OCRA is the acronym for OCcupational Repetitive Actions. It is a tool dedicated to the detection of MSD in the context of repetitive tasks mainly for the hands. Professors D. Colombini and E. Occipinti1, from the University of Bologna in Italy, developed this method in the years 2000.
The method is used only for frequent movements of hands bound to low loads, less than 3kg.
The method is based on 2 tools:
- An Index to calculate the ratio of the number of shares per minute performed during the Position to a reference value.
- A Checklist that allows you to evaluate the process in depth.
In the end, we get clues that will allow us to ” judge ” whether or not the situation is acceptable and implement the necessary actions.
1 – Calculation of the OCRA index
1.1 Calculating the ATA index
The ATA index, Actual technical actions, represents the number of technical actions that are performed throughout the duration of the Position. It depends on the frequency of the movement and the time of each of them. We’re going to be in two different cases:
The movement is always the same. The ATA coefficient is then calculated as follows:
ATA = f * t
- f: frequency of movement per minute. Attention, if both arms make a movement simultaneously, we count 2 movements.
- T: This is the total time when the task is performed during the time of the Position, expressed in minute.
The work is multitasking: This is the case for example where the person changes workstation during the day of work. The ATA index is then calculated for each task and the total ATA is deducted by making the sum of each of the ATA’s.
1.2 Calculation of the RTA
The RTA is a reference value that represents the number of technical actions per minute that could be performed during the entire Position in the working conditions that we have.
RTA = k * F * P * R * Fa * Rc * d
- k: Frequency constant equal to 30 per min.
- F: Force factor.
- P: Posture Factor.
- R: Repeatability factor.
- Fa: Additional factor.
- Rc: Recovery factor.
- D: Duration of the action.
1.2.1 Assessment of the Force factor
We define our level of effort linked to the activity. For this, we use our perception of the effort vis-a-vis the scale of Borg CR-10. This scale is particularly used in the field of sport. It defines levels of perception and the recommendation in its use is as follows (translation of the book)2 :
How heavy and tiring it is. Your perception of effort depends mainly on the degree of strain and fatigue experienced at the muscular level or shortness of breath or even a thoracic gene. But you have to take into account your own subjective impressions and not physiological signals or the current physical constraint.
10, “extremely strong-Max P” is the major graduation. This is the highest perception (P) you have ever felt. It is possible, however, to imagine something stronger. Also, “absolute maximum” is placed lower in the scale without setting a number defined and marked by a point. In case you percevriez an intensity of effort greater than 10, you will be able to offer a larger number.
Always start by reading verbal expressions and then choosing a number. If the perceived effort is “very low” (or “very light”), say 1; If it is moderate, say 3, and so on. You can of course give intermediate values such as 1.5 or 3.5, or decimals such as 0.3 or 0.8 or 2.3.
It is very important that your answer fits well with your own perception and not the one you think you have to give, and that you are as faithful as possible to your perceived effort intensity without doing any on and/or under estimation.
1.2.2 Posture Factor
The posture factor is dependent on the position of the upper body and the proportion of time we spend in this position during the cycle time. We will take into account the position of the:
1.2.3 repeatability Factor
We will also evaluate the level of repeatability of the movement. To do this, 2 ” valid ” criteria apply the coefficient:
- The cycle time is less than 15 seconds.
- A position is maintained for more than 50 of the cycle time.
If any of the criteria are validated, a coefficient of 0.7 will be applied. In all other cases, a coefficient of 1
1.2.3 Additive factor
We also take a set of criteria related to the operation itself. If one of the situations described below is encountered during the operation, then a coefficient will be applied that will depend on the time.
The criteria are:
- The operation requires the use of vibrating tools.
- “Shocks” are experienced or performed. This is the case for example where we use our hand as a hammer.
- The work requires a high concentration and precision in the movements (example of the watch fitter).
- Our bodies are undergoing significant cutbacks.
- It is exposed to surfaces or a cold environment.
- It is necessary for safety or other reasons to use gloves.
- The pace of work is clearly linked to an automatic machine.
If none of these criteria is validated, a coefficient of 1 will be applied. If this is not the case, the proportion of the time it takes with regard to the total cycle time will be defined:
- Between 25 and 50 of the cycle time: 0.95
- 51 to 80 cycle time: 0, 9
- Over 80:0.8
1.2.4 calculation of the interim RPA
In the same way as for the calculation of the ATA, we are always subject to 2 cases. Either we study a single-task cycle where the person always carries out the same cycle for the duration of the work, or the person changes workstation, range… And the movement is very different.
In the first case, the intermediate RPA will be calculated using the following formula:
RPA = k * F * P * R * Fa * f
- k: Frequency constant equal to 30 per min.
- F: Force factor
- P: Posture Factor
- R: Repeatability factor
- Fa: Additional factor
- f: Frequency of movement per minute. Attention, if both arms make a movement simultaneously, we count 2 movements.
In the case of a multitasking study, the formula is as follows:
RPAtot = RPA1 + RPA2 + … + RPAn
1.2.5 The recovery factor
Recovery during the working day is an important factor to take into account. Its considered as a rest period:
- Official stops or not, including time to eat.
- Visual control tasks.
- Periods during the cycle time allowing the muscles to be at least at rest for 10 sec, and this at regular frequency (a few minutes).
1.2.6 The time factor
Finally, the last factor to apply, the time factor. This is representative of the ” net ” time that we spend during a day of work to perform repetitive tasks. In the case of a change of workstation during the working time for a workstation without repetitive task, that time will not be taken into account.
1.2.7 Calculation of RTA
Last step, we’re going to deduct the RTA from the activity. This is calculated using the following formula:
RTA = RPA * Rc * D
- RPA: the coefficient calculated in step 1.2.4
- Rc: The recovery factor
- D: The time factor
2 – Use of the OCRA Check List
The Check List OCRA is the same as the index with two differences near:
- The first is the distinction between the left side and the right side, which allows us to fit in more detail. Thus, each type of constraint and effort is decomposed depending on whether it is the left or right side of it.
- The other difference is the fact that we get a little more into the details of the breaks, which will change the recovery factor. are taken into account in the calculation: “Unofficial” breaks lasting about 8 minutes, details of tasks that are not part of repetitive tasks (moving outside… and the ” rest ” included in the cycle time.
Thus, depending on the total number of breaks or ” micro ” breaks during the working day, the recovery coefficient changes.
3 – Interpretation
Once the results of the two tools have been obtained, they are interpreted according to the following table:
1-D. Colombini and E. Occipinti (1996) – Proposta di un index sinetico per la valutazione dell’esposizione a movimenti ripetitive degli arti superiori
2 – G. Borg (2001) – Borg’s Range Model Scale
N. J. Delleman, C. M. Haslegrave, D. B. Chaffin (2004) – Working postures and movements
D. Colombini and E. Occipinti (2004) – OCRA: Aggiornamiento dei valori di riferimento e dei modelli di predicte dell’occorenza di UL-WMSDs nelle popolazioni lavorative esPosition a movimenti e sforzi ripetuti degli arti superiori