What is the OWAS method?
OWAS originated in the Finnish steel industry and later spread to other industrial sectors and construction. Its approach differs from methods that analyze a specific posture: OWAS observes work at regular intervals and constructs a statistical picture of the postures adopted throughout the workday.
In its initial version, it was based on observation of the trunk, upper limbs, and lower limbs. In 1991, a computerized version was published that incorporates the effort exerted or the load handled, and since then it has been integrated into numerous ergonomic analysis applications.
The National Institute for Occupational Safety and Health describes it in Technical Prevention Note 452 and in its informative document on risk assessment by work postures.
What it evaluates and what it does not evaluate
- Evaluate. The position of the back, arms, and legs, along with the load or force used, at each sampled instant.
- Evaluate the temporal distribution. By repeating the observation, estimate the proportion of time that the back or limbs remain in each posture.
- It does not assess head position or wrist detail, so it is not suitable when the risk is concentrated in those segments.
- It does not measure angles: it works with broad categories of posture and with equally broad estimates of strength.
- It does not replace specific methods of manual handling of loads or repetitive movements.
Scope of application
OWAS is particularly well-suited for variable and long-cycle jobs where a single observation is insufficient to represent the entire task: industrial production, maintenance, construction, agriculture, and logistics. Its strength lies in quickly identifying the most frequent and strenuous awkward postures and alerting users to the most unfavorable situations.
The method has been validated primarily for assessing lumbar risk associated with work postures. It should not be used when awkward postures affect segments not considered by the method, such as the head, nor when the objective is a detailed analysis of the upper extremity.
Information needed to apply it
- Preliminary analysis of the work: phases, tasks, operations and duration of each.
- Decision on the number of observations and the interval between them, depending on how repetitive the task is.
- Back, arm and leg position in each observation.
- Load or force manipulated at that moment.
- Identification of the work phase to which each observation corresponds.
The recording is done by looking at the work to capture posture, force, and phase, and then looking away to take notes. Insufficient sampling or sampling concentrated on a single phase invalidates the representativeness of the result.
How to apply
Each observed posture is identified with a six-digit code: one for the back, one for the arms, one for the lower limbs, one for the load or force, and two for the phase of the work cycle in which the observation was made.
By combining back, arm, leg, and load positions, a table within the method determines the action category corresponding to that posture. A second table allows for assessing the acceptability of each posture based on the estimated maintenance time throughout the workday, which is particularly useful when the activity is frequent even with a light load.
The codes and decision tables are listed in Technical Prevention Note 452 and in the INSST information document cited in the references.
How to interpret the result
- Action Category 1. No corrective action is required.
- Action Category 2. Corrective measures are required in the near future.
- Action Category 3. Corrective action is required as soon as possible.
- Action Category 4. Immediate corrective action is required.
Once all the recorded postures have been evaluated, they are grouped by category and the intervention plan is developed based on the priorities obtained: correcting the most forced postures and acting on those that, without being extreme, are repeated a lot throughout the day.
Limitations and situations in which it should not be used
- Interpreting the categories as legal thresholds. They are not: no Spanish law imposes them.
- Apply it when the forced posture affects segments that the method does not cover, such as the head.
- Relying on a few observations: the validity of the result depends on the size and distribution of the sample.
- Ignoring the fact that the categories of posture and strength are very broad, which can obscure relevant differences.
- Use it as the sole method when handling loads or repetitive movements occur.
- Concluding the evaluation without verifying that the measures have modified the distribution of positions.
Practical example
In an industrial vehicle repair shop, tasks change throughout the day, and complaints are concentrated in the lower back. An assessment based on a single posture would not be representative.
- Sampling. Observations are made every two minutes for two full days, noting back, arms, legs, load and work phase.
- Result. Work under vehicle concentrates most postures in categories 3 and 4 due to twisted and bent back with arms above shoulder; work on bench remains in category 1.
- Measures. A column lift is installed to work standing up under the vehicle and the workbench is equipped with height-adjustable tools.
- Re-evaluation. A new sampling reduces the observations in categories 3 and 4 to a minority, and the remaining ones are concentrated on specific operations on which the next improvement is planned.
Differences with other methods
- Compared to RULA and REBA, these analyze a specific position in detail; OWAS statistically describes the day through sampling, with broader categories.
- Compared to OCRA. OCRA quantifies exposure to repetitive upper limb movements, a factor that OWAS does not address.
- In contrast to the NIOSH lifting equation, which calculates a recommended weight limit for two-handed lifting, OWAS only considers the load as a categorical variable.
- Compared to instrumental methods, goniometric techniques and motion capture systems measure actual angles, while OWAS estimates them by observation.
Technical and regulatory framework
- Law 31/1995, articles 15 and 16. It requires the assessment of unavoidable risks and the adaptation of work to the person.
- Royal Decree 39/1997, Article 5. Requires procedures that provide confidence in the outcome of the evaluation.
- Technical Prevention Notes on ergonomic risks . They include the postural load methods disseminated by the INSST, including OWAS.
- ISO 11226:2000 . International reference on static postures, complementary to postural sampling. It is under review.
OWAS is not a mandatory method in Spain. Its use is supported by Article 5 of Royal Decree 39/1997 and by its technical recognition.
