Anthropometry of Wheeled Mobility Project: Final Report

4.3.8 Operating forces

Operating forces are applied with a wide variety of grip forms (see Steinfeld, 1990). In this research we measured performance using three common grips – a power grip, lateral pinch and thumb-forefinger pinch grips. From these data, we can only make some broad conclusions for design and standards based on a large sample of people with relatively severe disabilities.

Designers should reduce the need for high precision grip postures and/or pinch forces given the lack of finger dexterity, motor control or pinching capabilities of persons with disabilities. This can be achieved by designs that allow operation using alternate grip configurations such as a flat hand, fist or a more convenient power grip or hook grip. Broader handles and larger gripping surface areas facilitate stronger grips.

When a precision pinch grip is required, a lateral pinch is recommended over a thumb-forefinger tip pinch grip because a lateral pinch grip provides a larger finger contact surface; the grip strength capabilities of our sample was higher for lateral pinch grip as compared thumb-forefinger pinch grip. Individuals with limited finger dexterity and strength are also more likely to be capable of forming this grip.

Use of operable parts that require fine grips preferably should not require exertion of lateral pinch grip forces in excess of 2 lbf (9 N) to accommodate the vast majority of WhMD users having at least some grasping capability. For other types of operable parts, the upper limit for operating force should be retained at a maximum of 5 lbf (22 N) to accommodate 95% of the sample.

Further, tasks and products that require operation of controls or object grasping should allow for both right and lefthanded operation, given that more than 25% of wheeled mobility device users in this study were lefthand dominant. Adequate clear floor space should be provided to access controls and switches from either the left or the right (see previous description of ‘Clear Floor Space for Reaching’).

A large proportion of wheeled mobility device users possess functional capabilities in only one hand. Hand-operated products and environmental features for use by the general public should be designed acknowledging that grip strength is significantly affected by gender, age and disability, and that a sizeable proportion of WhMD users (approx. 12% in our sample) have very limited or no grasping ability.

Grip strength data from our study can help designers employ more inclusive design criteria when developing new designs, as well as identify tasks that require pinch grip force exertion that exceed the capabilities of most users and need to be redesigned possibly through use of technological interventions and assistive devices. For example, many drawers or cabinets require tight pinching to open but adding a more ergonomically designed handle and rollers to the bottom of the drawer allows it to open with ease.

The ADA-ABA standards currently require operation with only one hand and without “tight grasping, pinching or twisting of the wrist”. This is a major step toward reducing demands on people with limitations of grip. However, the standards do not require left and right hand operation, and the maximum operating force allowed substantially exceeds the abilities of a large number of individuals in our sample for objects requiring a fine grip. Moreover, the standards preclude the use of a pinch grip, which is desirable to people who are able to use it effectively. Not allowing such grips at all may create usability problems for even more building users than it helps.

Our findings suggest developing a more detailed approach to grips and grip forces in the standards. This should include:

• Identifying maximum operating force requirements based on the types of grips and forces exerted.

• For small controls and hardware that normally would be operated with pinch grips, the maximum operating force should be 2 lbf (9 N).

• For larger controls and hardware that can be operated with a power grip, the maximum force requirements could be retained at 5 lbf (22 N) bearing in mind that this still does not address the needs of many WhMD users having very limited or no grasping ability.

• The 5 lbf (22.2 N) maximum force could be retained for operating forces for opening doors. This limit was based on the limitations of door closer technology.

• Developing specific requirements for alternative gripping options without precluding pinch grips, e.g. flat hand, knuckle push, based on existing research on hand anthropometry (e.g. Steinfeld, 1990)

• Retaining the requirement for use by only one hand.

• Utilizing standard terminology from anthropometry in reference to grip forms; the standards could list the types of grips and movements, including alternatives to standard grip forms that would be allowed. Information of this type is available from a previous Access Board study (Steinfeld, 1990; Steinfeld and Mullick, 1990).

Improving standards related to operating forces requires a careful examination of the availability of products that would be accessible to this population and the cost implications of new requirements. This would identify the limits to feasible improvements to the standards and may require adjusting the force limits to address feasible implementation.