ISO/TR 23076:2021
(Main)Ergonomics — Recovery model for cyclical industrial work
Ergonomics — Recovery model for cyclical industrial work
This document establishes an ergonomic model for any cyclical human work planned and executed in an industrial competitive environment. It also covers the process of measuring work based on the concept of normal work performance and of the assessment of risk factors commonly associated with body postures, body or hand forces, manual material handling of loads and handling low loads at high frequency. This document applies to the adult working population and is intended to give reasonable protection for nearly all healthy adults. Those areas concerning health risks and control measures are mainly based on experimental studies regarding musculoskeletal loading, discomfort or pain and endurance or fatigue related to work organization and methods. The scope of this document is any cyclical human work planned and executed in an industrial competitive environment. The most typical cases are within industries where there is the need to define an expected output (products or services) based on the optimization of the trade-off between labour productivity and health and safety. The most sensitive organizations to this proposal are those within labour-intensive manufacturing industries with series and batch production systems: — automotive (original equipment manufacturer and tier 1 and 2 suppliers); — industrial automotive (trucks, buses, agricultural and mining equipment); — industrial manufacturing (small domestic and industrial equipment or machinery); — domestic appliances and consumer goods (white goods); — plastic and rubber products (tires, doors, windows, shoes); — consumer electronics (PCs, televisions, printers, radios, hi-fis, alarm systems); — furniture; — textiles and apparel; — food preparation; — packaging; — aerospace and defence; — rail and shipping; — large domestic and industrial equipment or machinery; — logistics.
Ergonomie — Modèle de récupération pour les activités cycliques dans l’industrie
General Information
Standards Content (Sample)
TECHNICAL ISO/TR
REPORT 23076
First edition
2021-10
Ergonomics — Recovery model for
cyclical industrial work
Ergonomie — Modèle de récupération pour les activités cycliques
dans l’industrie
Reference number
© ISO 2021
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 2
4 Proposed approach . 5
4.1 The correct work content. 5
4.2 Design of a standard working method . 6
4.3 Work measurement . 7
4.3.1 General . 7
4.3.2 Standard work performance . 7
4.3.3 Real action . 8
4.4 Task assignment and work organization . 9
4.5 Biomechanical load measurement . 10
4.6 Ergonomic work allowance (EWA) . 11
4.6.1 General . 11
4.6.2 Traditional approach . 12
4.6.3 Proposed approach: EWA . 13
4.6.4 Design criteria . 18
4.7 Organizational solutions. 19
4.7.1 General . 19
4.7.2 Strategies to reduce the overall load index . 19
5 Ergonomic assessment worksheet (EAWS) .20
5.1 System overview .20
5.2 EAWS basic structure .20
5.2.1 General .20
5.2.2 Section 0: extra points . 23
5.2.3 S ection 1: body postures . 24
5.2.4 S ection 2: action forces . 25
5.2.5 S ection 3: manual material handling of loads . 26
5.2.6 S ection 4: repetitive motions of the upper limbs . 27
6 EWA effect on EAWS score.29
6.1 General .29
6.2 Effect of recovery time on typical workstations . 32
6.3 EAWS based EWA model . 33
Annex A (informative) EAWS scoring procedure .35
Bibliography . 144
iii
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 159, Ergonomics, Subcommittee SC 3,
Anthropometry and biomechanics.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
iv
Introduction
0.1 General
The literature contains numerous methodologies for measuring physical stress in manual work.
Studies from different disciplines and research groups have concentrated on diverse external factors,
workplaces, and jobs. Factors most often cited include forceful exertions, repetitive motions, sustained
postures, strong vibration and cold temperatures.
The ISO 11228 series, ISO 11226 and ISO TR 12295 establish ergonomic recommendations for different
manual handling tasks, repetitive movements and working postures. They apply to occupational and
non-occupational activities and provide information for designers, employers, employees and others
involved in work, job and product design, such as occupational health and safety professionals.
— The ISO 11228 series relates to manual handling, including lifting and carrying, pushing and pulling
and the handling of low loads at high frequency.
— ISO 11226 gives recommended limits for static working postures with no or minimal external force
exertion, while taking into account body angles and duration.
