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KINETIC CONCEPTS FOR ANALYZING HUMAN MOTION

This article discusses the different types of mechanical loads acting on the human body and explains their effects. It also explores the instrumentation used for measuring kinetic quantities. Additional topics covered include the concepts of mass, inertia, and force.

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KINETIC CONCEPTS FOR ANALYZING HUMAN MOTION

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  1. ENT 214 Biomechanics KINETIC CONCEPTS FOR ANALYZING HUMAN MOTION Mohd Yusof Baharuddin BBiomedEng(UM) MBiomedEng(Melbourne) myusof@unimap.edu.my Picture from Jim M Goldstein

  2. Identify and describe the different types of mechanical loads that act on the human body.Explain the effects of loading on human body. Identify and describe the uses of available instrumentation for measuring kinetic quantities. Objectives

  3. What are : Mass1, inertia2, force3? • Quantity of matter composing a body • Tendency to resist change in state of motion, proportional to mass, has no units! • A push or a pull acting on a body and characterized by magnitude, direction, and point of application.

  4. Classification of forces • External and Internal Forces • Internal forces : Forces acting within object (system) being considered. • External forces (contact force or distant force): Forces acting on an object (system) as a result of its interaction with the environment.

  5. Classification of forces • Contact force - Normal and Tangential Forces • If a force acting on a surface is applied in a direction perpendicular (normal) to that surface, then the force is called a normal force (Figure 3.8). • A tangential force is that applied on a surface in the direction parallel to the surface (Figure 3.9).

  6. Classification of forces • Tensile and Compressive Forces • A tensile force applied to a body will tend to stretch or elongate the body Figure 3.10 (a). • A compressive force will tend to shrink the body in the direction that it is applied Figure 3.10 (b).

  7. Classification of forces Coplanar Forces A system of forces is said to be coplanar if all the forces are acting on a two-dimensional (plane) surface Collinear Forces • A system of forces is collinear if all the forces have a common line of action. • To determine the magnitude of the resultant force vector for a collinear force system, all we have to do is arithmetically add and/or subtract the magnitude of forces forming the force system.

  8. Classification of forces Concurrent Forces • A system of forces is concurrent if the lines of action of the forces have a common point of intersection. • Figure 3.12 (traction device) • Resolve the forces into their components along the horizontal and vertical directions and subsequently determine the force resultant, and undertake an in-depth analysis of the entire system.

  9. Classification of forces Parallel Forces • A set of forces form a parallel force system if the lines of action of the forces are parallel to each other. • Ex: Figure 3.13 • The forces on the forearm are: • the weight of the object, • the weight of the arm itself, • the tensions in the biceps muscle, • and the joint reaction force at the elbow.

  10. HUMAN MOVEMENT • During motion, every contact of foot with the floor or ground generates an upward reaction force. F=mg - the vector that represents the force of gravity acting on the object. F=ma - the vector that represents the instantaneous inertial force acting on the object. In this diagram, inertial forces accelerate the body toward downward and to the right. Vector (Fr) - the resultant or sum of the gravitational and inertial forces http://moon.ouhsc.edu/dthompso/gait/kinetics/GRFBKGND.HTM

  11. HUMAN MOVEMENT • Vector Fr represents a force that is opposed by a ground reaction force of equal magnitude. • Because the ground reaction force is equal and opposite, its vector's line of application is the same as that of Fr, and it has the same effect on the body and its joints. • The GRFV combines both gravity's effect on the body and the effects of the body's movement and acceleration (change of velocity) in three planes of reference. • This makes the GRFV especially suitable for the study of gait, during which the body's various masses undergo complex accelerations. http://moon.ouhsc.edu/dthompso/gait/kinetics/GRFBKGND.HTM

  12. Force applied by racquet Ball being struck by a racquet Air resistance ball weight What is a free body diagram? A diagram showing vector representations of all forces acting on a defined system.

