The OCRA method, when computing the number of
reference technical actions,
considers several risk factors and corresponding multipliers.
The following general formula calculates the overall number of Reference Technical Actions within a shift (RTA)
More in detail consider the following sub-steps:
The force multiplier, FoM, is 1 if the following “optimal” conditions (see EN 1005-3) are met:
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a) the isometric force does not exceed 50 % of the values proposed for 15th force percentile for professional use in the healthy adult European population; |
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b) actions do not imply fast movements;
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c) the frequency of force exertions is no more than 1 in 5 minutes and the action time is no more than 3 seconds;
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d) the duration of the repetitive task is no more than one hour.
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If these conditions are not met, use Table 1 to determine the force multiplier (FoM) that applies to the average level of force,
as a function of time. The force level (upper row) is given as a percentage of Maximum Voluntary Contraction (MVC)
or as a percentage of the Maximal Isometric Force (Fb) as determined in EN 1005-3 (Step A).
If the percentage of MVC or the Fb are difficult to assess, a value derived from the application of the CR-10 Borg-scale [6, 7]
can be used (second row). The corresponding Force Multiplier (FoM) can be derived from the table. Use a FoM = 0,01
when the technical actions require 'peaks' above 50 % of force or a score of 5 (or more) in Borg-scale for almost 10 % of the cycle time.
The Annex 2 explaines how to determine the force level:
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The values in the Table 1 can be interpolated if intermediate results are obtained.
The EXAMPLE A1 describes how to determine the FORCE MULTIPLIER:
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The multiplier PoM is equals to 1 when one of the postures or movements, described in Table 2,
is present for less than 1/3 of the cycle time: otherwise use Table 2 to obtain the specific multiplier factor.
Choose the lowest multiplier PoM (that corresponds to the worst condition) between the posture and movements evaluated.
Also consider shoulder postures and movements by checking that:
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1.the arms are not held or moved at about shoulder level (flexion or abduction at about 80° or more)
for more than 10 % of cycle time and/or for more than 2 actions per minute [42];
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2.the arms are not held or moved in mild abduction (between 45° and 80°) for more than 1/3 of cycle time and/or for more than 10 actions/min.
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3.the arms are not held or moved in mild extension (more than 20°) for more than 1/3 of cycle time and/or for more than 10 actions/min.
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If one of those two conditions occurs, a risk of shoulder disorders exists and should be accurately considered
The Annex 3 explains how to analyse postures and movements of the upper limbs:
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The EXAMPLE A1 describes how to determine the POSTURE MULTIPLIER:
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The EXAMPLE A1 describes how to determine the REPETITIVENESS MULTIPLIER:
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Step 2.5. Determine of Additional Multiplier (AdM)
Besides the main risk factors (frequency and repetitiveness of technical actions, use of force, awkward postures and movements, lack of recovery periods, daily repetitive task duration), which are, examined
elsewhere, there are others factors, of an occupational nature, that should be taken into consideration when
exposure is assessed. They are defined here as additional risk factors. This is not because they are of
secondary importance, but because each one of them can, from time to time, be present or absent in the
contexts examined.
The list of these factors is not necessarily exhaustive and includes:
For each of the physical-mechanical risk factors, it is necessary to specify for how much time (as a portion of
the cycle/task time like 1/3, 2/3, 3/3) the factor is present, or to describe the frequency of occurrence of
actions where that factor is present (especially for sudden movements and movements with counter shocks).
The assessment of additional risk factors begins with a definition of optimum conditions, as represented by the absence, or by the very limited presence, of additional risk factors: in this scenario the additional multiplier
Ad
M equals 1. Any discrepancy with respect to this optimal condition represents a contribution of additional
risk factors to the overall exposure level, which grows with the growing portion of the cycle time during which
additional risk factors (one or more) are present. In those cases the additional factor Ad
M multiplier equals:
The Annex 4 explains how to analyse additional factors:
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The EXAMPLE A1 describes how to determine the additional factors :
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Step 2.6. Multiply the adjusted CF thus obtained for the net duration (in minutes) of the repetitive task (Dj) to obtain a Partial Reference number of technical Actions for task j (RPAj):
Step 2.7. Determine the Recovery period multiplier (RcM):
A recovery period is a period during which one or more muscle-tendon groups are basically at rest. The following can be considered as recovery periods:
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breaks (official or non official) including the lunch break
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visual control tasks
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periods within the cycle that leave muscle groups totally at rest consecutively for at least 10 seconds almost every few minutes.
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For repetitive task the reference condition is represented by the presence,
for each hour of repetitive task, of work breaks of at least 10 minutes consecutively or,
for working periods lasting less than one hour, in a ratio of 5:1 between work time and recovery time .
In relation to these reference criteria it is possible to consider how many hours, during the work shift,
do not have an adequate recovery period. It requires the observation, one by one,
of the single hours that make up a working shift: for each hour,
a check must be made if there are repetitive tasks and if there are adequate recovery periods.
For the hour preceding the lunch break (if present), and for the hour before the end of the shift,
the recovery period is represented by these two events.On the basis of the presence or absence of
adequate recovery periods within every hour of repetitive work, the number of hours with “no recovery“ is counted
( see Table 5 to define the specific Multipliers)
The Annex 6 explains more in detail how to identify the number of hours without adequate recovery:
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The EXAMPLE A1 describes how to determine RECOVERY MULTIPLIER:
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Step 2.8. Determine the Duration multiplier (DuM).
Within a working shift, the overall duration of manual repetitive tasks is important to determine the overall risk for upper limbs.
When repetitive manual tasks last for a relevant part of the shift, the Du
M is equal to 1.
In some contexts, however, there may be differences with respect to this more “typical” scenario
(e.g. regularly working over-time, part-time work, repetitive manual tasks for only a part of a shift);
the multiplier (Du
M) considers these changes with respect to usual exposure conditions.
Table 6 gives the values of Du
M in relation with the overall duration of manual repetitive tasks.
The EXAMPLE A1 describes how to determine DURATION MULTIPLIER:
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Step 2.9. Evaluation of RTA (Reference Tehnical Actions) adjusting RPAtot
in relation to the recovery distribution and the daily duration (in minutes) of all repetitive tasks
Once Rc
M and Du
M have been identified by Steps 2.7 and 2.8, the overall number of
Reference Technical Actions within a shift (RTA) can be computed by the following formula:
The EXAMPLE A1 describes how to determine RTA
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