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HUMAN FACTORS ENGINEERING – IEE437/IEE547
Reading Guide for Chapter 11*
Biomechanics
Readings: pages 269-274 and 286-296. This looks like a lot to memorize. You only have to
memorize the major points and understand the rest.
1. In chapter 10, we discussed the importance of ensuring the fit between the physical
dimensions of the products and workplaces and the physical dimensions of the users. A
mismatch can cause the worker to assume awkward postures or to not be able to use the
product at all.
2. In chapter 11, we look at the mechanical forces exerted by a worker in performing a task
such as lifting a load or using a hand tool. The two most prevalent musculoskeletal injuries
are low back pain and upper-extremity (UE) cumulative trauma disorders. UE involves the
fingers, hands, wrists, arms, and shoulders.
3. Occupational biomechanics plays a major role in studying and analyzing human performance
and musculoskeletal problems. It forms the fundamental scientific basis for ergonomic
analysis of physical work.
4. Human musculoskeletal system is a mechanical system that obeys the laws of physics, i.e.
Newton’s Three Laws.
a. A mass remains in uniform motion or at rest until acted on by an unbalanced
external force.
b. Force is proportional to the acceleration of a mass.
c. Any action is opposed by reaction of equal magnitude.
The Musculoskeletal System
5. Bones, muscles, and connective tissues, which include ligaments, tendons, fascia, and
cartilage.
a. The main functions of the musculoskeletal system are to support and protect the
body and body parts, to maintain posture and produce body movement, and to
generate heat and maintain body temperature.
b. Bones form the rigid skeletal structure.
i. Two or more bones are linked with each other at joints (3 types)
 Synovial joints
(a) Most joints are in this category.
(b) No tissue exists between the two bones.
(c) The joint is highly lubricated with synovial fluid.
 Other two types are fibrous joints (e.g. bones of the skull).
ii. Joints can also be classified by range of motion: no-mobility joints (e.g.
seams of the skull), hinge joints (e.g. elbow), pivot joint (e.g. wrist), and
ball-and-socket joint (e.g. hip and shoulder).
c. Connective tissue
i. Tendons are dense fibrous connective tissues that attach muscles to bones
and transmit the forces exerted by the muscles to the attached bones.
*Notes (C) 2005 by Rebecca W. Boren from Wickens, C.D., Lee, J., Liu, Y., and Gordon-Becker, S.E.
(2003) An Introduction to Human Factors Engineering ( 2nd ed.) Upper Saddle Hill, N.J.: Prentice Hall.
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ii. Ligaments are also dense fibrous tissues, but their function is to connect the
articular extremities of bones and help stabilize the articulations of bones
at joints.
iii. Cartilage is a translucent elastic tissue that can be found on some articular
bony surfaces and in some organs such as the nose and the ear.
iv. Fascia covers body structures and separates them from each other.
6. Muscles
a. Muscles make up about 40-50% of body weight
b. Muscles consume almost half of the body’s metabolism, which supplies the energy
for maintaining body posture , producing body motion, generating heat, and
maintaining body temperature.
c. Composed of bundles of muscle fibers, connective tissue, and nerves.
d. Connective tissue provides a channel through which nerves and blood vessels enter
and leave the muscle.
e. Muscles produce body movement, generate body heat, and maintain body
temperature.
f. Muscle activities are regulated by motor nerve fibers, which transmit impulses from
the central nervous system to the muscles. Each motor nerve fiber regulates a group
of related muscles fibers. A group of muscle fibers regulated by the branches of the
same motor nerve is called a motor unit, which is the basic functional unit of the
muscle. Function as all or nothing, on or off.
7. Muscles contract concentrically, eccentrically, and isometrically in response to motor nerve
impulses.
a. Concentric contraction (isotonic) the muscle shortens while contracting and
produces constant muscle force. (Arm flexor muscles when an object is lifted
upward.)
b. Eccentric contraction is one in which the muscle lengths while contracting, which
occurs when the external force is greater than the internal muscle force. When a
person picks up a heavy object and is unable to hold it in the desired position.
c. Isometric contraction produces muscle force or tension which is transmitted to
bones through tendons and is used to maintain body posture and perform physical
work.
