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Thumb force and muscle loads are influenced by the design of a mechanical pipette and by pipetting tasks.

Article Excerpt
INTRODUCTION

The plunger-operated mechanical pipette, which allows users to transfer precise volumes of liquid between containers, is one of the most commonly used hand tools in the laboratory. Use of the pipette often requires careful and precise control, which can be affected by discomfort, pain, or other musculoskeletal disorders. Bjorksten, Almby, and Jansson (1994) compared 128 Swedish female laboratory employees with 25,378 Swedish female state employees. They found an increased risk of hand (odds ratio = 5.0) and shoulder (odds ratio = 2.4) ailments associated with pipetting for more than 300 hr per year. The average amount of pipette work for laboratory technicians was 495 hr per year. Based on a survey of 80 pipette users (David & Buckle, 1997), 90% of users who pipetted continuously for more than 60 min reported hand complaints. Plunger operation and tip ejection were identified as features that made pipette operation difficult. Use of the pipette involves several risk factors associated with upper extremity musculoskeletal disorders, including repetition, force, and posture (National Research Council and Institute of Medicine, 2001). Fredriksson (1995) measured the force required to operate a pipette as a percentage of the maximum force with which participants could press on a pipette. She found that for female employees the peak force required to operate the pipette was 18.4% of maximum force, whereas for male employees it was 14.5% of maximum force.

Few studies have investigated the force applied to a manual pipette and the role of thumb muscles during pipetting, and no study, to our knowledge, has investigated how latch pipettes affect thumb muscle activity. The purpose of this study is to quantify thumb loading and muscle activity and determine whether they vary between common pipetting tasks. Although pipetting in general involves a high level of precision, some pipetting tasks require more precision than others. Several studies have demonstrated that the precision and attention required to complete a task can influence the level of muscle activity (Milerad & Ericson, 1994; Visser, Looze, Graaff, & van Dieen, 2004). This study will also investigate how latch pipettes, which are designed to reduce thumb activity, affect muscle activity. We hypothesize that high-precision pipetting tasks will involve greater muscle activity of the stabilizing muscles and an increase in duty cycle, that pipetting high-viscosity fluids will increase the thumb force applied to the plunger and that use of a latch pipette will lower the muscle activity of the muscles that move the thumb.

METHODS

In this laboratory study, participants completed nine pipetting tasks while the thumb force and muscle activity were measured.

Participants

Fourteen participants (9 men and 5 women) were recruited to the study. Their mean age was 34 years (SD = 7.8). All participants were experienced with the use of pipettes (7.4 years, SD = 5.9) and worked in jobs that involved an average of 5.5 hr (SD = 4.8) of pipette use per week. All participants used pipettes with their right hand. Five of the participants reported previously having discomfort in the right arm, wrist, or thumb during pipetting but were pain free at the time of the study. The study was approved by the University of California at San Francisco Committee on Human Research.

Electromyography

Surface electromyography (EMG) bipolar recordings were obtained using circular Ag/AgCI electrodes with an active diameter of 8 mm and a center-to-center distance of 21 mm. Sites on the right forearm for the placement of the electrodes were localized with the forearm in a pipetting posture, using recommended anatomical placement (Delagi, Perotto, lazzettim, & Morrison, 1981). Four thumb movers, muscles that create torque and articulate the joints of the thumb, were studied. These were the extensor pollicis brevis (EPB), abductor pollicis longus (APL), flexor pollicis longus (FPL), and abductor pollicis brevis (APB). The skin was abraded and cleaned with an alcohol pad and shaved if necessary. A ground electrode was placed over the lateral epicondyle. The EMG signals were amplified with preamplifiers and an adjustable gain amplitier and were high-pass filtered at 75 Hz (Therapeutics Unlimited, Iowa City, IA) in order to reduce 60-Hz noise.

Maximum and Reference Voluntary Electrical Activity

Both maximum voluntary electrical activity (MVE) and reference voluntary electrical activity (RVE) were obtained during isometric maximum voluntary contraction (MVC) and isometric reference voluntary contraction (RVC), respectively. Three MVC tasks were performed to generate the...

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