To Grip and Not to Slip
Hand function and protective gloves
Hand function and protective glovesIn work and play we use our hands to lift and move objects hundreds of times every day. Keeping objects stable while grasping them is crucial.
Stability depends on grip force directed into the grasp surface relative to the load force along the surface, and also the friction between the hand and the object. A combination of predictive and reactive control sets grip force appropriately and this allows us to keep a stable grip when manipulating objects that range widely in properties including mass, size, shape, surface texture. This article summarises methods used to investigate the control of grip and discusses the effects of gloves on handling objects.
The simplest way to hold an object is to use the hand to support it from underneath (see Figure 1a). If the downward load force due to object weight is matched by the upward lift force provided by the muscles of the arm, the object will be held safely in the hand. However, when an object is picked up from the floor or a table, the underside is not accessible and the object must be grasped using the sides. This commonly involves using one or more fingers and the thumb in precision grip with pads of the fingers pressing on one side of the object against an opposing force produced by the thumb on the other (see Figure 1b).
Grip force, perpendicular to the surface of the object at each point of digit-object contact, allows the development of friction which helps to overcome the weight of the object as lift force is applied tangentially along and up each contact surface. Provided the grip force is large enough for the coefficient of friction between the digits and the object, lift force summed over the contacting digits will exceed the load due to the weight and the object will be moved upwards.
From physics, the Coulomb model of friction tells us that the tangential lift force at each contact point with the object is limited to the coefficient of friction multiplied by the grip force perpendicular to the surface at that point, otherwise slip will occur. Thus the coefficient of friction between the digits and the points of contact with the object determines the size of the grip force required to hold it. The lower the friction coefficient, the greater must be the grip force.
People generally hold objects using a grip force that is about one third more than the minimum required to prevent slip. If friction is reduced, for instance because oil has made the grasp surfaces slippery, holding the object will require more grip force. Thus the work done in holding the object securely will be greater even though the weight is unchanged. Latex rubber gloves are often worn to protect the hands when handling objects under wet or oily conditions.
However, rubber gloves have lower friction when wet which makes grip less secure. One strategy to remedy this situation is to try and increase friction by adding texture to the grasp surfaces, that is, either on the glove or the object. Later in this article we consider an example of new glove making technology which has been used to improve the friction afforded by rubber gloves under wet or oily conditions and reduce the grip force required for stable grasp.
Swedish researchers have shown that using precision grip to lift objects involves increases in grip force that anticipate rises in load. People lift familiar objects of different weight in similar amounts of time, typically taking about one third of a second from contact to lift-off. This implies that they remember the weight of the object to generate the appropriate rise in lift force, since the weight cannot be sensed directly until the object is free of the support.
At the same time, people increase their grip force at a rate matched to the rise in lift force in order to prevent the digits slipping during the lift. Control of grip force is therefore said to exhibit predictive compensation for the changes in load during the lift.
In the Swedish studies it was also found that changing the surface texture (eg from higher friction sandpaper to lower friction suede) resulted in rapid adaptation in grip-load coordination. For instance, on experiencing suede after a series of sandpaper trials, grip force was initially too low, given the reduced friction, and this resulted in small slips which were quickly corrected by increasing grip force using sensory feedback from the cutaneous contact. After one or two trials of lifting with the new surface, the initial rise of grip force was increased to be appropriate to the lower friction surface. This shows that people predict frictional as well as load characteristics in lifting.
Subsequently a Belgian research group showed that when friction was lowered by applying talc people used slower actions with reduced lift force rates. The Coulomb friction model does not include any dependence on force rate so one might ask what is gained by lifting more slowly under slippery conditions? One possible answer is that it gives more time for the detection of slips and use of feedback to increase the grip force rate.
The coefficient of friction between skin and a grasped object depends on skin hydration. Too much hydration can result in loss of friction. Sweating, for example, can result in slippery contact and this is the reason absorbent surfaces are often used on sports equipment such as squash racquets. However, too little hydration can also result in low friction, as was the case in the talc application study. As people get older skin hydration is substantially reduced. Thus, people in their 50s experience a coefficient of friction that may be as little as half as that for those in their 20s. This means that older people have to grip harder to maintain the same safety margin against an object in the hand slipping under load.
