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OCR for page 68
5
General Discussion
CONCLUSIONS
As a result of the workshop, the members arrived at the
following three major conclusions.
1. There is a needfor an integrated model of the human body,
its performance characteristics and limitations, and its interactions
with technological systems. An integrated ergonomic model would
provide a valuable too! for the development of specifications for the
physical parameters of the work site based on the anthropometric,
biomechanical, and interface characteristics of the operators. A
valid mode} of the performance of people in technological systems
in the early conceptual and design stages could result in substantial
savings in terms of effort, time, and money. The development of
an integrated computer model that describes human traits and
limitations could prove useful to those who research basic human
qualities as well. Thus, the need has both theoretical and practical
implications.
2. The development of such a mode! appears to be feasible. Ad-
vances in research methods and instrumentation, many of which
are associated with the increasing sophistication in the use of
computerized systems, have made research feasible on the many
anthropometric, biomechanical, and interface details, as well as
their interactions regarding human performance capabilities and
limitations. The establishment of a standard protocol and nomen-
ciature is essential to the integration effort.
68
OCR for page 69
69
3. An integrated ergonomic mode! would be usefulfor guidance
for research, development, and engineering applications. While
current moclels indicate the usefulness of the approach, they also
reflect many of the shortcomings of the diverse approaches identi-
fied earlier. The approaches provide solutions to specific problems
but contribute little to a generaTizable model. Typical examples
in which an integrated ergonomic mode} would be very useful is
in applications to computer-aided design (CAD) and engineering,
which are fast evolving as major design tools.
Requirements
A study of the requirements for the development of a standard
ergonomic reference data system (SERDS) was prepared by the
National Bureau of Standards (Van Cott et al., 1978~. Although
this system was never implemented, the study provides informa-
tion relevant to the development of an integrated ergonomic model.
The following were some of the major findings of this study: (1) A
definitive survey of user needs and priorities is necessary in order
to define the scope of the system. (2) Standards must be devel-
oped for the definition of units, measures, measurement methods,
and data reporting. (3) An assessment of alternate technologies
for capturing, storing, and processing ergonomic data is needed
to identify a cost-effective approach. (4) Data derived from the
published ergonomics literature and from the national ergonorn~cs
survey (discussed in the SERDS report) must be evaluated criti-
cally.
A preliminary survey of potential users at that time identified
several important areas of needed research. These areas are shown
in Table 5-1.
Similar findings were identified at a conference on the the-
ory and application of anthropometry and biomechanics in 1980
(Emsterby et al., 1982) and at a conference on space workstation
human factors in 1982 (Montemerio and Cron, 1982~.
Criteria for the Development of an Integrated Ergonomic Mode!
Several general major criteria that should guide the develop
ment of an integrated ergonomic mode! require that the mode!
have the following characteristics (this list does not imply a rank-
ing by importance):
OCR for page 70
70
TABLE 5-1
Data Requirements of Potential SERDS Users
Area of Need Specific Date Needed Date Application
Static Basic human body Design of tools and other
anthropometry dimensions as function hard goods; development
of age/sex, etc. of clothing sising and
tariffs
Dynamic Bending and stooping Control location and
anthropometry capabilities; reach operation; workspace
dimensions design
Strength Static and dynamic force Equipment and job design -
characteristics measurements; lifting; for industrial workers
pushing and pulling product portability design
capabilities
Physiological Aerobic and anaerobic Environmental design, job
characteristics capacity; maximal heart specifications; toxicity
rate; expiratory volume levels
Sensory/perceptual Measures of visual and Design of controls, digital
processes auditory acuity, color displays; visual and
Derision auditory warning signals
Tolerance to Exposure tolerance to Protection of workers and
environmental physical and chemical environmental design
conditions agents, e.g., tolerance
to high-intensity light,
noise, temperature,
radiation
Reaction time Simple and complex Display-control
reaction time to a relationships; blade
variety of stimuli stopping time
Information Interpretation of symbols, Design of displays
processing/ learning processes, signals, instructional
cognitive functions memory materials, training
devices
Capabilities Anthropometric, sensory, Product and environmental
of special physiological measures design
populations of children, the aged,
the handicapped
SOURCE: Van Cott et al. (1978~.
OCR for page 71
71
be dynamic,
use a common notation system,
incorporate or simulate the real world,
have three-dimensional structure,
be predictive,
be capable of being validated,
be user-friendly,
be time- and cost-effective
be flexible,
permit rapid analysis,
permit on-line documentation,
be written in a standard language for transportability (use
on different systems),
have standardized segment and whole-body coordinate sys-
tems, and
;,
have graphical display capability.
Standardization
Standardization is a basic requirement for any kind of in-
tegrating ergonomic model. Such standardization is particularly
important in two key areas: common format of the input data and
standard language to make models and submodels compatible,
including their use on different computer systems.
