Markedly dissimilar risk assessments were obtained when two different approaches were used to identify task postural positions that were subsequently evaluated with a modified RULA analysis. In a prospective study involving 733 state of Washington working subjects, comparing the assessment of task/job postures picked out through an event-based method versus a time-based method revealed:
- For worst postural positions, the time-based method identified "more tasks in the worst postures for the highest risk levels"; for most postural positions, the difference was 44 percent to 74 percent
- For most common postural positions, the two methods lead to similar results – the most common postural position for most tasks was the neutral joint position; the exceptions revealed by event-based method were the elbow joint (50 to 60 percent of the time was between 60° and 100°) and the wrist joint (60 percent of the time was in extension range between 0° to 15°) while the exceptions identified by the time-based method were neck twisting (73 percent of the time was between 10° and 45°) and the wrist joint (47 percent of the time was in extension range between 0° to 15°)
- For both worst posture and most common postural positions, the percent agreement of RULA scores derived from the two posture analysis methods was poor
- For most common task/job postural positions, RULA scores generated from the time-based method were significantly higher than those derived from the event-based method
In the event-based method, the most common and worst task/job postures for each body part (neck, shoulders, arms, hands and trunk) were defined from task/job video tape evaluation.
In the time-based method, the most common and worst task/job postures for each body part were determined from video frames selected at random times (i.e., once every 75 video frames for a single task job).
The Bottom Line – How This Applies To Ergonomists
If you use RULA to classify the risk of an entire job, the way you identify postures for evaluation can influence your assessment. Disclosure and appreciation of your data gathering methods is prudent.
However, if you use RULA to quantify the risk of an easily observed worst task element (a specific event-based activity), the conclusions of this study are less applicable.
From 12 health care and manufacturing workplaces, 733 workers volunteered to participate in the study. The subjects included 350 females and 383 males who performed 201 different jobs.
The participants were divided into five categories based on job physical demands: sedentary work, light work, medium work, heavy work, and very heavy work.
Each subject was videotaped with two synchronized cameras as per work composition:
- 15 minutes for those performing cyclic single task jobs
- 20 to 40 minutes for those performing cyclic two to four task jobs
- 3 10-minute segments for those performing non-cyclic tasks
The video tapes were digitized and evaluated by one of two methods:
1) Event-based: for each job task, the most common posture and the worst posture for each body part (neck, shoulders, arms, hands and trunk) was determined and categorized into pre-defined classifications
2) Time-based: at randomly selected times, posture assessment was made of the body parts and assessed on a continuous angular scale; later, the continuous data was grouped and converted into the pre-defined classifications; the most common posture and worst posture for each body part were identified
A modified RULA approach was followed to score the postural positions. Muscle use, force/load, and lower extremity postures were ignored since this study was concentrating on the upper extremities. RULA scores were determined at the task level. At the job level, RULA scores were determined through a time-weighted averaging analysis.
Article Title: Two posture analysis approaches and their application in a modified Rapid Upper Limb Assessment evaluation
Authors: S Bao, N Howard, P Spielholz, and B Silverstein
Publication: Ergonomics, 50:12, 2118-2136, 2007
This article originally appeared in The Ergonomics Report™ on 2008-08-26.