Short term improvement in friction can be obtained by moistening the skin, as in wetting the finger when counting currency notes or using moisturising cream. In addition the provision of non-slip surfaces is important in older groups, especially as they experience reduction in muscle strength as part of normal ageing and, in the very old, this may be aggravated by impaired coordination in the muscles producing grip and lift forces.
Neurophysiology of grip
The coordination of grip and lift in object manipulation is ultimately the responsibility of the brain. Recently researchers have been turning to functional imaging to explore which areas of the brain are implicated in coordination. By contrasting normal lifting (where grip and lift have to be coordinated) with conditions in which the hand is used only to squeeze (pure grip) or the load is applied directly to the arm (pure lift) research groups in Sweden and Japan have shown brain areas responsible for the predictive coordination in object manipulation include the cerebellum (down towards the neck at the back of the head) and the posterior parietal cortex (on each side of the head near the crown).
In this context it is interesting to note that recent studies suggest that these brain areas may be particularly susceptible to ageing effects which may explain a tendency for impaired grip-lift coordination in the very old. It might also be expected that these areas of the brain would be critical in adapting to alterations in frictional conditions requiring different grip-lift coordination. Correspondingly, head injury affecting these areas of the brain could be expected to result in impaired grip-lift coordination.
Simplified anatomy of the hand and arm
The primary finger flexor muscles producing grip force are located in the forearm with tendons to the digits running through the carpal tunnel formed by a ligament at the wrist. The median and ulnar nerves conducting sensory information form the hand also pass through the carpal tunnel.
The muscles of the hand that are responsible for producing grip force are quite separate from the muscles responsible for lift force generated by the arm. There are many small muscles in the hand and these contribute mainly to movements of the fingers for shaping the hand. However, the major muscles of the hand producing the larger part of grip force are in the forearm. Tendons running in sheaths from the long flexors in the forearm pass on the palm side of the wrist through the carpal tunnel, formed by a ligament with the carpal bones of the w2. Synovial fluid allows easy running of the tendons in their sheaths.
The carpal tunnel is often associated with disorders of hand function. In tenosynovitis, inflammation due to repetitive finger and wrist movements, results in excess production of synovial fluid. Accumulation of the fluid causes the sheath to become swollen and painful. In Carpal Tunnel Syndrome (CTS), the swelling of the tendon sheath puts pressure on the median nerve, which also passes through the carpal tunnel, and this interferes with normal sensation from the hand. The symptoms of CTS include pain, numbness and tingling of the hands, especially around the base of the thumb and the first three fingers.
Tendon movements become less easy when pressure is elevated by the wrist being flexed or abducted away from the neutral position. Pressure is also increased when there is tension in the tendons due to flexor muscle contraction and it has been shown that this pressure increases with grip force. Reducing grip force, as well as keeping the wrist in a neutral position, are significant factors in avoiding elevated carpal tunnel pressure. As noted above, reduction in grip force depends critically on friction.
Choosing grasp points
The apparatus used in the research referred to above was designed to produce linear load forces with the weight of the object acting through the line (grasp axis) between the thumb and finger contact points. However, in everyday activities, the grasp axis is often not collinear with the load, so that there is an appreciable torque component. As a result, if there is a slip, this tends to cause the object to rotate, and not just slide in linear fashion, within the grasp.
It is therefore interesting to note that in studies carried out in my lab we have shown that grip force anticipates the effects of load torque in lifting or moving objects where torque arises because the object centre of mass falls at some distance from the grasp axis. In another study we showed that when people pick up irregularly shaped objects they are sensitive to visual information about the centre of mass location and appear to choose grasp points that minimise the distance between the centre of mass and the grasp axis. This reduces any tendency for torques to be generated and thus allows the use of lower grip force.
In general, several factors operate simultaneously to determine the way people choose where to grasp an object in picking it up. Thus for example, if regions of the object are concave to fit the thumb and index finger or have roughened texture, they may be selected in preference to points that define a grasp axis through the centre of mass.
