Download Chapter 4 Mechanical Work

Chapter 4
Mechanical
Work-Capacity
Evaluation
Introduction
• How to evaluate (screen) an individual’s potential
ability to perform mechanical tasks
•
•
•
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reaching to an object
squatting
exerting manual forces on a control or object
lifting
pulling
pushing
Introduction
• Interaction with environment depends on two
biomechanical properties of the intact
musculoskeletal system:
• joint motion
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•
•
•
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joint mobility
dexterity
flexibility
limberness
Range of Motion (ROM)
Introduction
• Interaction with environment depends on
two biomechanical properties of the intact
musculoskeletal system:
• joint motion
• muscle strength
• ability of a muscle to produce force
• ability of a muscle group to produce force
Introduction
• Interaction with environment depends on
two biomechanical properties of the intact
musculoskeletal system:
• joint motion
• muscle strength
• Evaluate the Performance of the system
• Normative criteria to assess model
calculations
Planes of motion and axes of rotation
• Planes:
• Sagittal
• Frontal
• Transverse (horizontal)
• Axes:
• Medio-lateral (frontal)
• Anterior-posterior (sagittal)
• Vertical
Standard Terminology
Raise arms or Arms up????
Standard description
of joint motions
• note errors
• hip/shoulder abduction & adduction
• Know such terms (as if you don’t)
•
•
•
•
flexion & extension
internal rotation & external rotation
adduction & abduction
pronation & supination
Standard Terminology
Raise arms or Arms up????
Standard description
of joint motions
• note errors
• hip/shoulder abduction & adduction
• Know such terms (as if you don’t)
•
•
•
•
flexion & extension
internal rotation & external rotation
adduction & abduction
pronation & supination
Why is flexibility important?
Standard Terminology
Raise arms or Arms up????
Standard description
of joint motions
• note errors
• hip/shoulder abduction & adduction
• Know such terms (as if you don’t)
•
•
•
•
flexion & extension
internal rotation & external rotation
adduction & abduction
pronation & supination
Why is flexibility important?
1.
2.
3.
Allow for greater choice of movements
Improve performance. How?
Reduce the risk of injury. How?
Standard Terminology
Raise arms or Arms up????
Standard description
of joint motions
• note errors
• hip/shoulder abduction & adduction
• Know such terms (as if you don’t)
•
•
•
•
flexion & extension
internal rotation & external rotation
adduction & abduction
pronation & supination
Why is flexibility important?
How much do you need?
Standard Terminology
Raise arms or Arms up????
Standard description
of joint motions
• note errors
• hip/shoulder abduction & adduction
• Know such terms (as if you don’t)
•
•
•
•
flexion & extension
internal rotation & external rotation
adduction & abduction
pronation & supination
Why is flexibility important?
What do you want to do?
How much do you need?
“Normal” joint ROM
• Descriptive statistics of
ROM at various joints
• Population: young healthy
males
• Useful in work design???
• interaction with segment
lengths
• Relationship of linear &
angular motion
• S = θr
Factors affecting ROM
• Type of joint (shape of surfaces)
• injury, degenerative disease
• Tissue mass
• muscle & fat
• Tissue arrangement
• tendons & ligaments
• Clothing
• Past injury
• Temperature
• Age: confounded with disuse, misuse, injury?
• Sex: confounded with tissue mass?
Workplace considerations
• Individual
• joint motion reflects lifestyle
• “tightens up” with extreme postures
• joint motion reflects injury history
• Joints have a “memory”
• Environment
• can be accommodated for reduced ROM
• Task
• can be assigned according to ROM
Why is it important to understand and
evaluate strength in the workplace?
• Many jobs
require high
levels of exertion
• This has a direct
impact on safety
& performance
Boeing Example
Thanks to Rush Green,
Boeing Commercial Airplane Group
May, 1994
Muscle Strength
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•
•
•
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a muscle’s ability to generate force
Maximum voluntary exertion
Maximum voluntary contraction
Maximum voluntary strength
Affected by . . .
Muscle Strength
• a muscle’s ability to generate force
• Neural factors.
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•
•
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a. synchronization.
b. recruitment.
c. summation.
d. neural quality (aging factors)
trainable
Muscle Strength
• a muscle’s ability to generate force
• Neural factors.
• Psychological factors.
• a. motivation
• ie rehab: desire to return to work
• b. hormonal release
Muscle Strength
•
•
•
•
a muscle’s ability to generate force
Neural factors.
Psychological factors
Physiological factors.
• a. fatigue.
trainable & diet
• b. available energy
Muscle Strength
•
•
•
•
•
a muscle’s ability to generate force
Neural factors.
Psychological factors
Physiological factors
Mechanical factors.
