1. Kinetic Concepts for Analyzing Human Motion
Chapter 3
Kinetic Concepts for Analyzing Human Motion
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.
© 2012 The McGraw-Hill Companies, Inc. All rights reserved.
McGraw-Hill/Irwin

2. Laws of motion Newton’s first law (law of inertia)
Body in motion tends to stay in motion at the same speed in the same direction; body at rest tends to stay at rest unless acted upon by an outside force.
What effect does gravity have on this?

3. Types of Motion
Linear (translation)
Curvilinear
Angular (rotary)

4. Second law of motion
Law of acceleration: change in acceleration of a body occurs in the same direction as the force that caused it, and is directly proportional to the force causing, and inversely proportional to the mass of the body.
Acceleration: change in rate of motion.
Can be + or -.
Mass: quantity of matter in a body

5. Third law of motion
Law of reaction: for every action, there is an equal and opposite reaction.
This allows propulsion.
Friction: force resulting from resistance between two objects.
Sometimes we want to increase this to enhance performance; sometimes we want to decrease it.

6. Basic Biomechanics, 6th edition
Basic Concepts Related to Kinetics
What is mass?
quantity of matter composing a body (dog, tree, desk, swimming pool, you)
represented by m
units are kg
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

7. Basic Biomechanics, 6th edition
Basic Concepts Related to Kinetics
What is inertia?
tendency to resist change in state of motion
proportional to mass
has no units!
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

8. Basic Biomechanics, 6th edition
Basic Concepts Related to Kinetics
Clearly, the weight bar will stay in place in the absence of being lifted because of it’s inertia.
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

9. Basic Biomechanics, 6th edition
Basic Concepts Related to Kinetics
What is force?
a push or a pull
characterized by magnitude, direction, and point of application
F = ma
unit is the Newton (N)
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

10. Force Force: a push or pull in a direction. F = m x a
Kinetic energy:1/2 mass x velocity²

11. Basic Concepts Related to Kinetics
What is a free body diagram?
ball weight
Force applied by racquet
Air resistance
Ball being struck by a racquet
(diagram showing vector representations of all forces acting on a defined system)
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

12. Basic Biomechanics, 6th edition
Basic Concepts Related to Kinetics
What is a net force?
the single resultant force derived from the vector composition of all the acting forces
the force that determines the net effect of all acting forces on a body
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

13. Basic Biomechanics, 6th edition
Basic Concepts Related to Kinetics
What is a torque (T)?
the rotary effect of a force
the angular equivalent of force
also known as moment of force
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

14. Basic Biomechanics, 6th edition
Basic Concepts Related to Kinetics
What is a torque?
T = Fd
T = (10N)(2m)
T = 20 Nm
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)
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

15. Basic Biomechanics, 6th edition
Basic Concepts Related to Kinetics
What is the center of gravity?
point around which a body’s weight is equally balanced in all directions
point that serves as an index of total body motion
point at which the weight vector acts
same as the center of mass
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

16. Center of Gravity
Point at which all body’s mass and weight is evenly distributed in all directions.
Center of gravity is dependent upon:
Body shape (anthropometrics, gender)
Body position
Base of support

17. Principles of balance and stability A person has balance …
When the c/g falls within the base
In direct proportion to size of the base
Depending on weight (mass).
Depending on height of c/g
Depending on how close c/g is to edge of base.
When c/g is nearer the center of the base.

18. Principles of balance and stability
To meet an oncoming force, stability is increased by:
moving c/g toward the force.
Enlarging base on the side of the force
Equilibrium enhanced by increasing friction between body and surface it contacts.

19. More principles of balance and stability
Rotation about an axis increases stability.
Locomotion (walking) is created by dislocating the c/g from the base and then reestablishing it within the base.
Movement in a direction may be enhanced by moving the c/g within the base in that direction.

