Guidance for Designing and Purchasing Accessible Medical Instrumentation

General Guidance 1.1: Functional Access Modes

Overview

This guidance on medical device access for individuals with disabilities consists of 25 items that can be divided into two parts: 12 functional performance criteria followed by 13 functional mode considerations. Of note is that these final standards were the output of a process that involved input from over one hundred stakeholder groups, and as such can be viewed as mature resources. Also of note is that for some products a solution may involve a simple change in mode of a similar type (e.g., audio or touch for visual) but in many cases it involves an integrated solution such as an interface that is universally designed (see Section 1.2 Design for All) or supports a specific alternative interface mode (e.g., provided by a user’s assistive technology) or alternative formats (see Section 1.3 Telecommunications and Personalized Interfaces) that address the needs of intended users with disabilities. The first part uses as its primary resource the U.S. Access Board’s regulations related to:

Nine of the first twelve guidelines introduced here are based on one or both of these existing guidelines, with two added that reflect the reality that some medical devices do not have an electronics and information technology (E&IT) component, such as certain examination tables. However, the wording for all twelve guidelines is closer to that used in Section 508, which identifies both the “mode of operation” and “information retrieval” criteria. This implies that even though a sensory or motor mode is the focus of the statement, the desired solution often involves conceptual integration of both motor and sensory capabilities, reflecting the inherently multimodal and two-way nature of interfaces.

The phrase “readily achievable” is used to mean “easily accomplishable, without much difficulty or expense” (U.S. Access Board) and in fact, many of the design recommendations that follow are readily achievable. It is acknowledged that certain types of medical procedures may require certain abilities on the part of the operator and thus incorporating all types of accessibility features into some medical devices is impractical. While in principle this should be the exception within an inclusive society, in some cases professional performance norms require that a device be designed to depend on certain sensory or motor modes.

1.1.1 Provide at least one mode of operation and information retrieval that does not require user vision, or support for assistive technology used by people who are blind or visually impaired, where readily achievable for the user population.

This is especially relevant for products that present a lot of visual information, unless professional performance norms require that a product be designed specifically to depend on vision. Text descriptions of visual information including images, graphs, charts, icons, and symbols are critical for gaining access to this information, and providing text descriptions is the most common approach for interfacing with assistive technologies such as screen readers that work with computer systems.  The user’s assistive technology can then provide the information in an appropriate alternative format, such as audio or Braille or large-font text.  Text descriptions can be considered for all images, and certain protocols for information layout and use of tab ordering for assistance with navigation through spatially presented information are provided in the Web Consortium Accessibility Guidelines 1.0 and 2.0 (World Wide Web Consortium 1999, 2006). For products that are not computer-based, such as vital signs monitors, one simple solution is to support use of audio to toggle through the displayed values, if the quantity of values is not too high.  The most common multimodal alternative is to provide audio output for the visual content. As another example, without considerable investment in two-way audio, a smooth touch screen control surface should be avoided because it is essentially useless to users without vision because they cannot rely on tactile-based exploration or memory of control locations.

Motivated by: § 1194.31(a), Section 508 Standards, Subpart C -- Functional Performance Criteria, http://www.section508.gov/index.cfm?FuseAction=Content&ID=12#Functional

1.1.2 Provide at least one mode of operation and information retrieval that does not require visual acuity greater than 20/70 in audio and enlarged print output working together or independently, or support for assistive technology used by people who are visually impaired, where readily achievable for the user population.

This is also relevant for products that present a lot of visual information. For example, a common strategy is to provide magnification, either electronically (using a third-party assistive technology or a built-in magnifier such as is available for certain computer operating systems), or an auxiliary magnifying tool.

Motivated by: § 1194.31(b), Section 508 Standards, Subpart C -- Functional Performance Criteria, http://www.section508.gov/index.cfm?FuseAction=Content&ID=12#Functional

1.1.3 Provide at least one mode of operation and information retrieval that does not require color vision.

A significant fraction of the population cannot distinguish between certain colors, with the most prevalent being green and red. Additionally, colors blend in low illumination. While for most persons green and red are considered to be excellent colors and are also important cuing colors, the designer should recognize this limitation of color as a mode. W3C’s Web Accessibility Guidelines state that color cannot be used as the only mode for sharing information content.

The means of conveying information, indicating an action, prompting a response, or distinguishing a visual element should not use color coding as the only coding mechanism. For example, traffic lights cue with location as well as color (the red light is always on top and the green light on the bottom), and some medical devices use the same strategy. Labeling can also solve this problem; for example, a green power button can also have an “On” label on its face or alongside the control.