— ISO TR 12295 serves as an application guide of the ISO 11228 series and ISO 11226 and offers a
simple risk assessment methodology for small and medium enterprises and for non-professional
activities. ISO/TR 12295:2014, C.5, is very relevant for this document, since there is a reference to
the EAWS system, which is extensively described in Annex A, being the first available ergonomic
tool meeting the requirements of the EWA model.
This document can be used by industrial engineers for the application of ergonomic work allowances
as a means to determine the correct quantity of cyclical work assigned to a worker in a manufacturing
plant in order to meet the definition of a fair day’s work. A fair day’s work is that length of working day,
and that intensity of actual work, which expends one day's full working power of the worker without
[26]
encroaching upon his or her capacity for the same amount of work for the next and following days .
In the old-fashioned production systems (piecework-based) the fair day’s work concept was used in
connection with the fair day’s wage. In this document, the studies about the definition of the fair day’s
work become fundamental to connect work-study with the most recent knowledge about biomechanical
load (occupational health and safety), with a special focus on the product-process design phase.
0.2 Recovery
In the field of ergonomics there is a special interest in predicting fatigue dependent on the intensity,
duration and composition of stress factors and to determine the necessary recovery time. Table 1 shows
those different activity levels and consideration periods, possible reasons for fatigue and different
possibilities of recovery.
Table 1 — Fatigue and recovery dependent on activity levels
Level of activity Period Fatigue from Recovery by
Work life Decades Overexertion for decades Retirement
Phases of work life Years Overexertion for years Holidays
Sequences of work shifts Months or weeks Unfavourable shift re- Weekend, free days
gimes
One work shift One day Stress above endurance Free time, rest periods
limits
Tasks Hours Stress above endurance Rest period
limits
Part of a task Minutes Stress above endurance Change of stress factors
limits
v
In ergonomic analysis of stress and fatigue for determining the necessary recovery time, considering
the period of one working day is the most important. In this document, this type of recovery is named
“recovery external to the work cycle” and is defined in ISO 11228-3.
In case of cyclical industrial work, where awkward static body postures are relevant, a strategy to
reduce the stress level is to allow short recovery periods within each work cycle. This type of recovery
is named “recovery within the work cycle”.
The proposed model concerns the quantification of recovery periods within the work cycle and
considers recovery periods outside the cycle (normally defined as pauses) as an exogenous variable,
evaluated within the factors characterizing the work organization.
0.3 Purpose and justification
The industrial sector is one of the sectors with the highest global employment rate (22,5 % of total
employment). Despite this, the most recent research efforts about the definition of a fair day’s work
date back to the 1980s. In the last 20 years a lot of research has been carried out on the biomechanical
load and many new standards have been created.
This document is a first bridge between two different fields of knowledge: work study (industrial
engineering) and occupational health and safety (ergonomics). the objective is to improve the work
study tools by leveraging the knowledge made available by the most recent studies about work-related
musculoskeletal disorders (WMSDs).
This document provides a methodological reference for the procedures to determine the fair quantity
of work within a working day in industrial operations with repetitive manual work cycles.
The goal of the model is to guide industrial engineers to keep the biomechanical load or local muscle
fatigue generated by the planned cyclical work within the limits defined in the ISO 11228 series and
ISO 11226.
This document proposes neither new work measurement techniques nor new ergonomic techniques
or standards. Rather, it aims at merging the best available knowledge (industrial engineering and
ergonomics) about human capacity of accomplishing a manual task, following a pre-defined work cycle
(method description and related standard time) without generating an excess of biomechanical load
(fatigue).
Present issues:
— Ergonomic allowance is neglected or assigned based on a partial evaluation of the physical load
(usually body postures and forces). The calculation is not influenced by:
— load duration (action frequency and duration of static actions);
— work organization (shift duration, duration and distribution of the break periods) and work
measurement.
— Lack of a well-recognized standard work performance to measure manual work.
— Available ergonomic evaluation systems work on different measurement scales and the difficulty of
assessing the overall physical stress.
— The ergonomic approach tends to be used reactively in the industry rather than proactively
(preventive ergonomics).