8. No device to measure tension within the muscle directly. We can measure the amount of
force or torque it exerts. We can measure electrical activity of the muscles
(electromyography or EMG).
9. In performing physical work, excessive loading can cause musculoskeletal problems such as
bone fracture and muscle fatigue.
Biomechanical Models
10. Based on the Newton's three laws.
11. Mathematical models represent the mechanical properties of the human body.
12. The musculoskeletal system is analyzed as a system of mechanical links; and, the bones and
muscles act as a series of levers.
*Notes (C) 2005 by Rebecca W. Boren from Wickens, C.D., Lee, J., Liu, Y., and Gordon-Becker, S.E.
(2003) An Introduction to Human Factors Engineering ( 2nd ed.) Upper Saddle Hill, N.J.: Prentice Hall.
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13. Allows one to predict the stress levels on specific musculoskeletal components
quantitatively.
14. Serve as an analytical tool to help job designers identify and avoid hazardous job situations.
Low-back Problems
15. One of the most costly and prevalent work-related musculoskeletal disorder in industry.
16. Accounts for one-third of all workers' compensation payments.
17. Prevalent in two common situations
a. Manual material handling
b. Sedentary work environments requiring a prolonged static sitting posture.
18. NIOSH lifting guide – just know that there is a formula to calculate acceptable lifting loads.
Seated work and chair design
19. Sitting is preferable to standing for reasons mentioned in chapter 10.
20. A new set of problems comes with sitting.
a. Low-back pain
b. Designers must pay particular attention to three areas.
i. Backrest inclination angle (1100 is preferred over 900 because it retains
the body’s natural state of lordosis).
ii. Lumbar support should be adjustable
iii. Arm rests support the body weight and reduce the load on the spine.
c. Other factors can help reduce back stress, but are not as critical (tiltable seat surface,
properly adjusted seat height, use of cushioned seat surfaces, and adequate leg
space)
d. Sloughing is OUT. It causes kyphosis (the opposite of lordosis).
e. Workers must take frequent breaks.
Upper-extremity cumulative trauma disorders (CDTs)
21. Also called repetitive motion injuries or repetitive strain injuries (RSIs)
a. Can be even more costly than low-back problems.
b. Common forms of CTD
i. Tendon-related CTD: inflammation of the tendons
ii. Neuritis (nerve pain caused by nerves rubbing against bones): tingling
and numbness
iii. Ischemia (obstruction of the blood flow to a body part, e.g. pressure
forces from the handle of a hand tool): tingling and numbness.
iv. CDT’s of the fingers: caused by vibrating hand tools; can cause
numbness, tingling, or pain when the hands are exposed to cold. Also
trigger finger and gamekeeper’s thumb
v. CDTs of the hand and wrist (carpal tunnel syndrome): conventional
keyboards bend the wrist and narrow the carpal tunnel)
vi. CDTs of the elbow: tennis or golfer’s elbow.
*Notes (C) 2005 by Rebecca W. Boren from Wickens, C.D., Lee, J., Liu, Y., and Gordon-Becker, S.E.
(2003) An Introduction to Human Factors Engineering ( 2nd ed.) Upper Saddle Hill, N.J.: Prentice Hall.
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Hand-tool design
a. Keep a neutral position
b. Do not bend at the wrist
c. Shape tool handles to assist grip
d. Provide adequate grip span
e. Provide finger and gloves clearance
*Notes (C) 2005 by Rebecca W. Boren from Wickens, C.D., Lee, J., Liu, Y., and Gordon-Becker, S.E.
(2003) An Introduction to Human Factors Engineering ( 2nd ed.) Upper Saddle Hill, N.J.: Prentice Hall.
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