If this is not the case, each model or module needs a "transla-
tor" that allows data exchange by software modulation. Graphics
input and output should be in accordance with the International
Graphics Exchange Standard Format (IGES). If certain assump-
tions are made for the various data bases of submodels, these must
be known to the user to make the system usable and reliable.
Given the conclusions that an integrated ergonomic model
and its submodels, that is, anthropometric, biomechanical, and
interface models, are needed, feasible, and useful, methods for
accomplishing the modeling goals need to be determined.
APPROACHES TO THE DEVELOPMENT 01? AN
INTEGRATED MODEL
Two approaches to integrated ergonomic modeling evolved
from the discussions at the workshop on which this report is based.
The first approach is to develop one "supermodel~ that integrates
OCR for page 72
72
the best qualities of all or most other models, while the second is
a "modular approach, which would incorporate various existing
models or submodels to develop an integrated ergonomic modeling
system. The pros and cons of each approach were reviewed.
Sup ermo de! Approach
Some current interface models such as PLAID-TEMPOS,
CAR, COMBIMAN, and Crew Chief appear to be moving in
the direction of a supermode! approach. These models have been
developed individually and, as a rule, are not compatible (e.g.,
there is no interface between COMBIMAN and SAMMIE). This
incompatibility is usually a result of different data formats, de--
grees of modeling complexity, technically different computers, and
different modeling theories or techniques. Furthermore, the data
and assumptions in data collection may not be appropriate for
specific models or sets of data.
These models are similar in many of the respects that meet
the criteria for integrated ergonomic models. An evaluation of
these existing models regarding their potential for integration into
a supermode! is needed.
Modular Approach
A modular approach to an integrated ergonomic modeling
system is a building block process of joining compatible modules
with a standard structure. This allows flexibility for the user to
incorporate those aspects of the ergonomic mode! or its component
modules into as simple or complex a system as desired. For exam-
ple, an engineer interested} in fixed base activities on the ground
might have no interest in a module that describes characteristics
of reduced gravity or a module that incorporates platform motions
and dynamics and their effects on the human operator.
The modular approach requires that the modules fit together.
They need to be designed and structured according to common
principles and nomenclature. A modular approach thus requires
a superstructure to make each module a true component of the
general system. Thus, even a modular approach constitutes one
form of an integrated ergonomic model.
A flow chart illustrating a process for integrated ergonomic
OCR for page 73
73
models was provided by Joe W. McDaniel (Figure 5-1~. It contains
the following features:
1. Data bases are structured in a standardized protocol so
that a new mode} could retrieve a data base or a subset of a
data base. This minimizes the number of data bases required to
be maintained on a system and makes it easier to update the
data bases. (Researchers and mode} users could develop data to
use with the mode} without having to be progr~runers or model
developers.)
2. Each mode} has a translator or data exchange standard on
the front end to access the required data. This feature would make
the mode] less system dependent.
3. Mode! users have a library of programs to use in their
specific designs that communicate indirectly through shared data
bases, permitting the use of smaller computers. This also prevents
obsolescence by allowing individual models or data bases to be ac-
quired, replaced, or upgraded individually and as needed. In this
concept the user can select items specific to a particular modeling
analysis. Data and computer graphics systems should be standard-
ized to allow interchange among systems. This approach is similar
in concept to the workspace design analysis system proposed by
Evans (1985~.
One approach to the development of an integrated workspace
design system based on the modular approach is shown in Figure
5-2. This system may be suitable to integrate operator analyses
with existing computer design models (Evans, 19853. The sys-
tem components, outlined in Table 5-2, provide a more detailed
explanation of the data requirements (Evans, 1985~.
DATA REQUIREMENTS
Users of anthropometric and biomechanical data have tended
to rely on existing data bases, adapting, inferring, and making
assumptions to meet their needs. For the most part, these are
limited to the measurements made on the male U.S. soldier. The
civilian anthropometric data base, particularly that for women, is
extremely weak. For other subsets of the population (e.g., the ag-
ing and the handicapped), the data are virtually nonexistent. Yet,
OCR for page 74
74
DATA I ~ ~ ~ | SOLUTION
L
RAW
DATA
PROCESSED
DATA
FUTURE
DATA
WORKPLACE
TRANSLATOR
DATA
EXCHANGE
STANDARD
MODEL A
B
C
USER
SYSTEM 1
USER 1 ~ IGES
WORK J ~
USER
SYSTEM 2
CAD
(a) - International Graphic Exchange Standard
or Data Excharl<3e Standard
1 -{is
GRAPHICS
PRINTOUT
FEASIBI LITY
(VALIDATION:
SENSITIVITY)
ANALYSIS J
FIGURE 5-1 Graphic display of an integrated ergonomic modeling system.
SOURCE: Joe McDaniel (unpublished data).