0.4 Expected benefits
— Support the adoption of the ISO 11228 series and ISO 11226 in the industrial manufacturing sectors.
— Support the definition of a standard work performance to standardize the work measurement.
— Improve working conditions, safety and ergonomics of workers in manufacturing industries.
vi
— Complement the traditional set of experts’ capabilities on time and motion with the ergonomic
skills necessary to design safe and efficient work stations and sustain continuous improvements in
productivity and ergonomics during the entire product life cycle.
— Support the ergonomic evaluation in the earliest stages of product or process development, when
changes are still feasible and the cost of such changes is affordable (preventive ergonomics).
— Link ergonomic improvements with labour cost reduction (improve ergonomics – reduce costs –
justify investments in ergonomic improvements).
— Reduce cost and deviation of the ergonomic risk-mapping process by linking the biomechanical load
measurement with work measurement and organization.
— Be an objective reference for employers and unions when setting up gainsharing contracts based on
labour productivity (industrial relations).
vii
TECHNICAL REPORT ISO/TR 23076:2021(E)
Ergonomics — Recovery model for cyclical industrial work
1 Scope
This document establishes an ergonomic model for any cyclical human work planned and executed
in an industrial competitive environment. It also covers the process of measuring work based on the
concept of normal work performance and of the assessment of risk factors commonly associated with
body postures, body or hand forces, manual material handling of loads and handling low loads at high
frequency.
This document applies to the adult working population and is intended to give reasonable protection
for nearly all healthy adults. Those areas concerning health risks and control measures are mainly
based on experimental studies regarding musculoskeletal loading, discomfort or pain and endurance
or fatigue related to work organization and methods.
The scope of this document is any cyclical human work planned and executed in an industrial
competitive environment. The most typical cases are within industries where there is the need to
define an expected output (products or services) based on the optimization of the trade-off between
labour productivity and health and safety.
The most sensitive organizations to this proposal are those within labour-intensive manufacturing
industries with series and batch production systems:
— automotive (original equipment manufacturer and tier 1 and 2 suppliers);
— industrial automotive (trucks, buses, agricultural and mining equipment);
— industrial manufacturing (small domestic and industrial equipment or machinery);
— domestic appliances and consumer goods (white goods);
— plastic and rubber products (tires, doors, windows, shoes);
— consumer electronics (PCs, televisions, printers, radios, hi-fis, alarm systems);
— furniture;
— textiles and apparel;
— food preparation;
— packaging;
— aerospace and defence;
— rail and shipping;
— large domestic and industrial equipment or machinery;
— logistics.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
basic motion
manual motion performed with fingers, hands, arms, eyes, feet, legs or body, no longer decomposable as
regards to content time
3.2
technical action
elementary manual action to complete operations within the cycle basic motion in which a segment of
the upper limb (shoulder, elbow, wrist or finger) is involved to reach a target or to hold an object or a
posture
EXAMPLE Grasp, reach, move, turn, apply pressure, hold, turn, push or cut.
3.3
real action
combination of basic motions (technical actions) performed to achieve a finite and planned state of an
object
EXAMPLE Get and place an object, place a tool, activate (reach and press a button), micro finger cycle as
fastening a screw with fingers.
3.4
standard work
work with the most efficient method to produce a product (or perform a service) at a balanced flow to
achieve a desired output rate
3.5
standard working method
method to break down of the work into elements (operations), which are sequenced, organized and
repeatedly followed
Note 1 to entry: Standard conditions as part presentation, distances, geometries, weights or tools and equipment
are clearly described.
3.6
work measurement
application of techniques designed to define the time for a qualified worker to carry out a specified job
at a defined level of performance
3.7
standard work performance
effort level that could be easily maintained year in, year out, by a worker with average physical
capabilities, without drawing upon his or her reserves of energy
Note 1 to entry: Working at standard performance brings the worker to the end of the fair day’s work without an
excess of physical stress.
3.8
time allowance
time added to the basic time
Note 1 to entry: The amount of the allowance depends on the nature of the work and the working environment,
and it is often assessed using an agreed set of guidelines and scales.
