Lifting experience had the greatest influence on subject perceived discomfort when performing a variety of lifting tasks. Novice lifters reported much higher discomfort levels as the moment arm increased and as task duration increased compared to experienced subjects. Parakkat et al. felt this finding was due to a combination of characteristics among experienced lifters:
- a learned motor pattern (conditioned and efficient use of muscle to perform a lift)
- association of reporting discomfort with complaining about work conditions
The authors also found that under similar spinal compression loads, the novice lifters reported much higher levels of discomfort compared to experienced lifters. Hence, biomechanical measurements may be misleading relative to degree of worker physical complaints.
Study Design
Twenty-four volunteers (12 experienced with manual material handling from local grocery stores and distribution centers/average of 23; 12 inexperienced with manual material handling from the university community/average age of 24) with no prior history of low back pain were the study subjects.
A common industrial style lifting task was performed in the laboratory simultaneously by two subjects with the same experience and characterized by:
- an origin vertical height of 88 cm
- a destination vertical height of 121 cm
- an origin moment arm distance of 74 cm
- an asymmetry motion of 90 degrees
The subjects handled only one of three initial static load moment levels (8, 36, or 85 Nm generated by loads of 1.1, 4.9, or 11.7 kg respectively) but were tested at six frequency levels (2, 4, 6, 8, 10, and 12 lifts/minute). The frequency level was applied for 8 hours with two 15-minute breaks and a 30-minute lunch break. Although the frequency of lift was controlled, the speed of lift was determined by the subject. At least one day of rest separated the six test sessions.
A discomfort survey was administered every hour during a test session. The subject was asked to rate level of discomfort on a scale of 0 (no discomfort) to 6 (high level of discomfort). Although all body areas were evaluated, the upper back and lower back were scores of interest for this study and in analysis, scores were developed from two hour time blocks.
Other Findings
For all study variables:
- inexperienced lifters reported higher levels of discomfort compared to experienced lifters
- similar levels of discomfort were described by experienced lifters
At the 85 Nm moment level, there was a significant association between discomfort and:
- task duration within each of the six lift frequency rates
- select two hour time blocks (the 2-4, 4-6, and 6-8 hour periods) as lift frequency increased
Study Limitations
Reservations effecting of this research include:
- the impact of deep lumbar muscles was excluded when estimating spinal loading
- subjects were adjacent to each other during the testing times; their conversation was not controlled and may have influenced their discomfort scoring
Article Title: The influence of lift frequency, lift duration and work experience on discomfort reporting
Publication: Ergonomics 50(3): 396-409, 2007
Authors: J Parakkat, G Yang, A-M Chany, D Burr and W S Marras
This article originally appeared in The Ergonomics Report™ on 2007-05-11.