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Many tasks in office, manufacturing, health care, and housework require workers to grip something. This could be the mouse, a pen, a gardening tool, a syringe, or a broom. In all these examples, workers are interacting with equipment that requires force to be applied with some of the smallest muscles in the human body, those in the hands and fingers.
Studies have linked forceful grips, prolonged grips, and grips performed with awkward postures to musculoskeletal disorders (MSDs) like DeQuervain's syndrome, "trigger finger", arthritis, tendonitis, and even carpal tunnel syndrome (CTS). Increased awareness of this relationship has led to the development of analysis tools to better quantify risk exposure and also new designs in products like hand tools. For instance, in 1975 researchers Pheasant and O'Neil investigated handle design in gripping and turning tasks, such as those that might be performed with a screwdriver. They found that the force a worker could exert decreased as the handle diameter increased past 5cm. In 1997, Ducharme found that women working in what was previously a male craft with the US Air Force reported that tool grips were too wide for their hands and often required two hands to operate.
In addition to having a grip that is too big, awkward postures affect the amount of force that a worker can produce. When the wrist is in extreme flexion, extension, or deviation, grip strength diminishes. While this is not widely know to those who design workspaces and equipment, the knowledge that flexion in the wrist decreases grip strength is a key element in martial arts and self defense teachings. Before disarming an attacker, the hand holding a weapon must first be twisted (pronated) and the wrist flexed.
Wearing gloves can also affect grip strength. While some gloves are designed to increase friction thereby providing a better grip, gloves that are too big or of an inappropriate material may cause the worker to apply more force while gripping.
In a study by Stover H. Snook and Vincent M. Ciriello at the Liberty Mutual
Research Center for Safety and Health, Psychophysics was used to determine
maximum acceptable torques at the wrist using pinch and power grips. A
summary of results below shows that 1.9 pounds of downward force at the end
of the fingers is acceptable to 90 % of the female population while
performing a wrist flexion using a pinch grip at a frequency of 5 times a
minute. However, using a power grip, the maximum acceptable downward force
at the middle of the palm goes up to 3.3 pounds. The table also shows the
pounds of force acceptable to 10% of the female population.
Flexion
Using A Pinch Grip
Population | 5/min Repetition Rate |
90% Female
Population | 1.9 |
10% Female
Population | 6.3 |
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Flexion Using A Power Grip
Population | 5/min Repetition Rate |
90% Female
Population | 3.3 |
10% Female
Population | 11.2 |
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This data is available from Snook, S.H. and Ciriello, V.M., "Psychophysical studies of repetitive wrist flexion and extension." Ergonomics, 1995, vol.38, No.7, 1488-1507.
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