Note 2 to entry: Time allowances are used to cover personal needs, technical and organizational planned losses
and learning effect. This document refers to time allowances meaning the additional time to recover from an
excess of fatigue generated by the work cycle.
3.9
basic time
time set through a given work analysis system
Note 1 to entry: Predetermined time measuring systems (e.g. methods-time measurement) provide basic times
of manual elementary motions (e.g. reach, grasp, move).
Note 2 to entry: Basic time does not include any allowance.
3.10
standard time
time required by an average skilled operator, working at a normal pace, to perform a specified task
using a prescribed method
Note 1 to entry: The difference between standard time and basic time is that basic time is the time when work
should be done without any delays. Standard time is the time taken by the worker to complete the work with
some unavoidable and therefore planned delays (time allowances).
Note 2 to entry: Standard time includes time allowances.
3.11
methods-time measurement
MTM
procedure which analyses any manual operation or method into the basic motions required to perform
it and assigns to each motion a basic predetermined time, which is determined by the influencing
factors under which it is made
Note 1 to entry: Examples include reach or move distance, type of grasp, object weight.
3.12
cycle time
time available at each workstation to accomplish the tasks assigned for each unit of output
Note 1 to entry: Cycle time corresponds to the pace at which an assembly line delivers its output.
Note 2 to entry: In the case of a single workstation, cycle time and standard time coincide, since there is no idle
time caused by the imperfect synchronization of a sequence of workstations (balancing losses).
Note 3 to entry: Cycle time is expressed as the sum of standard time and idle time.
3.13
task assignment
line balancing
manufacturing-engineering technique, in which the production line operations are divided into tasks,
which are assigned to the minimum number of workstations
Note 1 to entry: A production line is said to be in balance when every worker's task takes approximately the same
amount of standard time. Well-balanced lines minimize labour idleness and improve productivity.
3.14
work organization
way that tasks are distributed among the individuals in an organization and the ways in which these
are then coordinated to achieve the final product or service
Note 1 to entry: Work organization typically encompasses the total shift duration, the quantity and distribution
of the breaks, the type of man-machine interface and the level of allowed flexibility.
3.15
worker saturation
percentage of non-idle time within a cycle time
Note 1 to entry: Worker saturation is expressed as the fraction of standard time and cycle time.
Note 2 to entry: See Figure 1.
Key
CT cycle time
T time
A, B workstation A, B
1,2,3,4,5 operation 1,2,3,4,5 (basic time)
AA, BB allowance A, B
IT idle time (unsaturation)
1 + 2 task assigned to work station A (standard time)
3 + 4 + 5 task assigned to work station B (standard time)
Figure 1 — Industrial engineering terminology
3.16
biomechanical load
physical stress acting on the body or on anatomical structures within the body
Note 1 to entry: Loads originate from the external environment (e.g. the force generated by a power hand tool) or
are the possible result of voluntary or involuntary actions of the individual (e.g. lifting objects).
EXAMPLE Kinetic (motion), kinematic (force), oscillatory (vibration) stress and thermal (temperature)
energy sources.
3.17
transition time
duration of the movements for changing from one body posture to another
3.18
overall load index
OLI
index compounding the overall biomechanical load generated by the different types of physical stress
4 Proposed approach
4.1 The correct work content
The determination of the correct work content for a given activity is a fundamental task for a company
in order to be competitive on the market, as well as to safeguard workers’ health and to guarantee a
proper quality of the performed activity. The setting of a standard time of a manual task is based on the
following steps (see Figure 2; T is the cycle time and T is used to indicate the standard time):
c std
a) design of a standard working method;
b) work measurement;
c) task assignment and work organization;
d) biomechanical load measurement;
e) ergonomic work allowance calculation (applying the model).
