Mechanical WorkCapacity Evaluation

1
Chapter 4

• Mechanical Work-Capacity Evaluation

2
Introduction

• How to evaluate (screen) an individuals
  potential ability to perform mechanical tasks
• reaching to an object
• squatting
• exerting manual forces on a control or object
• lifting
• pulling
• pushing

3
Introduction

• Interaction with environment depends on two
  biomechanical properties of the intact
  musculoskeletal system
• joint motion
• joint mobility
• dexterity
• flexibility
• limberness
• Range of Motion (ROM)

4
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

5
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

6
Planes of motion and axes of rotation

• Planes
• Sagittal
• Frontal
• Transverse (horizontal)
• Axes
• Medio-lateral (frontal)
• Anterior-posterior (sagittal)
• Vertical

7
Standard Terminology

• Raise arms or Arms up????
• Standard description
  of joint motions
• note errors
• hip/shoulder abduction adduction
• Know such terms (as if you dont)
• flexion extension
• internal rotation external rotation
• adduction abduction
• pronation supination

Why is flexibility important? How much do you
need?
8
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 ?r

9
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?

10
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

11
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

12
Boeing Example
Thanks to Rush Green, Boeing Commercial Airplane
Group May, 1994
13
Muscle Strength

• a muscles ability to generate force
• Maximum voluntary exertion
• Maximum voluntary contraction
• Maximum voluntary strength
• Affected by . . .

14
Muscle Strength

• a muscles ability to generate force
• Neural factors.
• a. synchronization.
• b. recruitment.
• c. summation.
• d. neural quality (aging factors)

trainable
15
Muscle Strength

• a muscles ability to generate force
• Neural factors.
• Psychological factors.
• a. motivation
• ie rehab desire to return to work
• b. hormonal release

16
Muscle Strength

• a muscles ability to generate force
• Neural factors.
• Psychological factors
• Physiological factors.
• a. fatigue.
• b. available energy

trainable diet
17
Muscle Strength

• a muscles ability to generate force
• Neural factors.
• Psychological factors
• Physiological factors
• Mechanical factors.
• a. length-tension relationship
• b. force-velocity relationship
• c. angle (posture) gt Torque

18
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

19
Another way to look at muscle force
20
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)

21
Moment arms of the arm
From Houk et al, 2000
22
Ankle moment arms
23
Finger flexion effects on tendons
24
Finger flexion effects tendon force on other
structures
25
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
26
Reality of the situation

• Not a single muscle, but the muscle group
• ie elbow flexion strength
• biceps brachii
• brachialis
• brachioradialis
• wrist flexor muscles
• Individual muscles (tension and moment arms)
  interact as a group to produce torque at a joint

27
What we know about strength

• 1. Maximum force (torque) producing capability
  varies considerably between people and between
  tasks

28
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))

29
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.

30
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

31
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

32
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

33
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.

34
Psychophysical Limits

• note voluntary decrease in max with increased
  dimensions
• note decrease with increased height
• note female - male difference

35
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
• Task
• redesign, additional workers