OCR for page 75
75
I N PUTS
Percent
, Population
Tool of Interest
Characteristics ~
/ /
Work -
Workspace
Data Structure,
Workspace
Geometry,
and Commands
DESIGNER 1~
~ OUTPUT{;
I _ . .
space
Dimensions
\ I
Ta k \
Demands \
\ \.
1
Reach
Envelop:
l
\
,
Interface Among Human Performance Models
and Workspace Geometry
Stren)\\
Profile \ \
Time \
Predi 7ions 1
/ Fatigue
Al lowances
-
Endurance
_ _
Methods
Analysis
Strength
Prediction
Reach
Analysis
Data Base Interface
,
~ t_
EXISTING
W -—
I\4etabo!;c NIT PI
T—
Opera tor
Graphic
FIGURE 5-2 System structure for an integrated computer-aided workspace
design system. SOURCE: Evans (1985~. Reproduced with permission.
OCR for page 76
76
TABLE 5-2
System Components for a CAD Approach
to the Design of Manual Workspaces
System Components
Implementation Approaches
User dialogue
interface
System modes (states)
Design modes
Operator performance
prediction models
Design data bases
System input and output
Menu-based command language
Display windows showing parameters and options
Default--task entry
Optione--workepace entry, object definition,
operator definition, task evaluation
Prelirn~nary design--pro~rides design guidelines
with incomplete workspace or task information
Single exertion, posture prediction
and biomechanical analysis
Repeated trials--biomechanical and physiological
effects on lifting
Time prediction based on MTM-2 get and place
elements
Static data bases--eystem files of generic
operator, workspace, or task data
Dynamic data bases--user defined files, which
vary with the application, and the stage of
design
System defaults for posture, gender, and
analysis modes
Task input syntax similar to current process
descriptions
Output in graphic format--workspace and operator
three-dimensional graphics; two-dimensional
graphs and charts of analysis results
Output formatted to comply with designer-mated
preferences
SOURCE: Evans (1985~.
these populations must be accommodated in the design of hun-
dreds of products and workspaces. For example, clothiers, pattern
makers, and other product designers have a need for anthropomet-
ric and biodynamic models.
It is difficult to determine whether data from different sources
are comparable. The names used for the same dimension can vary
from study to study, and measurements with the same name can
be entirely different as a result of the use of differing landmarks or
measuring techniques (Garrett and Kennedy, 1971~.
OCR for page 77
77
In general, anthropometric measurements are static and pos-
ture-dependent. While a static subject facilitates measurement,
the technique creates a data set that describes the body only in
relatively artificial poses and provides no direct information on
how body size and shape change with motion. Changes in size
and shape of the proximal ends of body segments are particularly
pronounced as a result of muscle dynamics. In adclition, body
shape data have generally been inferred from traditional anthro-
pometric body size data and have not been quantified. Most body
dimensions are measured independently of each other. While tra-
ditional anthropometric analysis has a solid statistical foundation
based on normally distributed variables, a comparable statistical
methodology must be established for three-~rnensional models.
When a person ~ viewed in the lateral plane, for example, mea-
surements for stature, shoulder height, and length of leg are all
taken from different measurement positions, with no established
relationships among them. Therefore, it is difficult to generate
a three-~unensional model from these molated measurements in a
systematic manner without making artistic assumptions regarding
these relationships.
Loadings on internal structures in the body change signifi-
cantly under dynamic conditions (Marras et al., 1984~. There is a
need to measure the dynamic loadings in viva, however, since most
models involving ergonomic analysm of activities have been based
on static conditions. Three-dunensional models are needed that
describe the acute as well as cumulative wear and tear in a joint
caused by the dynamic motion of the body and the synergistic
action of internal forces (e.g., muscles, ligaments, and pressures).
Current methods such as electromyography and disc pressure mea-
surements are questionable under true dynamic conditions.
New transducers are needed based on noninvasive measure-
ments such as ultrasound to identify this wear and tear. These
devices must be capable of producing quantitative data regarding
the load components. Dynamic biomechanical models are needed
that do more than simply describe the position of the body or the
body components. Optimization techniques may provide a useful
approach.
OCR for page 78
78
Analysm of Muscle and Joint Dynamics
Three-~nnensional models of the body are useful for accu-
rate descriptions of joint loadings. Therefore, more information
regarding the position and the line of action of agon~t-antagonist
pairs of muscles is needed. Several mode! approaches using these
data have been described by Schultz and Andermon (1981~. These
models determine the compression and lateral and anterior shear
components of stress on joints and replace the simple compression
estimates that are currently used.
