More Grip, Less Force

And less grip force can lower the risk of Carpal Tunnel Syndrome.

GRIP is a safety issue in many industrial environments. During in-depth analyses conducted at more than a thousand workstations in more than 50 major manufacturing industries in Europe, grip in either wet or dry conditions was perceived as a need by workers at some 89 percent of the workstations.1

Manufacturers surveyed often cited poor grip as a serious safety hazard because objects that slip or are dropped could result in cuts and other injuries and damage to expensive equipment. Musculoskeletal disorders, including Carpal Tunnel Syndrome, also were cited as concerns relative to grip and the force required for workers to maintain a secure grip.

Grip Involves Many Factors
Gripping or holding an object involves a balance between the downward load, which results from the weight of the object, and the upward lift force provided by a worker's arm muscles, including the shoulder, elbow, and wrist.2 Grip relies on sufficient friction between the object and the hand. When friction is insufficient and objects are not supported from beneath, the worker will compensate by exerting greater grip force. Grip friction between the object and the hand usually decreases when excessive moisture is present--especially lubricants and oils.

Because workers are concerned about maintaining a safety margin when they grasp an object, they tend to predict how the object they are handling is going to behave. This behavior depends on the interface between the object and the hand. When the object begins to slip from the worker's grasp, the worker must apply more grip force to regain control.

Workers have a tendency to normally apply 30 to 40 percent more grip force than the minimum required to prevent slip. In effect, they are maintaining a safety margin against slip, which probably reflects uncertainty about the slipperiness of the surface.

If the object is slippery due to the lubricating effects of water, oil, or other chemicals on the grasp surface, the worker has to apply even more grip force--even though the load force has not changed. This increase in grip force may occur after the object slips, drawing attention to inadequate grip force. Reacting to a slip, however, takes time. In many instances, workers will anticipate the potential for slip and will exert a higher grip force as a precaution.

Individuals also are likely to exert more force if they anticipate that certain actions while handling the object, such as climbing stairs or raising the arms, will result in an inertial load due to accelerating the object upward, which adds to the normal downward pull of gravity.

In industrial facilities, workers often wear rubber gloves to protect their hands while performing tasks in wet conditions. While the gloves serve their protective function, when they become wet they have substantially less friction than when they are dry, making it necessary to use a higher grip force to maintain a stable grasp.

For employees working bare-handed, the friction developed when grasping objects depends on the hydration of the skin. Younger workers tend to have more adequate skin hydration to facilitate gripping. As workers age, however, skin hydration decreases, so that a worker older than 50 may have less than half of the grip friction of workers in their twenties. Thus, older workers usually must grip objects harder to maintain the same safety margin as their younger counterparts. Paradoxically, too much hydration (such as when workers sweat) also can present a problem in lowering grip function.

Whether workers are wearing rubber gloves or performing tasks bare-handed, grip could be improved by increasing the friction on the grasp surface, either by changing the texture on the gloves or on the objects.

Force and CTS
According to the U.S. Centers for Disease Control and Prevention, evidence exists linking forceful work with CTS.3 CTS can result when the median nerve, which extends from the forearm into the hand, becomes compressed or squeezed at the wrist. The carpal tunnel is a narrow, rigid passageway located at the base of the hand that houses most of the nerves and tendons in the hand.

When workers perform repetitive tasks that cause exertion in the hand and/or wrist, the tendons may become irritated and swell or thicken, resulting in a narrowing of the tunnel and compression of the median nerve. The situation is worsened when force is applied.

Individuals who have CTS experience pain, weakness, or numbness in the hand and wrist, which travels up the arm. They also have decreased grip, making it difficult to form a fist, grasp small objects, or perform other manual tasks. CTS, which develops over months and even years, is the second-most-prevalent musculoskeletal disorder in the United States after sprains and strains. While women are three times more likely than men to develop CTS because the carpal tunnel itself may be smaller, CTS is not limited to a specific industry or job. It is, however, especially prevalent among workers performing assembly-line tasks in manufacturing, sewing, finishing, meatpacking, and cleaning operations.

CTS is painful for workers and costly for companies in terms of medical expenses and lost time and productivity. Injuries related to CTS result in an average 25 days away from work per employee, which is longer than usually required for amputations and fractures. The total direct costs for all musculoskeletal disorders in the United States exceed $50 billion each year.

New Technology for Gripping
A new glove technology allows workers to apply significantly less force and still effectively grip wet or oily objects. The technology involves adding a roughened surface comprised of microscopic channels in a patented ultra-thin nitrile coating to gloves to direct fluids away from the grip surface.

The technology works in a manner similar to tire treads cutting through water on a roadway, with the channels propelling oils and other lubricants away from the pressure surface. This provides a relatively dry contact area that allows workers to maintain almost the same grip in oily applications they would have in dry conditions. Validation tests were conducted on gloves embodying the new technology at the University of Birmingham's Behavioural Brain Sciences Centre in Birmingham, England. During the studies, engine oil was applied to lubricate the surface of a steel shaft. When test participants wore standard high-quality nitrile gloves, they found it difficult to maintain control of the shaft and were unable to keep the object from slipping. Conversely, when participants wore gloves with the advanced-technology gripping surface, they were able to maintain a secure grip, with significantly less muscle tension required to prevent the shaft from slipping. Similar tests carried out by the manufacturer showed that gloves incorporating the new technology required from 34 to 69 percent less grip force to lift a 10-pound object. As illustrated in Figure 1, 7.7 pounds of grip force were required to lift a 5.5-pound oily round bar when workers wore gloves with the advanced gripping surface. This compared to 14.3 pounds of grip force required to lift the same object when comparable gloves without the new gripping surface were worn. Figure 2 reveals the results when similar tests were conducted using an oily plate.

During laboratory tests, measurement of the force needed to lift a 2.5-pound weight showed that test participants wearing the gloves with the new gripping technology were able to accomplish the task using significantly less grip force. Comparisons were made with both power and precision grasps on oily surfaces, with similar results. The studies were replicated in industrial applications to confirm the results.

Muscle recordings showed that the reduction in grip force is linked to significantly reduced activity in the finger flexor muscles when workers lift and grip objects while wearing gloves with the advanced gripping surface. The studies revealed that workers who wore gloves with the new gripping surface were not only able to achieve a more secure grasp initially, but they also were able to maintain that grasp when there was an unexpected increase in the object's weight.

The tests also confirmed the advanced gripping surface made it easier for workers to regain control of lubricated objects that slipped in their grasp, which is projected to increase hand comfort and elevate workers' confidence.

Overall, more than 83 percent of workers who participated in the tests conducted at 25 major industrial companies throughout Europe indicated they felt they could perform tasks better when wearing gloves that incorporated the new gripping technology. Lab tests showed participants were quick to adapt to the advanced gripping surface and were not as likely to exert excessive grip force once they felt confident in their ability to grasp the objects securely. This same behavior is expected to translate to the workplace.

Studies have shown that exerting excessive force while gripping objects is an aggravating factor that can lead to increased worker stress, fatigue, muscle strain, and musculoskeletal disorders. A new advanced gripping surface is available that eases the grip force required by workers when grasping slippery objects, whether they are working with oil, oil-based chemicals, water-based lubricants, or other greasy coatings. Tests suggest the new gripping technology increases safety by reducing the incidence of CTS and preventing slips.

This article appeared in the April 2006 issue of Occupational Health & Safety.

This article originally appeared in the April 2006 issue of Occupational Health & Safety.

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