Motivated by: §1194.25(g), Section 508 Standards, Subpart B -- Technical Standards, http://www.section508.gov/index.cfm?FuseAction=Content&ID=12#Self

1.1.4 Provide at least one mode of operation and information retrieval that does not require user hearing, or support for assistive technology used by people who are deaf or hard of hearing, where readily achievable for the user population.

This is applicable for products that present audio information, unless professional performance norms require that a product be designed specifically to depend on hearing. An example is a monitor alarm with a multimodal interface that presents both an audible tone and a flashing visual display. Because medical devices rarely depend only on hearing, addressing this guidance is rarely a challenge.

Motivated by: § 1194.31(c), Section 508 Standards, Subpart C -- Functional Performance Criteria, http://www.section508.gov/index.cfm?FuseAction=Content&ID=12#Functional

1.1.5 Provide at least one mode of operation and information retrieval in an enhanced auditory fashion, or support for assistive hearing devices, where readily achievable for the user population.

This is important for products that present audio information, unless the abilities of all intended users explicitly demand the requirement of hearing without any device level enhancement. One possible approach is to have a volume adjustability associated with an audio channel that spans a range of at least 20 dB and goes to at least 65 dB, or 20 dB above anticipated environmental background noise, whichever is higher. This can be accomplished by providing built-in speakers and/or an easily identified standards-compliant audio output jack interface on the device that can interface with a user’s headset (e.g., wireless Bluetooth) or other auxiliary audio device. Note that under Section 508, a user’s personal headset is not legally classified as an “assistive technology,” but rather as part of the device.

Motivated by: § 1194.31(d), Section 508 Standards, Subpart C -- Functional Performance Criteria, http://www.section508.gov/index.cfm?FuseAction=Content&ID=12#Functional

1.1.6 Provide at least one mode of operation and information retrieval that does not depend on tactile sensation, such as a video or audio mode, where readily achievable for the user population.

For example, buttons should be labeled, and may have a sound cue associated with their control operation.

1.1.7 Provide at least one mode of operation and information retrieval that does not require user speech, or support for assistive technology used by people with disabilities, where readily achievable for the user population.

An example is a home-based telehealth product that integrates vital sign monitoring and voice-based phone calls where the user periodically interacts with a telenurse to share information. Examples of reasonable accommodations that meet this guidance include providing an instant messaging or TTY interface, or a videoconferencing system that supports a shared whiteboard under the T.120 standard that is part of the International Telecommunications Union (ITU) videoconferencing standards for both switched-circuit (telephone, ISDN) and packet-based (Internet) telecommunication.

Motivated by: § 1194.31(e), Section 508 Standards, Subpart C -- Functional Performance Criteria, http://www.section508.gov/index.cfm?FuseAction=Content&ID=12#Functional

1.1.8 Provide at least one mode of operation and information retrieval that does not require fine motor control or simultaneous actions, where readily achievable for the user population.

An example is a computer keyboard with the “sticky keys” option that enables a user to press keys such as “alt” or “command” and a letter key sequentially rather than simultaneously. Other examples are adequate spacing between keys and speech recognition systems that support sequential actions for controls. See also 2.25 below, on guidelines for using mechanically operated controls.

Motivated by: § 1194.31(f), Section 508 Standards, Subpart C -- Functional Performance Criteria, http://www.section508.gov/index.cfm?FuseAction=Content&ID=12#Functional

1.1.8.1 Controls or keys should be operable with one hand, either the right or the left, and should not require tight grasping, pinching, or twisting of the wrist. The force required to activate controls and keys should be 5 lbs (22.2 N) maximum

Motivated by: §1194.23(k2), Section 508 Standards, Subpart B -- Technical Standards, http://www.section508.gov/index.cfm?FuseAction=Content&ID=12#Telecommunications

1.1.8.2 Controls or keys should be spaced far enough apart to allow operation by a person with limited fine motor control. (To test this, the designer may try to operate the controls while wearing moderately thick gloves.)

1.1.9 Provide at least one mode of operation and information retrieval that is operable with limited reach and strength or stamina, or support for assistive technology that enables alternate use by people with disabilities, where readily achievable for the user population.

An example is a remote control for a device that can be used with one hand. Ideally, this control would enable individuals to operate a device in their personal “ability” workspace or from a region of their choice. Operable controls should be located within easy reach. Furthermore, devices that are freestanding, non-portable, and intended to be used in one location that have operable controls should comply with the following:

1.1.9.1 The position of any operable control should be determined with respect to a vertical plane, which is 48 inches in length, centered on the operable control, and at the maximum protrusion of the product within the 48 inch length

Motivated by: § 1194.25(j1), Section 508 Standards, Subpart B -- Technical Standards Criteria, http://www.section508.gov/index.cfm?FuseAction=Content&ID=12#Self

1.1.10 Provide at least one mode of operation and information retrieval that enables relative positioning and orienting between the user and the medical device without requiring the user to walk, stand, or maintain a specific arm or head orientation posture, including full access for users of assistive technologies such as wheelchairs.

