1. Kinetic Concepts for Analyzing Human Motion
Chapter 3
Kinetic Concepts for Analyzing Human Motion
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.

2. 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.

3. 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.
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).

4. 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).

5. 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.

6. 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.

7. 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.

8. 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

9. 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.

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

11. 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.

12. What is the FBD of the following activities?
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

13. 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.

14. 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/s a = 0 m/s2
Stay at rest Stay in motion
(same speed
and direction)

15. Newton’s Second Law Forces are unbalanced
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.
In terms of an equation, the net force is equated to the product of the mass times the acceleration
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

16. 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.

17. What is a net force?
Vector sum of all acting forces
When all forces are balance, cancel out each other – net force – 0. Newton 1st law.
When a net force is present – Newton 2nd law.

18. ANGULAR KINETICS
Concentric And 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.

19. ANGULAR KINETICS What is a torque?
The rotary effect on a body as a result of an eccentric force or a couple.

20. What is a torque?
T = Fd
T = (10N)(2m)
T = 20 Nm
axis
d = 2m
F = 10N
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. Any different between moment and torque?

21. Between torque and moment?
Open the door ? Torque.
Bend the board with fix end using body weight? Moment.
Torque associated 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.

22. 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)
a : angular acceleration a : linear acceleration.

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

24. 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

25. 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.

26. 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.

27. MECHANICS OF DEFORMABLE BODY

28. 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.

29. 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.

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

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

32. (force directed parallel to a surface)
What is shear?
Original
Shape
Shear
(force directed parallel to a surface)

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

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

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

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

37. 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.
Deformation
Plastic
Region
Ultimate
Failure
Point
Yield
Elastic
Load

38. 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.

39. 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

40. 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

41. 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

42. 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

43. Repetitive vs. acute loading – finish on 08/01/08
Likelihood of Injury
Frequency of Loading
Load Magnitude

44. 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.

45. 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.

46. PRESENTATION FOR NEXT TUTORIAL
In a group of five write a 5 minuts presentations of currently available equipments for measuring kinetics quantity.
Also write the task of each group members to do the findings and presentation.
Please include the following items:
Name, types and classification (if available)
Each major component in the equipment.
How it is used? Purpose or objectives?
Advantages and disadvantageous of the system describe