Microcentrifuge Tube Ergonomics and the Required Force of Opening

A microtube that requires less force to open, coupled with a more ergonomic, perpendicular, and vertically extending tab, could reduce the occurrence of RSIs in laboratory personnel.

Repetitive Strain Injury or Repetitive Stress Injury (RSI), Cumulative Trauma Disorder (CTD), Repetitive Motion Disorder (RMD), and Overuse Syndrome are synonymous terms referring to a condition that can result in any environment where the operator of equipment continually performs the same motion with the necessary force to strain a particular body part. Most commonly, it is seen in the hand or forearm.1

This strain can vary in seriousness from slight, occasional discomfort to a compressed nerve. Treatment ranges from support aids to surgery. The Occupational Safety and Health Administration identified RSIs as one of the largest classes of workplace injuries.2 In a laboratory environment, handlers of pipettes and microcentrifuge tubes (microtubes) perform repetitious cycles of extracting/ejecting solutions and opening/closing microtubes during their day. These continual tasks can lead to dynamic ailments that affect the hand and forearm, such as carpal tunnel syndrome (CTS), DeQuervain's Syndrome or Tenosynovitis, and Tendonitis.3

Several studies have been conducted regarding the strain exerted on the thumb when working with automatic pipettes. These studies quantify the forces exerted on the users thumb.4,5,6 Results showed that for some individuals (often female), thumb strain is unacceptably high. Recommendations were to construct pipettes with as little button resistance as possible to reduce peak forces.5,7 Similarly, there is a need for laboratory microtubes that also reduce RSIs. Many of the most popular types can induce RSI during repeated opening. The purpose of this study is to quantify and compare the force exerted by the thumb while opening several microtubes currently available.

Materials and Methods
A microtube requiring less force to open, with a more ergonomic tab, could reduce occurrence of RSIs in laboratories.The 1.5 mL microcentrifuge tube was chosen for this study. To achieve a diverse sampling, several brands of microtubes were used in the experiment. Table 1 shows the characteristics of 1.5 mL microtubes tested. Microtubes were tested for force of opening, in newtons (N), with the following equipment: a Vernier Dual-Range Force Sensor (set for ±50N), Vernier LabQuest acquisition software, Logger Pro Data manipulation software, and a standard PC with Microsoft Excel.

All tested microtubes had approximately the same outside diameter of 10.65 ± 0.15 mm, with a length of approximately 40 mm. Microtubes were placed in holes with a tolerance of -0.25 mm to simulate a hand-held microtube. The force sensor, with a flat-tip hook, was positioned to exert an opening force on the microtube lip or tab. The hook was then steadily advanced until the lid opened.

The resulting maximum force of opening (sampling rate = 10 sec-1) was simultaneously recorded. The force of opening was either parallel (front lip type) or perpendicular (rear lid tab type) to the microtube and applied to the outermost location on the lip or tab. Positioning at the outermost point ensured the longest torque arm possible, allowing the fullest benefit of the lip or tab. Five trials of 25 new microtubes of each brand were tested.

The results of the experiment are tabulated with the standard deviation and mean in table 2. The microtube with the highest required force of opening was that of sample 1. The microtube with the lowest required force of opening was that of sample 5. The microtube that performed the best was sample 5. This tube had the least required force of opening (28 percent less than its closest competitor, sample 4) and the second-lowest standard deviation with a 1.3N degree of variability. This tube also ranked the highest qualitatively with regards to the ergonomics of use.

The data showed variability in microtubes. While all performed their intended functions well, some required a higher force of opening than other microtubes (see figure 1). A significantly increased force was seen when comparing the sample 1 microtube to the other brands. These microtubes required an average force of 33N to open, an average of 40 percent more force than their closest competitor. When comparing averages from samples 2, 3, or 4 with sample 5, there is a significant reduction in force (28-38 percent). When opening the sample 1 microtube, high resistances equate to a significant strain on the user's thumb and/or forearm. When continually cycled through exceedingly forceful openings, this can lead to an RSI.5