Key
input
process
output
Figure 2 — Standard time setting process
4.2 Design of a standard working method
The design of a standard working method is the key driver to achieve operational excellence in levels of
productivity and safety. This task is one of the main responsibilities for industrial engineers, who have
to blend wisely several fields of knowledge to coordinate humans, machines and materials to attain a
desired output rate with the optimum utilization of energy, knowledge, money and time. It employs
key techniques (such as floor layouts, personnel organization, time standards, wage rates, incentive
payment plans, production scheduling) and technologies (ICT, digital devices, data and analytics) to
control the quantity and quality of goods and services produced. The design and planning of a working
system largely determines the ergonomic conditions of the worker and therefore it is fundamental to
bring the ergonomic knowledge into the earliest stages of the product and process development process
and the ergonomic constraints into the planning process (see Figure 3).
Figure 3 — Preventive ergonomics in the new product development process
To achieve such a sophisticated level of product or process development and planning process, the most
advanced industrial companies use a predetermined motion-time system (PMTS). A PMTS is a set of
data of elementary human motions, of which a basic time is predetermined, which is used as a reliable
language to design, plan and measure a manual task.
The last developments among available PMTSs aim at creating specific tools for designing work systems
in the earliest stages of product and process development, rather than simply measuring them once they
are up and running. In this way, it is possible to find the most efficient and ergonomic solutions when
it is still feasible to make product and process changes and the cost of such change is still affordable
(metal has not yet been cut). Indeed, in the early phases of product or process development, investments
in tools and equipment have usually not yet been released and changing a CAD file or a design is not too
expensive. Standard times play a key role in setting transformation process costs and purchasing costs
of goods and services.
World class companies’ purchasing departments monitor direct purchasing or outsourced service
costs thanks to an analytical calculation based on the most appropriate PMTSs. As far as ergonomics is
concerned, if there is a tool to pre-calculate the biomechanical load based on a planned working method,
it becomes economical and effective to preventively reduce the risk due to an excessive workload.
4.3 Work measurement
4.3.1 General
The definition of the basic time (T Step 2 in Figure 2) is built on the concept of standard work
b,
[23]
performance , strictly related to the fair day’s work. As mentioned previously, the standard work
performance represents an effort level that could be easily maintained year in, year out by a worker
with average physical capabilities without in any way asking him or her to draw upon his or her reserves
of energy. Working at standard performance allows the worker to get to the end of the fair day’s work
without an excess of physical stress.
Most accurate work measurement techniques (stopwatch and PMTS) make use of performance rating
to ensure that times calculated or derived are times for "an average qualified worker" to carry out the
work being measured. Since this average qualified worker is not actually observed, performance rating
is used to modify what is observed and thus convert it to basic time (see Figure 4).
Figure 4 — Stopwatch procedure to set a basic time
Some measurement techniques, such as the PMTS, are not based on the observer to rate the worker’s
performance. PMTS developers use performance rating in the derivation of the original data to
calculate the basic times of each single elementary motion. Therefore, PMTSs, once the method has
been set (sequence of elementary motions), directly provide the basic times, without the need to rate
the operator’s working performance and, even more important, without the need to observe. This is the
reason why PMTSs are strongly recommended for designing and planning a new work system, making
a preventive approach to ergonomics possible.
Currently, there are a number of different performance rating systems and scales available and in use
(no reference standard is defined) and this makes it difficult to define a standard norm performance.
Using different performance scales leads to setting different basic times for the same quantity of work,
causing critical deviations in the ergonomic evaluation of the work load (e.g. a different basic time per
motion would generate different motion frequencies in a cycle).
4.3.2 Standard work performance
Due to increasing globalization, many organizations are currently using several different work
measurement techniques in different geographies of the organization. This happens because different
techniques have gained a greater degree of usage in specific countries. Global organizations are
willing to set comparable standard times of the same piece of work to simplify planning and control
processes and to manage properly their manufacturing footprint and production allocation. That’s why
it is important to support the definition of a global work performance reference, exploiting the large
quantity of knowledge about ergonomics, which became available mainly in the last 20 to 25 years
(while the most common definitions of standard work performance date back to the 1940s).
Each of the rating systems or scales starts from a different conceptual viewpoint. For example, the
Bedaux System assumed that 'normal' performance was 60 'minutes of work' per hour, that 80 'minutes
of work' per hour was incentive performance and that 100 was the theoretical maximum.