If torque is then experienced a second finger may be used to make contact at a distance from the grasp axis in order to provide a steadying influence against the torque component. This finger can then also provide a useful means for intentionally varying the torque to achieve manipulation of the object’s orientation in the hand. An example of this is using the middle finger to rotate a pencil into a usable position for writing after picking it up from the table with thumb-index finger precision grip.
The preceding example of manipulating a pencil makes the point that, sometimes, the function of precision grip is to allow controlled slip around the grasp axis. However, generally when we consider slips in the context of stable grip they are unintended, and often unexpected. The slip may be due to inadequate grip for the current load and friction conditions. For instance, the load in lifting may turn out to be higher than anticipated because the mass is underestimated. In the case of an object on a table, size is usually a reliable clue to weight. But, suppose the item is small but unexpectedly heavy; it is then a fairly common experience for the hand to slip off the top of the object.
Although overly low grip force can lead to slipping, an object will not necessarily escape from the grasp because there are rapid feedback loops which can detect the slip and produce a reflex increase in grip force in about one fifteenth of a second. This is often fast enough to catch the object and prevent further slipping. Such reflexes are fast but not as quick as the spinal reflex such as the knee jerk, which takes only one thirtieth of a second. The grip force reflexes, which experiments with skin anaesthesia have shown depend on mechanoreceptors in the skin, are thought to involve neural pathways extending up to the brain. When slip is required the brain can then override the reflexive increase in grip force which would, otherwise, tend to prevent slip.
Latex rubber gloves are required in manual handling operations whenever contact with the surroundings has to be avoided. As noted previously, sensory input from the skin is important in providing information about the grasped surface and in triggering reactive responses to slip. Thus, it is important to ask what effects do rubber gloves have on touch sensation and on the control of grip? Recent research has shown that people use higher grip force with thicker gloves, even when friction is constant. It seems likely that the thicker gloves reduce sensation levels and so tend to impair people’s ability to detect a slip in order to take appropriate action. If that is the case, then the observed increase in grip force probably reflected people’s attempts to compensate for the reduction in touch sensitivity by increasing the safety margin in order to reduce the likelihood of a slip. Here it is interesting to note that other research has shown that sensory impairments associated with CTS can lead to elevated grip force. Since CTS is associated with impaired sensation, the reason that sufferers increased their grip force may also have been to improve the safety margin and reduce the possibility of a slip occurring. However, from what was said earlier, it can be seen that this would have the undesirable side-effect of increasing carpal tunnel pressure, which, in turn, could be expected to aggravate CTS.
While latex rubber is an effective chemical barrier, it does have the problem that it becomes slippery when wet. As discussed above, any reduction in friction coefficient means additional grip force needs to be used in order to stabilise the grasped object. This may be attention demanding, perhaps taking concentration off other aspects of the task in progress, but it can also be fatiguing and, even worse, potentially injurious since elevated grip force may contribute to CTS. However, despite the potential importance of effective levels of friction, surprisingly, there are no standards for assessing the grip afforded by rubber gloves under functional conditions, although there are standards for other physical aspects such as cut resistance and degree of chemical penetration. In my lab we have recently been carrying out collaborative research with a leading manufacturer of latex rubber gloves to evaluate methods for assessing quality of grip afforded by different gloves. For instance in one study we evaluated the grip afforded by two contrasting sets of gloves produced by leading glove manufacturer Ansell. One set (SolVexTM) was produced by traditional methods, whereas the other (AlphaTecTM) was produced with new technology for improved friction when carrying out handling operations under oily conditions. Our tests showed that grip force used by volunteer participants was reduced in proportion to the improved friction offered by the AlphaTecTM glove and that this was the case for both single and repeated pulling actions.
In summary, in unfamiliar environments where object characteristics, such as weight or friction, or the forces acting on the object are unknown, grip reacts to the outcome of lifting with sensed slip driving increase in grip force (feed-back control). Under predictable conditions, grip force is set in anticipation of expected outcome (feed- forward control) so that slip is prevented. The level of grip force used in any particular situation reflects these factors. Good task design may be expected to reduce grip force levels with potential benefits in reducing the incidence of such disorders as CTS. This understanding of grip force control is leading to new tests of the functionality of glove technology.
Published: 10th Oct 2006 in Health and Safety International