Hand forces required to perform simulated laboratory tasks were found to be two to six times greater when using an axial designed pipette compared to a non-axial pipette in a recent United States based study. Thumb forces were, in general, one third less when employing the non-axial model.
When evaluating upper extremity posture, the non-axial pipette required significantly less:
- wrist ulnar deviation for most of the pipetting task cycle
- wrist extension when performing the "aspirate sample" activity
- shoulder flexion/abduction above 45º for most of the pipetting task cycle
Subjects had to assume a constant forearm pronation position to operate the non-axial pipette (for most of the task cycle, the forearm was pronated 2.5 times greater than when the axial pipettes were used). The authors felt this was a favorable finding by citing other researchers who determined the lowest pressure within the carpal tunnel occurred at the postural position of 45º forearm pronation and 45º metacarpophalangeal flexion (Rempel et al.).
The mean total cycle time was less with the non-axial pipette (8.8 seconds compared to 9.8 and 9.1 seconds for the axial pipettes) suggesting slightly greater efficiency in the alternative model.
The Bottom Line – How This Applies To Ergonomists
The design of a tool can lessen ergonomic risk factors as well as optimize cycle time. The non-axial pipette appears to require less force and promote more favorable postural positioning – significant concerns in reducing the risk of forearm/wrist/hand musculoskeletal injury/illness. Also, the non-axial pipette may improve the business performance by increasing production.
However, it should be realized that this study involved only two axial pipettes. Other axial pipettes may require lower hand forces than the models used in this research and impact conclusions regarding non-axial superiority.
Also, this study considered only pipette-related force/posture and cycle time. Laboratory technicians may prefer a specific axial pipette for reasons not evaluated in this experiment – i.e., a specific feature (easy to hold while performing complex hand activity) or task characteristic (electronic device to perform many repeat aspirations).
Other Key Study Findings
1) Regardless of pipette design, tasks required:
- wrist ulnar deviation in the range of 12º to 22º
- wrist extension in the range of 23º to 41º
2) Significantly greater ulnar deviation was necessary to perform the "eject tip" activity with the non-axial pipette.
3) The highest hand grip force was needed when using the axial pipettes for the "pick up tip" activity.
4) One of the axial pipettes, the Eppendorf AG, required much greater thumb force upon tip ejection and overall, greater hand force to operate.
Study Design
Subjects
Twelve, right-handed, laboratory technicians with an average of 16 years experience volunteered for the study. No subject had a history of hand injury.
Pipettes Tested and their Characteristics
|
Features
|
Pipettes
|
||
|
Oxford Benchmate II
|
Eppendorf AG
|
Ovation BioNatural
|
|
|
Design
|
Axial |
Axial |
Non-axial |
|
Volume range
|
200-1000 µL |
200-1000 µL |
200-1000 µL |
|
Separate tip ejection button?
|
Yes |
No – used one plunger for both aspirating sample and ejecting tip |
Yes |
|
Length of model without tip
|
24.5 cm |
24.1 cm |
14.3 cm |
|
Weight
|
113 g |
98 g |
156 g |
|
Handle diameter
|
2.47 cm |
2.48cm |
4.16 cm |
A leather glove with 19 profile sensors was worn to detect the contact pressure between a subject’s hand the pipette. The glove was available in three sizes and wires did not interfere with hand/wrist motion during the testing procedures.
Total hand force was calculated in a time series manner by summing the forces recorded from all 19 sensors. Total thumb force was determined by adding the forces from two thumb sensors captured at time intervals.
Wrist and forearm postural positions were measured with an electrogoniometer and a torsiometer.
Subjects were video-recorded from a sagittal view and to assess shoulder posture following an observational method.
Tasks Performed
Each volunteer performed two simulated pipetting tasks (that were actually done on a daily basis) with each pipette at two pipette sample volumes (200µL and 1000 µL) and from two positions (standing and sitting).
The workstation was located within the subjects’ laboratories with a work surface height of approximately 91 cm. For the sitting test, a volunteer could adjust the vertical height of the chair while for the standing test, subjects assumed a comfortable standing position.
Data Analysis
A task cycle was divided into five activities:
- Pick up pipette
- Pick up tip
- Aspirate sample
- Overblow sample
- Eject tip
For each task cycle activity, forces at the hand and thumb were co-related with the postural positions of the wrist (flexion/extension, ulnar/radial deviation) and forearm (pronation/supination). Also, both the percent of cycle time the humerus was elevated greater than 45º and the total task cycle duration were calculated.
Article Title: An investigation of hand forces and postures for using selected mechanical pipettes
Publication: International Journal of Industrial Ergonomics, 38, 18-29, 2008
Authors: M-L Lu, T James, B Lowe, M Barrero, and Y-K Kong Measurements
This article originally appeared in The Ergonomics Report™ on 2008-07-30.