  13. What is A Free Body Diagram? • Free-body diagram are constructed to help identify the forces and moments acting on individual parts of a system and to ensure the correct use of the equations of statics. In a free-body diagram, all known and unknown forces and moments are shown. • A force is unknown if its magnitude or direction is not known. For the known forces, the correct direction is indicated. • If the direction of a force is not known, a direction is chose for it. If this initial guess is not correct, it will appear in solutions as negative force.

  14. Consider a person trying to push a block to the right on a rough surface as illustrated in figure 5.3 • There are three parts constituting this system: • The person • The block • The horizontal surface representing the earth What is the FBD of the following activities?

  15. Figure 5.4 shows the free-body diagram of the system. • F = magnitude of the horizontal force F applied by the person on the block. • F also is the magnitude of the force applied by the block on the person (Newton’s third law) • W1 = weight of the block • W2 = weights of the person • N1 = magnitude of force on the horizontal surface applied by the block • N2 = magnitude of force on the horizontal surface applied by the person • f1 and f2 are the magnitudes of frictional forces f1 and f2 • between the block, the person and the horizontal surface.

  16. Newton’s First Law States that a body that is originally at rest will remain at rest, or a body in motion will move in a straight line with constant velocity, if the net force acting upon it is zero. i.e F=0 Forces are balanced Objects at Rest Objects in Motion (v = 0 m/s) (v  0 m/s) a = 0 m/s2 a = 0 m/s2 Stay at rest Stay in motion (same speed and direction)

  17. Newton’s Second Law States that a body with a net force acting on it will accelerate in the direction of that force, and that the magnitude of the acceleration will be proportional to the magnitude of the net force. Fnet = m*a Forces are unbalanced There is an acceleration The acceleration depends The acceleration depends directly upon the “net force” inversely upon the object’s mass

  18. Newton’s Third Law • States that to every action there is always an equal reaction, and that the forces of action and reaction between interacting bodies are equal in magnitude, opposite in direction, and have the same line of action.

  19. What is a net force? • Vector sum of all acting forces • Single resultant force derived from the vector composition of all the acting forces • Force that determines the net effect of all acting forces on a body

  20. ANGULAR KINETICS Concentric & Eccentric Forces Eccentric forces – does not pass through the centre of the body, body experience translation and rotation at the same time. Coupling Effect – 2 eccentric forces of equal magnitude act at both sides of a stick, body rotate without any translation.

  21. What is a TORQUE? • the rotary effect of a force • the angular equivalent of force • also known as moment of force

  22. F = 10N d = 2m axis T = Fd (the product of force and the perpendicular distance from the force’s line of action to the axis of rotation) Also known as moment of force. T = Fd T = (10N)(2m) T = 20 Nm What is a torque?

  23. Between torque and moment? Open the door ? Torque. Bend the board with fix end using body weight? Moment. Torqueassociated with rotational and twisting action of applied force, whereas Moment is related to the force bending effect. However, their mathematical definition is the same. Therefore it is sufficient to use moment to describe both parameters.

  24. What is moment of Inertia? • A quantity that describes a body’s reluctance to change its state of angular motion. The moment of inertia must be specified with respect to a chosen axis of rotation. • For a point mass the moment of inertia is just the mass times the square of perpendicular distance to the rotation axis, I = mr2. It is analog of mass for linear motion. M=Iα F=ma M: moment; F : Force I : moment of inertia m: mass (linear inertia) (rotational inertia) α : angular acceleration a : linear acceleration.

  25. What is the center of mass? x1 d: density; V: volume Mass is concentrated on particular point

  26. Sit up, torque and centre of mass • Extending the arms and keep them behind the head. The trunk centre of mass is far from the hip. • Extending the arms and keep them by the side of the body. The trunk centre of mass is closer to the head. • Sufficient muscular contraction to produce muscle torque to overcome resistive torque due to trunk weight

  27. What is impulse? The resulting motion is not only depend on force (magnitude, direction) also the duration of force application. Impulse = Ft (Ns) A large impulse may resulted from a small force acting for a relatively long time or from large force acting for a relatively short time.