Current methods for determining the strain in muscles and
joints measure the net output of all actions involved. This net
output, however, is the sum total of a number of individual muscle
efforts. For example, in elbow flexion, in which both extensors
and flexors are active, the combined torques around the elbow
joint partially nullify each other. The result shows the net joint
loading, but does not measure the forces that are contributed by
the individual muscles. The problem involved is of practical im-
portance because under the combined torques, for example, those
created by concurrent contraction of flexor and extensor muscles,
the intermediate body joint may be overloaded. This cannot be
predicted from the net result, which ~ the only information that
current methodologies yield.
In addition, gross estimations are required for the lever arms
of muscles acting around body articulations. The geometry of
these muscle attachments with respect to their lever arms may
be quite different for different people. This produces uncertainty
about the actual torques developed around joints, the strength
to be exerted by the muscles involved, and the loading on the
intermediate joints.
Most information about human muscle strength capabilities
assumes a 1-9 constant force field condition. We know little about
the effects of higher or lower constant and transitory force fields
on the ability to exert muscle strength. It is difficult to determine
the muscle strength that is available, for example, in the reduced
gravity of space or in an airplane that flies a path that generates
accelerations on the pilot. While there have been a few isolated
experiments (e.g., Kroemer, 1974), there have been no systematic
measurements made under controlled laboratory conditions.
There is a need to evaluate existing systems for measuring the
properties of the biomechanical system and to develop a system
OCR for page 79
79
for measuring the mechanical impedance of the body during the
performance of normal activities. Then it ~ necessary to mea-
sure the extent of impedance modulation as a function of the task
being performed and to correlate these data with other measur-
able variables such as the electromyogram activity of antagonist
muscles.
Body Segments and Effects of Mamma
Data are needed to develop models of specific body segments
such as head and neck, arm and hand, and leg and foot, beyond
those required for a total body model. An an example, a total body
model describing the behavior in an impact situation does not
usually require specific information about the biodynamic charac-
teristics of the wrist-finger subsystem. This specific information
also would be useful for the design of controls to be operated with
small motions of the wrist and fingers, as in high-performance
aircraft under loading in excess of normal gravity.
Available information on the susceptibility of the body to sin-
gle (acute) or cumulative trauma ~ limited. Individual excursions
or positions in body articulation that occur in activities such as
force exertion or the direction of certain motions may not pro-
duce immediate trauma, but the injuries may be cumulative. We
know that accompanying conditions such as temperature may in-
fluence the occurrence of certain cumulative trauma items. It is
not known, however, how the combination of these, that is, the
magnitude of excursion, directions of excursions, and accompany-
ing force or torque generations, together may generate cumulative
trauma injuries to tendons or tendon sheaths or impingement on
nerves. Although the phenomenon is known, the conditions under
which it may occur are not fully understood.
Bone and Fink D~nam;ce
Stressed bones behave differently when anisotropic assume
tions are made. The knowledge gained from bone modeling will
apply to link models of the human body. The geometrically com-
plex features of bone should be included in these link models since
stress within a joint would most certainly change as the geometric
characteristics of interacting bone surface areas change. Studies
of human body linkages, which are basic to the majority of human
OCR for page 80
80
body analogs, require the precise location of specific skeletal land-
marks. Current methods in anthropometry can only approximate
the location of these landmarks in a three-dimensional system be-
cause of the varying thickness of overlying tissue. Finally, link
models of the human body cannot assume that the spine is a rigid
link, but must acknowledge its flexibility.
Finite-element models of the an~sotropic features of bone may
provide useful data for the development of a complete index of
bone characteristics of the body. Typically, models are needed to
describe the characteristics of the spinal column and of body joints
in general (Hakim and King, 1979~.
Motivation and Fatigne
Factors such as motivation and fatigue play important roles
in human biomechanical actions. The control that the operator
exerts over muscles because of transitory motivation or fatigue are
biasing factors that have been largely ignored in biomechanical
modeling. We know that when motivation is present, people are
capable of exerting force which far exceeds that predicted by most
biomechanical models. The effects of fatigue on muscular perfor-
mance have not been quantified. Some of the recent literature
has also indicated that when workers are subjected to unexpected
loadings they are at a greater risk of musculoskeletal injury. There
is a lack of suitable theory and experimental data regarding inter-
nal nervous control with respect to feed-forward generated in the
brain and to the rearrangement of CNS motor signals according to
the feedback that is received (Kroemer et al., 1986~. Consequently,
quantification of the effects of learning and adaptation and of psy-
chomotor behavior while a person is fatigued is difficult since the
internal processes in the central nervous system are not readily
accessible. However, much of this adaptive behavior manifests
itself in changes in the mechanical parameters of the biological
system, specifically the unpedance (i.e., mechanical stiffness and
effective viscosity) about the joints. These quantities are under
voluntary control (e.g., elbow stiffness may be changed by a factor
of 100 or more) and dramatically influence the behavior of the
biomechanical system and its response to external loads.
Representative terms from entire chapter:
modular approach
