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.

Chapter 3

Kinetic Concepts for Analyzing Human

Motion

1. Quantity of matter composing a body

2. Tendency to resist change in state of motion, proportional to mass, has no units!

3. A push or a pull acting on a body and characterized by magnitude, direction, and point of application.

What are : Mass1, inertia2, force3?

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).

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).

Coplanar ForcesA system of forces is said to be

coplanar if all the forces are acting on a two-dimensional (plane) surface

Collinear ForcesA 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.

Classification of forces

Concurrent ForcesA 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.

Classification of forces

Parallel ForcesA set of forces form a parallel force

system if the lines of action of the forces are parallel to each other.

Ex: Figure 3.13The 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.

Classification of forces

HUMAN MOVEMENTDuring 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

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

What is a free body diagram?

A diagram showing vector representations of all forces acting on a defined system.

ball weight

Force applied by racquet

Air resistance

Ball being struck by a

racquet

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.

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 5.3There are three parts constituting this system:

•The person•The block•The horizontal surface representing the earth

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.

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)

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.

• 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

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.

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.

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.

ANGULAR KINETICS

ANGULAR KINETICS

The rotary effect on a body as a result of an eccentric force or a couple.

What is a torque?

What is a torque?

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?

axisd = 2m

F = 10NT = Fd

T = (10N)(2m)

T = 20 Nm

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.

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=maM: moment; F : ForceI : moment of inertia m: mass (linear inertia)(rotational inertia)a : angular acceleration a : linear acceleration.

What is moment of Inertia?

What is the center of mass?

x1

n

1iimM iii Vdm d: density; V: volume

n

1iiVdM

i

n

1ii

i

n

1ii

xmM

1x

xmxM

Mass is concentrated on particular point

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

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.

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.

MECHANICS OF DEFORMABLE BODY

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.

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.

Between Pressure and Stress?

What is compression?

(pressing or squeezing force directed axially through a body)

OriginalShape Compression

What is tension?

(pulling or stretching force directed axially through a body)

OriginalShape Tension

What is shear?

(force directed parallel to a surface)

OriginalShape Shear

What is bending?

(asymmetric loading that produces tension on one side of a body’s longitudinal axis and compression on the other side)

Compression

Tension

What is torsion?

Load producing twisting of a body around its longitudinal axis.

Neutral

axis

Torsion?

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

Combined Loading

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

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.

Deformation

PlasticRegion

UltimateFailurePoint

YieldPoint

ElasticRegion

Loa

d

Small load - deform, elastic - get back to its original shape. If exceed elastic limit, deformation is permanent. Exceed ultimate failure point – fracture.

INJURIES

Several factors influencing injuries in human:

Magnitude and direction of forcesThe area over the force is distributed.The material properties of the loaded

body tissues.

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

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

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

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

Repetitive vs. acute loading – finish on 08/01/08

Likelihood of Injury

Frequency of Loading

Loa

d M

agn

itud

e

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.

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.

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