Table 1

Trial Manufacturer, Microtube Name Type of Opening Material Max. R.C.F. (G) Temp. Range (degrees C)
1 Robbins Scien., Locking Front lip Polypropylene Not reported Not reported
2 BioPlas, G-Tube Front lip Polypropylene 13,000 -80 to 121
3 VWR, Economy Front lip Polypropylene 20,000 -90 to 122
4 Eppendorf, Flex-Tube Front lip Polypropylene 25,000 Not reported
5 Microstein, Microstein Rear lid tab Polypropylene 20,000 -80 to 120

Bjorksten (1994) concluded that hand ailments among laboratory technicians are correlated to the amount (or, as he states, "dose") of pipetting an individual performs.6 He suggested that more repetition leads to increased occurrences of ailments. Because workload reduction is not an acceptable solution, reduction of button resistance and better hand fit (ergonomics) seem to be the only answers for pipettes, as suggested by Fredriksson (1995).

Table 2. Results of microcentrifuge tube force-of-opening experiment
Listed are the average (mean) force-of-opening and the standard deviations. This data was calculated from the data taken from 25 individual tests of each manufacturer's microtubes.

Robbins-Scien., Locking (N) BioPlas, G-Tube (N) VWR, Economy (N) Eppendorf, Flex-Tube (N) Microstein (N)
Trial 1 2 3 4 5
Standard Deviation (N) 1.538 0.570 2.314 1.537 1.338
Mean (N) 32.95 20.25 18.40 17.55 12.57

As for microtubes, an ergonomically designed opening is of great value. In this study, the ergonomics of the microtube opening were considered qualitatively and determined to be more beneficial when the opening force was applied perpendicular to the holding force (friction) than when parallel to it. Current discussion suggests that opposing parallel forces are not ergonomically correct for the human hand and thumb. This situation requires a strong grip in the hand while, simultaneously, the thumb is pulling the tube out of the grip. The sample 5 microtubes, with their unique rear lid tabs, that allowed for a more ergonomically oriented opening. This was attributed mainly to the fact that the hand's grip was not opposing the thumb's motion. Instead, the force of opening of sample 5 was exerted perpendicular to the direction of friction (grip). The tab was positioned for a more natural application of pressure at the back of the tube without the need for an awkward abducting, "side-flicking" thumb motion, a potential cause of DeQuervain's Tenosynovitis.1

In conclusion, a microtube that requires less force to open, coupled with a more ergonomic, perpendicular, and vertically extending tab, could reduce the occurrence of RSIs in laboratories. This is due to a more favorable positioning of the hand and thumb while opening this type of microtube. Accordingly, a lower force of opening and more ergonomically placed tab can offer significant relief to laboratory personnel who experience strain. These data could be useful in designing standard laboratory protocols to minimize RSIs.

1. National Institute of Environmental Health Sciences (NIEHS), National Institutes of Health (NIH). Ergonomic Disorders Commonly Found Among Laboratory Personnel, Chapter 1. Health and Safety Guide to Laboratory Ergonomics. http://www.chem.purdue.edu/chemsafety/SafetyClass/Injury/lecture/NIEHS.htm

2. McGlothlin, J. D., Hales, T. R. 1996. Health Hazard Evaluation Report; HETA 95-0294-2594; Science Applications International Corporation; NIOSH, U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, Frederick, MD.

3. Hoskins, D. B., Erickson, J., 1998. Laboratory Ergonomics, The Wake-up Call: A Case Study. Chemical Health & Safety, American Chemical Society 5(1): 20-23, 38.

4. Erickson, J. G., Smith, A. V. 2007. A Biopharmaceutical Breakthrough. Occupational Health & Safety magazine, 76(6).

5. Fredriksson, K. 1995. Laboratory work with automatic pipettes: a study on how pipetting affects the thumb. Ergonomics 38(5): 1067-1073.

6. Bjorksten, M. G., Almby, B, Jansson, E. S. 1994. Hand and shoulder ailments among laboratory technicians using modern plunger-operated pipettes. Applied Ergonomics 25(2): 88-94.

7. Asundi, K. R., Joel, B. M., Rempel, D. M. 2005. Thumb force and muscle loads are influenced by the design of a mechanical pipette and by pipetting tasks. The Journal of the Human Factors and Ergonomics Society, 47(1): 67-76.


This article originally appeared in the July 2010 issue of Occupational Health & Safety.

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