Examples include a mammogram where a patient can sit in a chair or wheelchair for the duration of the exam; or a powered exam chair with controls for positioning of height, seatback, headrest, and so on, which could accommodate the postural needs of the healthcare professional as well as the patient.

1.1.11 Provide at least one mode of operation and information retrieval that minimizes the cognitive, memory, language and learning skills required of the user, where readily achievable for the user population.

For example, medical devices may provide full prompting for selected procedures during training periods to guide new users through use of the device, which may be turned off for proficient users. For devices used by patients with a limited ability to read or comprehend text in English, alternative modes such as picture images, audio or an alternative language that relay the same content are encouraged. Certain Principles of Universal Design are also applicable, as described in General Guidance 1.2: Design for All.

Motivated by: § 1193.41(i), TAAG, Subpart C —Requirements for Accessibility and Usability, http://www.access-board.gov/telecomm/rule.htm#41.

1.1.12 Provide at least one mode of operation and information retrieval that does not require a timed response for tasks that are not time-critical, where readily achievable for the user population.

For example, while there are clearly time-critical functions for persons in certain roles, for some people access is the key issue, and they are willing to spend extra time to achieve it. This is especially true for homecare products, as independence is greatly valued by many persons with disabilities. For E&IT products, Section 508 requires that users have the option of changing or turning off the timeout setting, and also suggests that users have the option of having up to 2 seconds per character during operations such as text entry. This guidance also makes sense from the perspective of medical device safety, as there is little reason to place, for instance, in-home users under time pressures that may result in use errors.

Motivated by: § 1193.41(g), TAAG, Subpart C – Requirements for Accessibility and Usability, http://www.access-board.gov/telecomm/rule.htm#41.

1.1.13 When the user is expected to respond within a certain amount of time, the device should provide sufficient time for the user to indicate more time is needed.

Ideally, when a timed response is required, users should be alerted and given sufficient time to indicate that they need more time.

Motivated by: §1194.25(b), Section 508 Standards, Subpart B -- Technical Standards Criteria, http://www.section508.gov/index.cfm?FuseAction=Content&ID=12#Self.

The Web Consortium Accessibility Guidelines require that for each time-out that is a function of the content, at least one of the following is true (see http://www.w3.org/TR/WCAG20/guidelines.html#time-limits):

1.1.14 Provide text descriptions of key visual information, where readily achievable for the user population.

This is the most common approach used by computer systems, and it is an acceptable accommodation for most software applications. For instance, the W3C Web Accessibility Guidelines 1.0 (see http://www.w3.org/TR/WAI-WEBCONTENT/) allow, and indeed encourage, the use of text descriptions for all images, certain protocols for information layout, and the use of tab ordering for assistance with navigation. This may be unnecessary for all information provided by a medical device, but it should be offered for essential information.

Motivated by: W3C Web Accessibility Guidelines 1.0, http://www.w3.org/TR/WAI-WEBCONTENT/

1.1.15 Standard computer-based monitors should be used, where readily achievable for the user population.

For medical devices for which it is appropriate, the trend toward computer-based monitors is viewed generally as a positive development. While this is true for reasons such as familiarity, from the perspective of accessibility there is an even more important reason for this preference for using a standard operating system rather than either a computer-based monitor using proprietary operating system or a conventional monitor: intended users with disabilities, and on their behalf the designer, can take advantage of the considerable accessibility support infrastructure that already exists. For example, developers of major software operating systems (e.g., Microsoft, Apple, Sun and IBM) all have a long history of participation in computer accessibility activities that led to guidelines such as Section 508. Indeed, without such a commitment, these companies might not have been able to continue selling their operating systems to federal agencies. For example, the most popular operating system, Microsoft Windows, has a suite of accessibly capabilities that can be set by users (e.g., visual contrast, preferred fonts, pointer speed and image, built-in magnifier), and a large number of third party assistive technology products are available specifically for individuals with disabilities (e.g., screen readers, Braille printers). When a product permits a user to adjust color and contrast settings, a range of color selections capable of producing a variety of contrast levels should be offered (Section 508, §1194.25(h)). Note, however, that software systems that use touch screens lack tactile information and cannot be used alone by people with vision disabilities.

