In further refining manual material handling guidelines determined through psychophysical methods (Liberty Mutual Tables developed by Snook and Ciriello, 1991), Ciriello recently tested the impact of the following variables on female maximum acceptable lifting weight:
1. Box size (in the original study, a small box with a 17cm chest-hand distance was used; this study used a large box with a 38 cm chest-hand distance).
2. Extended horizontal reach (chest-hand distance of 44.6 cm) in the mid lift and center lift vertical height zones.
3. High frequency (1 lift every 3 seconds) in the mid lift zone.
Key study findings included:
· The 1991 Liberty Mutual table (by Snook/Ciriello) is accurate for acceptable female lifting weight for large as well as small boxes when performing fast lifting frequencies (1 lift every 5 seconds up to 1 lift every 2 minutes)
· The large box produced weight deviations from the 1991 Liberty Mutual table at slower lifting frequencies including:
v 7% higher acceptable lifting weight for lifting frequency of 1 lift every 5 minutes
v 15% higher acceptable lifting weight for lifting frequency of 1 lift every 30 minutes
v 13% higher acceptable lifting weight for lifting frequency of 1 lift every 8 hours
· An extended horizontal reach produced weight deviations from the 1991 Liberty Mutual table of:
v 22% lower acceptable lifting weight for lifting frequency of 1 lift every 1 minute performed in the mid lift range (9.9 kg)
v 18% lower acceptable lifting weight for lifting frequency of 1 lift every 1 minute performed in the center lift range (9.8 kg)
· A high lifting frequency produced a weight deviation from the 1991 Liberty Mutual table of:
v 47% lower acceptable lifting weight when lifting with a frequency of 20 lifts every 1 minute compared to a frequency of 1 lift every 1 minute performed in the mid lift range (6.0 kg)
Study Design
The volunteers were 10 females with manual material handling experience and no history of significant low back/extremity musculoskeletal symptoms or cardiovascular disorders.
Tote Box Size
Two different sized tote boxes were used with dimensions described in Table 1.
|
|
Width (cm) |
Length (cm) |
Depth (cm) |
|
Small box |
33.4 |
56.2 |
16.0 |
|
Large box |
76.1 |
56.5 |
22.0 |
Width = box distance in the plane away from the body
Length = distance between box handles
The handle placement produced a minimum horizontal hand distance from the chest of 17 cm for the small box and 38 cm for the large box.
Table 1: Description of tote boxes.
Lift Location and Range
A tote box was placed on a movable shelf. After the subject lifted the box from the shelf, the shelf automatically repositioned a vertical distance of 51 cm in the middle of one of three ranges:
-
low lift – between floor level and knuckle height
-
mid lift – 25.5 cm above and below knuckle height
-
center lift – between knuckle height and shoulder height.
The subject then placed the box on the shelf at the new vertical height.
Extended Horizontal Reach
Through use of wooden and plastic barriers, a horizontal reach of 44.6 cm (from the front of the body to the hands) was applied when lifting the small box for select lift frequencies and lift ranges.
High Frequency Lifts
Using the small box, a frequency of 20 lifts/minute through the mid lift range was tested.
Summary of Tested Variables
Several variations relative to box size, lift range, lift frequency, and reach were tested. Table 2 describes the tests performed.
|
|
Large Box |
Small Box |
||
|
Lift Range |
Low lift |
Low lift |
Mid lift |
Center lift |
|
Frequency (one lift per) |
5 sec, 9 sec, 14sec, 1 min, 2 min, 5 min, 30 min, 8 hr |
1 min |
1 min |
1 min |
|
Frequency with extended horizontal reach (one lift per) |
– |
– |
14 sec, 1 min |
14 sec, 1 min |
|
High frequency (one lift per) |
– |
– |
3 sec |
– |
Table 2: The tested combinations of lift ranges, frequency, and modified reaches for a given tote box size.
Protocol similar to prior psychophysical Liberty Mutual (Snook/Ciriello) lifting procedures to determine maximum acceptable weight was followed. Subjects:
· worked as hard as they could without straining themselves
· could vary tote box weight by adding or removing steel shot
· had minimal visual cues as to tote box weight due to the existence of a false bottom that contained weight
Maximum acceptable weights from this study were compared to findings from prior Liberty Mutual (Snook/Ciriello) research.
Article Title: The effect of container size, frequency and extended horizontal reach on maximum acceptable weights of lifting for female industrial workers
Publication: Applied Ergonomics 38: 1-5, 2007
Author: V M Ciriello
This article originally appeared in The Ergonomics Report™ on 2007-07-06.