20. Basic Biomechanics, 6th edition
Basic Concepts Related to Kinetics
20N
10N 1m 2m CG
The weights are balanced, creating equal torques on either side of the fulcrum.
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

21. Basic Biomechanics, 6th edition
Basic Concepts Related to Kinetics
What is weight?
attractive force that the earth exerts on a body
wt. = mag (product of mass and the acceleration of gravity: m/s2)
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

22. Basic Biomechanics, 6th edition
Basic Concepts Related to Kinetics
What is weight?
the point of application of the weight
force is a body’s center of gravity
since weight is a force, units of weight are units of force: N
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

23. Basic Biomechanics, 6th edition
Basic Concepts Related to Kinetics
What is pressure?
force per unit of area over which the force acts
commonly used to describe force distribution within a fluid (e.g. blood pressure, water pressure)
units are N/m2
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

24. Basic Biomechanics, 6th edition
Basic Concepts Related to Kinetics
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
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

25. Pressure and Stress Pressure = force ÷ area
The greater the area over which a force is distributed, the lower the pressure (amount of stress).
Muscles and bones adapt to stress by hypertrophy.

26. Basic Biomechanics, 6th edition
Basic Concepts Related to Kinetics
What is stress?
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

27. Bone adaptations to stress
Hypertrophy: bones increase in density when subjected to continuous stress.
Bone atrophy and osteoporosis
Bony insertion points for tendon attachments (muscles) also increase in size to accommodate for increased tensile stress.

28. Tendon and ligament responses to stress
Tendons and ligaments increase in size and strength to accommodate for increased tensile stress.

29. Basic Biomechanics, 6th edition
Basic Concepts Related to Kinetics
What is volume?
space occupied by a body
has three dimensions (width, height, and depth)
units are m3 and cm3
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

30. Basic Biomechanics, 6th edition
Basic Concepts Related to Kinetics
What is density?
mass per unit of volume
represented with the small Greek letter rho: 
units are kg/m3
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

31. Basic Biomechanics, 6th edition
Basic Concepts Related to Kinetics
What is specific weight?
weight per unit of volume
represented with the Greek letter gamma: 
units are N/m3
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

32. Basic Biomechanics, 6th edition
Basic Concepts Related to Kinetics
What is impulse?
the product of force and the time over which the force acts (Ft)
units are Ns
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

33. Mechanical loading Types of forces: Tension Compression Shear Bending
Muscles pulling on bone.
Compression
Gravity compressing spine or long bones.
Shear
Getting hit in the side of the knee.
Bending
Greenstick fractures
Torsion
Twisting knee

34. Basic Biomechanics, 6th edition
Basic Concepts Related to Kinetics
What is compression?
Original
Shape
Compression
(pressing or squeezing force directed axially through a body)
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

35. Basic Biomechanics, 6th edition
Basic Concepts Related to Kinetics
What is tension?
Original
Shape
Tension
(pulling or stretching force directed axially through a body)
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

36. Basic Biomechanics, 6th edition
Basic Concepts Related to Kinetics
What is shear?
Original
Shape
Shear
(force directed parallel to a surface)
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

37. Basic Biomechanics, 6th edition
Basic Concepts Related to Kinetics
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)
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

38. Basic Biomechanics, 6th edition
Basic Concepts Related to Kinetics
What is torsion?
Neutral
axis
(load producing twisting of a body around its longitudinal axis)
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

39. Basic Biomechanics, 6th edition
Basic Concepts Related to Kinetics
What is deformation?
Deformation
Plastic
Region
Ultimate
Failure
Point
Yield
Elastic
Load
(change in shape)
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

40. Basic Biomechanics, 6th edition
Basic Concepts Related to Kinetics
What are repetitive and acute loading?
repetitive: repeated application of a subacute load that is usually of relatively low magnitude
acute: application of a single force of sufficient magnitude to cause injury to a biological tissue
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

41. Basic Biomechanics, 6th edition
Basic Concepts Related to Kinetics
Repetitive vs. acute loading
Likelihood of Injury
Frequency of Loading
Load Magnitude
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

42. Basic Biomechanics, 6th edition
Vector Algebra
What is vector composition?
(process of determining a single vector from two or more vectors by vector addition)
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.

43. Basic Biomechanics, 6th edition
Vector Algebra
Vectors may be resolved into perpendicular components. The vector composition of each pair of components yields the original vector.
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.