Dexterity

Dexterity covers the ability to grasp and hold objects, and to perform fine finger movements to manipulate small objects.

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Most products require physical manipulation of controls and manual handling. The hands are used to grasp, move and exert forces to use and operate various products. Objects can be grasped, pushed and pulled. The human hand is composed of four fingers and an opposable thumb, which is key to many dexterity tasks.

We can exert clamping forces between fingers and also clamp and hold larger objects in the palm of the hand. Individual fingers can be used to exert pulling and pushing forces. We often use both hands at the same time to manipulate objects. This coordinated movement requires strength and dexterity in the fingers together with sensory capability and motor control.

There are many causes of pain that can limit dexterity. Arthritis is one example that is particularly prevalent for older people, causing stiffness, swelling and pain in the joints. The satisfaction level for using a product is seriously affected when it causes pain, even if the product is still usable.

Many able bodied people will experience temporary bruising or breakages that can affect their dexterity capability. This causes particular frustration when attempting to use products that require unusually high levels of strength or two-handed co-ordination.

The most critical dexterity functions for product interaction are now considered. A single hand can be used to generate a

• Push force using any part of the hand
• Pinch grip using the index finger and thumb to generate opposing forces
• Power grip using the palm and thumb together with all four fingers

A push force requires the least amount of dexterity capability because there is no need for an opposing grip. Two-handed tasks require the use of both hands to perform a combination of pushing or gripping. One hand is often required to hold or stabilise the object, while the other performs fine precision movements.

The simplest dexterity function is when the fingers or palm are used to exert forces without grasping. The movement and force are aligned in the same direction, so there is no requirement for a friction contact. There are three possible perpendicular directions in which a force can be applied.

Large forces can be achieved by using the body weight to help push away from the body, for example when opening a door. If the surface to be pushed is vertical, then either the palm of the hand must be aligned with the vertical surface, or the fingers must be used to transmit the force. Both of these can be a source of pain. The pushing force can be generated more comfortably if the surface is contoured to fit the hand with a neutral wrist position.

Pushing up and down is used to depress buttons and sliders, such as that required for a toaster. It is more comfortable to push down than up, because the wrist has to rotate 180 degrees if the palm is required to push upwards.

Pushing left or right can be required to push a button on the side of a product, or to push a product from side to side. It is easier to push things towards the body centre-line rather than away from it, because this wrist position is more natural.

Pinch gripping is the ability to develop opposable forces between the thumb and fingers of the hand. Pinch grips are often used to manipulate controls such as sliders and knobs, which only require minimal forces. Pinch grips should not generally be used to generate large forces. Instead, more hand muscles can be employed by a power grip, thereby increasing the strength that can be brought to bear.

Careful provision of shape and surface texture of controls should allow the user to operate the product with a loose grip formed with his or her preferred combination of thumb and fingers.

The maximum rotational force that can be generated by a pinch grip is dependent on the size and shape of the object to be turned. Circular objects such as door knobs are the hardest to grip and turn as the fingers tend to slip on the surface. The best shape for pinch grip turning allows the thumb and fingers to grasp either side of long strip.

A power grip is formed with the thumb, all four fingers, and the palm of the hand. The large number of muscles used means that large forces can be generated with comparatively little effort, although the precision is reduced when compared to a pinch grip.

This type of grip is used for grasping handles and picking up objects such as mugs. This grip is most effective when the object is shaped such that the hand wraps completely around the object so that the contact area is maximised.

When this grip is used to provide rotational motion, the ease of use depends on the shape of the object and whether frictional contact is required. A handle or lever is the best way to generate rotational motion because it is easy to grip and does not rely on frictional contact.

The most difficult knobs to use are circular, have a hard surface and do not allow the whole hand to be used.

Many tasks with everyday products require the use of both hands at the same time. In some cases this dexterity demand is unnecessary, and an alternative design can allow the product to be used one handed, which will typically reduce design exclusion and increase user satisfaction.

For products that can be used one-handed it is also prudent to cater for those who wish to use the product two-handed. People can suffer from temporary or permanent impairments that reduce the capability of one or both hands, in which case maximum inclusion is obtained when the product’s operation is as flexible as possible.

The ability to perform dexterity tasks that rely on friction can be impaired by the environmental context. A higher strength is required if the hand is sweaty, wet or covered in lubricating substances that are common in workshops or kitchen environments.

Cold temperatures can also affect hand function. The flexibility and sensitivity of the fingers decreases with temperature reducing the dexterity capability. Wearing insulating or protective clothing such as gloves can also make it harder to operate controls on products.

Vibration, motion and the visibility of the hands can also affect the ability to grip or make precision movements. Touch is the only sense that can be employed in low light levels or when the hands and product are not visible.

• Try to facilitate product interaction that only requires the user to generate pushing forces
• Carefully consider the force and precision requirements for gripping operations, and ensure that these are appropriate given the size and shape of the surface together with the type of grip that will be used
• Note that controls requiring simultaneous movements in different directions (such as combined pushing and twisting) are particularly difficult for those with reduced motor control
• Enable easier gripping by providing a slightly deformable surface, and maximising the available contact area where possible
• Try to ensure the product can be used left or right handed, and one or two-handed
• Arrange gripping tasks so that they can be performed with the wrist in a neutral and straight position in order to improve user comfort, and minimise pain for those with conditions such as arthritis
• Provide loops, handles or sudden changes in surface contours to help minimise the gripping strength required
• Avoid connection slots that require both vision and dexterity to align

Dexterity ability level (in increasing order)

No demand

• The user is not required to pick up and carry objects, or perform fine finger manipulation, or grasp objects

Low demand: The user is required to have sufficient ability to do things like:

• Perform simple fine finger manipulation with either the left or right hand; e.g. pickup a safety pin
• Pick up and carry a light object with either the left or right hand; e.g. carry a pint of milk
• Grasp an object with either the left or right hand; e.g. squeeze a sponge

Moderate demand: The user is required to have sufficient ability to do things like:

• Perform complicated fine finger manipulation with either the left or right hand; e.g. use a pen without difficulty
• Pick up and carry a heavy object with either the left or right hand; e.g. carry a 2.5 kg (≈ 5 lb) bag of potatoes
• Use both hands to grasp something lightly; e.g. unscrew the lid of an instant coffee jar

High demand: The user is required to have sufficient ability to do things like:

• Perform two-handed fine finger manipulation; e.g. tie a bow in laces without difficulty
• Pick up and carry a heavy object in each hand; e.g. carry a 2.5 kg (≈ 5 lb) bag of potatoes in each hand
• Grasp an object in each hand; e.g. squeeze a sponge with each hand