All work measurement systems use time units to represent work content – the quantity of work
involved in carrying out a particular task, operation or job. Thus, the unit, such as 'standard minute',
is an expression of quantity of work, rather than of time. It only converts to an equivalent time by
assuming that the operator works at standard performance (with reference to the performance rating
scale in use) and takes the agreed level of allowances built into the work content value (standard time).
Different rating systems claim to rate different factors – commonly these are some combinations of
speed, effort, skill, dexterity, consistency and conditions.
One of the common problems of rating is that it is often linked to remuneration, through setting
'daywork' rates or through graduated incentive payment schemes. This results in pressure from
employees and unions on work study practitioners to 'slacken' their ratings to give 'looser' time values
for jobs.
Thus, even though the same rating system and scale is in use in different organizations, there is no
guarantee that the concepts of normal and incentive performance are the same in each one of them –
this is especially true if the organizations carry out no rating validation through rating clinics.
In some countries or organizations, trade unions have a right to observe time studies or to carry out
parallel studies to check on the times produced by industrial engineers. Where incentive payment
schemes are involved there is understandably a desire to challenge ratings and allowances used by the
practitioner – since most rating systems are based on subjective judgment, this debate is difficult to
resolve in the absence of some means to validate ratings.
The choice of a well-known level of standard performance is crucial for the process of designing safe
and ergonomic work systems, especially as far as the upper limbs risk evaluation is concerned. Indeed,
a higher level of standard performance would bring to shorter basic times for each elementary motion
and consequently an expected increase in action frequency of the upper limbs planned motions. When
most of the work measurement systems were developed, there were no ergonomic standards available
and the good ergonomic solutions were left to the individual experience of the industrial engineers.
Nowadays, the correlation between biomechanical load and the probability of incurring a work-related
musculoskeletal disorder is proved and relevant ISO and CEN documents set clear references.
One objective of this document is to take a formal position against the use of the different standard
performance levels to set basic times in the industries. The availability of different performance rating
scales is not an issue. When measuring a temperature, regardless of the scale used, if the water starts
boiling, the value read on each scale is different but well known and equivalent (100 °C or 212 °F or
373,15 °K indicate the same level of heat). In the same way, it is important to establish a fair reference
level of work performance, which keeps the biomechanical load under given limits. Several tests have
been run by the technical committee of the International MTM Directorate, using the MTM scale as a
reference, and the results are summarized in Annex A.4.
4.3.3 Real action
A real action (RA) is a combined movement of the upper limb (fingers, hand, wrist, elbow or shoulder)
aimed at achieving a planned state (e.g. get and place an object to a specific destination). The exact
[24]
definition of the RA is based on the movement definitions of the building blocks of MTM-UAS .
ISO 11228-3 sets the maximum number of actions at 70 technical actions per minute, equivalent to 40
[19]
real actions per minute . Considering the durations shown in Figure 5, the average duration of one
action is in the range of 31–35 TMU (time measurement units) (100 000 TMU = 1 hour), equivalent to
1,2 s and generating a frequency of 50 real actions per minute (equivalent to c. 87 technical actions/
min).
In a real workplace, consider that there is usually a distribution of motions between the two upper
limbs (left and right) and some body motions and visual controls, which do not generate any real action
and therefore dilute the frequency of actions. Consequently, there is a good chance that, adopting
the MTM standard work performance, the resulting frequencies of action will not cause an excessive
biomechanical load. Of course, to obtain a complete load evaluation, further influencing factors have to
be considered (e.g. force levels, weights, postures).
Key
d duration range of real actions (TMU)
p percentile
NOTE 1 Histogram bars represent the frequency.
NOTE 2 Points in the line represent the percentile.
