle length versus isometric force during active and passive isometric contraction

38. Who Use Biomechanics • Sport scientists • Improving wind resistance in skiing, cycling, and sailing • Physical therapists • Analyze gait defects • Muscle function • Ergonomist • Assembly line injury • Rehabilitation • Design new prosthetic devices • Artificial hearts

39. Summary Points

40. Discussion • What’s the simplistic way to explain baseball pitcher’s kinetics and kinematics. • How could you devise a test to quantify long distance runners’ running economy (movement efficiency) that has been considered as such an important factor in athletic performance. • How you scheme to apply a force plate to investigate the effects of balance (equilibrium) on athletic sport. • How you explain the pitcher’s forkball underlying Newton law of motion.