• a. length-tension relationship
• b. force-velocity relationship
• c. angle (posture) ==> Torque
Torque (moment of force)
Tendency of a force to cause rotation
• Torque = Force x moment arm
• moment arm: perpendicular distance from
line of action of the force to axis of rotation
Torque (moment of force)
Tendency of a force to cause rotation
• Torque = F * MA
• MA varies through ROM
• Feel it with your fingers.
• Torque potential varies
throughout ROM
• change in moment arm
• change in muscle force
production (L/T)
Moment arms of the arm
From Houk et al, 2000
Ankle moment arms
Finger
flexion
effects on
tendons
Finger
flexion
effects:
tendon
force on
other
structures
Torque (moment of force)
Tendency of a force to cause rotation
• Torque = F * MA
• MA varies through ROM
• Feel it with your fingers.
• Torque potential varies
throughout ROM
• change in moment arm
• change in muscle force
production (L/T)
• Change in muscle force
(F/V)
Fuglevand, 1987
Reality of the
situation
• Not a single muscle, but the
muscle group
• ie elbow flexion strength
•
•
•
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biceps brachii
brachialis
brachioradialis
“wrist flexor muscles”
• Individual muscles (tension
and moment arms) interact
as a group to produce
torque at a joint
What we know about strength:
1. Maximum force
(torque) producing
capability varies
considerably
between people and
between tasks
Another way to look at muscle force
What we know about strength:
1. Maximum force
(torque) producing
capability varies
considerably between
people and between
tasks
• differences in training
level
• differences in age (aging
workforce; downsizing
of workforce (union
priority))
What we know about strength:
1. Maximum force (torque)
producing capability varies
considerably between
people and between tasks
• differences in training level
• differences in age (aging
workforce; downsizing of
workforce (union priority))
• differences in gender/
anthropometrics
• difference in motivation, etc
• strongest 6-8 times stronger
than weakest.
Additional gender issues with strength
• Difference in upper body
greater than lower body (?)
• smaller muscle moment
arms of average women?
• smaller muscle mass?
• social influence?
• Type of measurement?
• Differences almost entirely
explained by differences in
muscle size
What we know about strength:
• 1. Maximum force producing
capability varies considerably
between people and between tasks.
• 2. Static strength is not necessarily
correlated with dynamic strength
• static strength: measured
isometrically
• What position to measure in?
(position specific)
• dynamic strength: body segments
move
• which point to measure at?
• effect of technique
What we know about strength:
• 1. Maximum force producing capability
varies considerably between people and
between tasks.
• 2. Static strength is not necessarily
correlated with dynamic strength
• 3. Use of strength data in screening must be
done cautiously
• poor association with injury or performance
Psychophysical strength method for
screening employees
• Requires simulating specific task
• subjects are allowed to adjust the load
(unknown amount) after each attempted
performance.
• 30-45 minutes to simulate workday
• ends with subjective maximum
• involves cooperation and motivation to
obtain valid results.
Psychophysical Limits
• note voluntary decrease
in max with increased
dimensions
• note decrease with
increased height
• note female - male
difference
Workplace considerations
• Individual
• screen people for sufficient
strength to a task
(workplace rotation)
• load should not exceed
capacity of least capable
employee.
• Environment
• reduce package wt, alter
layout, new equipment ,
glove use
• Task
• redesign, additional
workers
"When I put a glove on my hand to protect me from the cold, I've now changed the grip
posture of my hand," says Budnick, who is also president of the Board of Certification in
Professional Ergonomics. "In most cases, I'm not getting good tactile feedback, so I don't
have the proper closed loop in my musculature system to know how hard I'm gripping. In
other words, I'm trying to grip as hard as I can, yet the actual result is probably lower grip
strength against whatever I'm grasping because of the intervening bulk of the glove. It
depends on the fit and type of the glove, but some research suggests that up to a 40 percent
reduction in grip strength is common from glove use, and as far as a risk factor for working
in cold environments, that's probably the biggest effect I see.”
Thesis Topic
• Individual
• screen people for sufficient
strength to a task
"When I put a glove on my hand to protect me from the cold,
I've now changed the grip posture of my hand," says
(workplace rotation)
who is also president of the Board of Certification
• load should not exceed Budnick,
in Professional Ergonomics. "In most cases, I'm not getting
capacity of least capable good tactile feedback, so I don't have the proper closed loop
employee.
in my musculature system to know how hard I'm gripping. In
• Environment
• reduce package wt, alter
layout, new equipment ,
glove use
• Task
• redesign, additional
workers
other words, I'm trying to grip as hard as I can, yet the actual
result is probably lower grip strength against whatever I'm
grasping because of the intervening bulk of the glove. It
depends on the fit and type of the glove, but some research
suggests that up to a 40 percent reduction in grip strength is
common from glove use, and as far as a risk factor for
working in cold environments, that's probably the biggest
effect I see.”
The Complexities of Cold
By Ronnie Rittenberry · February 1, 2008
http://www.ohsonline.com/articles/57758/