Figure 5 — Distribution of real actions duration
4.4 Task assignment and work organization
Task assignment in the manufacturing industry is very important, especially when dealing with
assembly lines (line balancing, see Figure 6). Indeed, once the total work content is calculated (total
basic time of all the actions necessary to accomplish the complete task), given a targeted quantity of
units to produce and the net working time available in a shift (shift duration minus breaks and non-
productive time), it is possible to set the pace of our production flow (cycle time, T ). Cycle time then
c
becomes the maximum capacity of each workstation along the flow if the operators are to work at a
controlled performance and to produce the planned output. T is like the capacity of a glass, the water
c
poured into it is the set of tasks assigned to a workstation and T (standard time) is the quantity of
std
litres of water poured into the glass. Without an accurate work measurement, it would not be possible
to balance the line evenly and production would not flow smoothly along the line. Consequently, there
would be lower productivity levels and an uneven distribution of work among the workers, forcing the
most saturated workers to work harder and faster to cope with the line pace (T ).
c
Key
CT cycle time
T time
A, B, C, D workstation A, B, C, D
1,2,3,4,5 . operation 1,2,3,4,5… (basic time)
AA, BB, CC, allowance a, b, c, d
DD
Figure 6 — Line balancing
Once the tasks are assigned to a workstation and the T is set, the duration of each action (times per
c
minute in the case of dynamic actions or seconds of duration per minute in the case of static actions) is
determined and the calculation of the workload results accurately accomplished.
4.5 Biomechanical load measurement
Load results from the intensity and the duration of the work and from the working conditions in which
it is carried out. A load describes the objective demand of work, which is to be fulfilled in a period of
time. It is independent from the individual who performs the activity.
At present, several ergonomic analysis systems are available to measure the workload. Each system
was designed to deal with a specific risk area and it works with its own measurement scale (e.g. NIOSH
Lifting Index, OCRA Index, ACGIH TLV, HAL, RULA, Strain Index). To apply the EWA model, it is necessary
to compound all type of loads (postures, forces, manual material handling of loads, vibrations and
repetitive upper limb motions) on a unique scale. In Figure 7 a comprehensive approach is represented.
Clause 6 provides a first available solution (EAWS), which meets all the rules of the application of the
EWA model.
Figure 7 — Approach to an overall load index assessment
The required ergonomic measurement tool provides an overall load evaluation that includes all
biomechanical risks to which an operator can be exposed during a cyclical work task. All loads are
measured and compounded on a unique scale and the resulting load expressed through a final index,
the overall load index (OLI), which is then used in the EWA model to determine a proper allowance
factor.
The load is given by the result of the following formula:
Load = Intensity × Duration
Intensity is mainly driven by the awkwardness of postures (body, upper limb and grip), intensity of
forces (force exertion and manual material handling) and vibrations.
Duration is driven by the action frequency (dynamic actions) and action duration (static actions).
ISO 11226 and ISO 11228 offer models to assess the level of exposure to ergonomic risk by providing
a measure of the biomechanical load. These provide a means of measuring loads that are not simple to
measure given the numerous and related influencing factors (e.g. the intensity of an upper limb motion
depends on the force level with respect to the type of grip used to get the control over the object being
moved and to the direction of the movement).
The requirements of the EWA model are even higher, since it needs in input the measurement of the
total load generated by the composition of all types of load.
4.6 Ergonomic work allowance (EWA)
4.6.1 General
An allowance is the adjustment of the basic time to obtain the standard time for the purpose of covering
the time spent for personal needs, recover from fatigue and unavoidable delays. By providing a small
increase in the basic time for each cycle, the “non-productive” time becomes planned and a worker can
still be able to complete the work assigned to him or her.
There are two types of interruption: (1) interruption related to the work; (2) interruption not related to
the work. For example, a machine breakdown, rest break to overcome fatigue, and receiving instructions
from the manager are the interruptions related to the work, but personal needs and lunch breaks are
interruptions not related to the work. However, the two types of interruption are both essential for the
worker because it is almost impossible to work in a continuous manner during a regular shift.
Fatigue allowance is intended to cover the time given to the worker to overcome fatigue due to work-
related stress and conditions. There are three factors that cause fatigue: (1) physical factors, like
standing and the use of force; (2) mental and cognitive factors, like mental strain and eye strain; and
(3) environmental and work factors, like poor lighting, noise and heat.
This document deals only with (1) physical factors and partially with (2) mental and cognitive factors
for workers assigned to cyclical manual tasks in an industrial manufacturing environment. Specifically,
EWA means the allowance coping with physical factors. A few mental and cognitive factors of manual
repetitive tasks are only evaluated in the most advanced wor
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