  28. Momentum • A vector, mass (m) multiplied by velocity (v), p =mv. • Newton 1st law state that p remain constant ( in the absence of external forces), conservation of momentum. • Newton 2nd law can be express in terms of momentum as well, provided the mass is constant. • ma = mdv/dt = d/dt (mv) = dp/dt = F; which can be expressed as the rate of change of the momentum of a body equals the net external force acting on it.

  29. MECHANICS OF DEFORMABLE BODY

  30. What is stress? Force per unit of area over which the force acts. Commonly used to describe force distribution within a solid, units are N/m2. Related to deformable body.

  31. Between Pressure and Stress? • Stress is an internal resistance to external load where as pressure is external force applied upon the surface. • We know that every materials is elastic in nature , so when a force is applied it tends to deform and due to this deformation , there is an internal resistance produced which tends to spring back the molecules to their original position and this internal resistance per unit area is know as Stress. • Pressure is nothing but force acting on a unit area.

  32. Sample Problem 1 Is it better to be stepped on by a woman wearing a spike heel or a woman wearing sport shoe? If a women weight is 55 kg, the surface area of spike heel is 4 cm2 and the surface area of sport shoe is 175cm2, how much pressure exerted by each shoe?

  33. Compression Original Shape What is compression? pressing or squeezing force directed axially through a body

  34. Tension Original Shape What is tension? pulling or stretching force directed axially through a body

  35. Original Shape Shear What isshear? force directed parallel to a surface

  36. Compression Tension What is bending? asymmetric loading that produces tension on one side of a body’s longitudinal axis and compression on the other side

  37. Neutral axis What is torsion? Load producing twisting of a body around its longitudinal axis.

  38. Torsion Torsional fractures of Tibia are not uncommon in football injuries and skiing accidents – foot fix, body twist.

  39. Combined Loading • The presence of more than one form of loading – most common type of loading on the body.

  40. Sample Problem 2 How much compressive stress is present on the L1, L2 vertebral disk of a 55 kg woman, given that 45 % of body weight is supported by the disk a) when she stands in anatomical position? b) when she stands erect holding a 222 N suitcase? Assume that the disk is oriented horizontally and that its surface area is 20 cm2.

  41. Yield Ultimate Point Failure Load Point Elastic Plastic Region Region Deformation THE EFFECTS OF LOADING ON HUMAN BODY Force applied – acceleration and deformation. • The amount of deformation depends on the stiffness of the objects. What is stiffness? Steeper slope – high stifness. Small load - deform, elastic - get back to its original shape. If exceed elastic limit, deformation is permanent. Exceed ultimate failure point – fracture.

  42. INJURIES Several factors influencing injuries in human: • Magnitude and direction of forces • The area over the force is distributed. • The material properties of the loaded body tissues.

  43. Elastic versus viscoelastic behaviors • For a constant applied strain • An elastic material has a unique material response • A viscoelastic material has infinite material responses depending on the strain-rate

  44. VISCOELASTICS • Definition: time-dependent material behavior where the stress response of that material depends on both the strain applied and the strain rate at which it was applied! • Examples • biological materials • polymer plastics • metals at high temperatures

  45. What are acute loading? • When a single force of sufficient magnitude to cause injury acts on a biological tissue, the injury is term acute. • Microtrauma: causative force • Fall, rugby tackle, automobile accident

  46. What are repetitive loading? • A repeated application of relatively small forces, the injury is term as chronic injury or a stress injury. • Macrotrauma: causative force

  47. Likelihood of Injury Load Magnitude Frequency of Loading Repetitive vs. acute loading

  48. THE AVAILABLE INSTRUMENTATION FOR MEASURING KINETIC QUANTITIES Electromyography • Use to study neuromuscular functions • Two concepts: • Muscle developed tension when electrically stimulated • Muscle produce voltage when developing tension.

  49. THE AVAILABLE INSTRUMENTATION FOR MEASURING KINETIC QUANTITIES Dynamography Objectives: • Measurement of forces and pressure on the plantar surface of the foot. • Provide graphical time history of recorded force.

  50. SAMPLE PROBLEM 3 (VECTOR) Ali 350 N If the frictional force is 700N, can they move the refrigerator? 30o Refrigerator 700 N 20o Abu 400 N

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