Applying five different analysis methods to a repetitive, multi-vertical height lifting task led to mixed consistency in predicting the level of task musculoskeletal exposure risk. According to Russell et al., the different exposure risk assessments reflected the strengths, limitations and underlying assumptions of each tool.
Despite moderate variation, similar results occured when applying:
- The revised 1991 National Institute of Occupational Safety and Health lifting equation (NIOSH)
- The American Conference of Governmental Industrial Hygienists lifting threshold limit values (ACGIH TLV)
- The Liberty Mutual “Snook” lifting Tables (Snook)
The Washington State ergonomics rule lifting calculator (WA L&I) predicted a lower level of risk compared to the prior three tools mentioned. The authors noted this was likely due to the intended application of the WA L&I as a regulatory tool with the purpose of identifying only moderate to high exposure lifts.
Most divergent of the five analysis methods was the University of Michigan 3D static strength prediction program (3DSSPP). The tool indicated that the task had a much lower exposure risk than NIOSH, ACGIH TLV, and Snook. This difference in risk assessment, the authors pointed out, may have resulted from the analysis method’s lack of considering:
- metabolic load
- shoulder strength capabilities
- cumulative effect of repetitive compressive forces
- dynamic compressive force impact
Study Design
The Subject
The study volunteer was a 95th percentile (height – US) male employee who was experienced in performing the task. The subject’s height and weight were recorded while other measurements (i.e., shoulder height, waist height, knuckle height, knee height, mid-shin height, ankle to toe distance) were based on the 95th percentile values of US males.
The Task
Stacked cases of milk were pulled off of a pallet, slid across a floor, and re-stacked up to seven cases high in a grocery store dairy cooler. This study evaluated the restacking of the milk cases in the cooler. There was a possibility of seven vertical origin heights and seven vertical destination heights. Control of the case at the destination was not needed.
Two different milk case sizes were used in this study as described in Table 1.
|
Characteristic
|
23 1iter case
|
15 1iter case
|
|
Weight
|
26 kg
|
17 kg
|
|
Wide
|
33 cm
|
33 cm
|
|
Long
|
48 cm
|
33 cm
|
|
Tall
|
28 cm
|
28 cm
|
|
Handle distance from bottom of case
|
25 cm
|
25 cm
|
|
Cases to a pallet
|
30
|
45
|
|
Pallet stacking
|
3 rows wide/5 cases high/2 rows deep
|
3 rows wide/5 cases high/3 rows deep
|
Table 1: Characteristics of containers used in the study.
Selected Tools and Analytical Method Inputs
As mentioned, five commonly used lifting analysis methods (NIOSH, ACGIH TLV, Snook, 3DSSPP, and WA L&I) were applied to the lifting tasks.
Measurements and values (i.e., horizontal distance, load/trunk asymmetry value relative to mid-sagital vertical plane, vertical zone of the lift, vertical distance traveled, and lifting frequency) as needed by the different tools were collected in accordance with how the lifting tasks were performed. Vertical height/horizontal distance data (where the hand grasped the case) are provided in Table 2.
|
Vertical Height in cm
(floor to handle height)
|
Horizontal Distance in cm
(as defined by the tool)
|
|
25
|
23
|
|
53
|
23
|
|
81
|
30
|
|
109
|
30
|
|
137
|
30
|
|
165
|
30
|
|
193
|
30
|
Table 2: Hand grasp task measurements of horizontal distance for a given vertical height.
Key Outcome Measurement
To allow for analysis/comparison of a tool’s risk assessment of the lifts, an Exposure Index (EI) quantification was created (very similar to the NIOSH Lifting equation Lifting Index).
For a given analytical method (i.e., ACGIH TLV), a recommended weight was calculated for each of the seven lifting origins by averaging the recommended weight of all possible lifting combinations (relative to that origin). The average recommended weight for each lifting origin was used as the denominator value relative to the milk case weights (numerator values of 17 kg and 26 kg) to produce the EI. An EI greater than 1.0 reveals the extent to which the load exceeds a recommended weight. This approach was used for NIOSH, ACGIH TLV, Snook, and WA L&I.
For the 3DSSPP, the EI was calculated by using a denominator value of 349 kg (amount of force indicated as acceptable for 75% of the female population and 99% of the male population). The numerator value was the average back compressive force (kg) for all possible lifting combinations for a given origin.
Table 3 presents the EI calculations.
|
Vertical Height
Lift Origin (cm)
|
Tool
|
|||||||||
|
NIOSH
|
ACGIH TLV
|
Snook
|
3DSSPP
|
WA L&I
|
||||||
|
15 l
|
23 l
|
15 l
|
23 l
|
15 l
|
23 l
|
15 l
|
23 l
|
15 l
|
23 l
|
|
|
25
|
1.17
|
1.79
|
1.23
|
1.88
|
1.58
|
2.15
|
1.05
|
1.25
|
0.57
|
0.87
|
|
53
|
1.14
|
1.74
|
0.96
|
1.46
|
1.58
|
2.15
|
0.85
|
0.88
|
0.57
|
0.87
|
|
81
|
1.15
|
1.75
|
1.23
|
1.88
|
1.58
|
2.15
|
0.54
|
0.76
|
0.44
|
0.68
|
|
109
|
1.18
|
1.80
|
1.08
|
1.64
|
1.58
|
2.15
|
0.43
|
0.53
|
0.44
|
0.68
|
|
137
|
1.24
|
1.89
|
1.08
|
1.64
|
1.58
|
2.15
|
0.36
|
0.50
|
0.57
|
0.87
|
|
165
|
1.34
|
2.05
|
2.46
|
3.75
|
2.11
|
2.76
|
0.41
|
0.54
|
0.61
|
0.93
|
|
193
|
1.52
|
2.32
|
2.46
|
3.75
|
2.11
|
2.76
|
0.41
|
0.55
|
0.61
|
0.93
|
Table 3: EI calculations of each tool, at each vertical height origin, for both the 15 and 23 liter cases.
Other Findings
Significant study points included:
- All analytical tools predicted a greater musculoskeletal risk with the heavier load (23 liter/26 kg) versus the lighter load (15 liter/17 kg).
- The greatest risk exposures were associated with the higher vertical origin lifts for all tools except the 3DSSPP.
- The authors felt if they had included shoulder strength capabilities into the 3DSSPP model, the tool’s exposure predictions would have been more consistent with the other analytical models.
- Although ACGIH TLV and Snook were easier to apply in the work setting, NIOSH risk predictions were more sensitive to incremental changes in lift input values.
Study Limitations
Reservations effecting of this research include:
- The study population was limited to one 95th percentile male
- The nature of the task was limited to multi-vertical height, repetitive lifting
Article Title: Comparing the results of five lifting analysis tools
Publication: Applied Ergonomics 38: 91-97, 2007
Authors: S J Russell, L Winnemuller, J E Camp, and P W Johnson
This article originally appeared in The Ergonomics Report™ on 2007-09-20.