1.1.16 Appropriate visual displays should accommodate captioning, where readily achievable for the user population.

All digital monitors that are at least 7.8 inches in vertical dimension, and all analog displays of 13 inches or larger, should be equipped with caption decoder circuitry, and visual information supplied on them should provide space at the bottom of the screen for captioning that duplicates the audio information, including tones as well as speech. (Section 508 already applies to such products, and under the Television Decoder Circuitry Act of 1990 (Public Law 101-431), all U.S. televisions 13 inches or greater in diagonal screen size are already required to have this circuitry; thus the technology is readily available)

Motivated by: § 1194.24(a), Section 508 Standards, Subpart B – Technical Standards Criteria, http://www.section508.gov/index.cfm?FuseAction=Content&ID=12#Video

1.1.17 Multimedia video should follow standard multimedia formats, where readily achievable for the user population.

Video feeds that include audio should be compliant with common multimedia standards. This helps, for instance, to assure lip synching, as well as to accommodate the transfer of video feeds to larger displays.

The useful range for medical devices is about 45-85 dB (Vanderheiden, 2004). Without use of visual cues such as lips, the “signal” of interest normally needs to be at least 6 dB greater than environmental noise, and preferably more than this so as to increase access (e.g., 20 dB).

1.1.18 Modes of hearing should be integrated with modes of vision and touch, where readily achievable for the user population.

The audio mode tends to integrate well with other modes, especially vision but also manual operation (e.g., sounds that are expected to occur along with certain operations). As such, it often requires lower attentional resources than vision or touching. This can make it a very effective multimodal option. If specific information is of a simple form and is needed quickly, audio may be a preferred mode, and at least should be available as an alternative or supplement. Possible approaches include both a continuous tone or a speech synthesizer, or an audio response that is triggered by the recognition of a cue in the user’s speech.

As an example, videoconferencing standards purposely aim to maintain lip-synching by slightly delaying the lower bandwidth audio signal to be synchronized with video, as this is a clear user preference.

Hearing should be one of the redundant modes for many medical devices. Audio signals should have a unique character, but should use more than one tonal frequency, as it is common for some hearing disabilities to be more pronounced at certain frequencies. The following, from the ADAAG standard, already applies to healthcare facilities and provides useful guidance for all medical equipment, including equipment for in-home use. Audible emergency alarms must have an intensity and frequency that can attract the attention of individuals who have partial hearing loss. Such alarms shall produce a sound that exceeds the prevailing equivalent sound level in the room or space by at least 15 dB or exceeds any maximum sound level with a duration of 60 seconds by 5 dB, whichever is louder. Sound levels for alarm signals shall not exceed 120 dB. (ADAAG §4.28.2 Audible Alarms). People over 60 years of age generally have difficulty perceiving frequencies higher than 10,000 Hz. Audio signals that have a periodic element, such as single stroke bells (clang-pause-clang- pause), hi-low (up-down-up-down) and fast whoop (on-off-on-off) are best. Avoid continuous or reverberating tones. Select a signal which has a sound characterized by three or four clear tones without a great deal of "noise" in between (ADAAG Appendix Note A4.28.2).

1.1.19 Performance advantages of hearing should be optimized within designs, where readily achievable for the user population.

Audio often requires less attentional cognitive focus and fewer physical constraints than vision or touching. For applications where one channel of information is presented (e.g., time-based signal), one potential advantage is that the reaction time associated with hearing a signal and making a simple response averages 150 milliseconds, which is about 50 milliseconds quicker than for vision. Another advantage is that the user can perform other tasks while listening to an audio channel, because the user’s body does not have to be oriented in a general direction as for video, or within arm’s reach as with tactile. Single channels can be coded by acoustic frequency or magnitude, though frequency is more common. Generally, humans have excellent relative tonal resolution but poorer absolute recognition, making hearing especially effective for dynamic signals or for hearing beats such as heart rate.

Classic examples are ultrasound and EMG signals, where one signal is provided at a time but it is possible to multiplex between signals. For absolute values that change less frequently, it is better for a speech synthesizer to speak the value.

Designers should make every effort to anticipate noisy environments, as well as distracting environments, when designing an audio mode interface. Many individuals with disabilities may be easily distracted. An example is a vital signs monitor intended for home use, which must be heard over a TV at high volume.

1.1.20 Provide embedded speakers and a standards-compliant audio jack, where readily achievable for the user population.

While proving embedded speakers is not appropriate for all medical devices, they offer convenience for many users. The advantage of a standards-compliant interface jack is that it is easy to locate and offers private listening with reduced environmental noise and takes up little physical space, enables wireless control by use of a remote headset, and the ability for personalized filtering by use of a customized assistive listening device. When products provide auditory output, the audio signal should be provided at a standard signal level through an industry-standard connector (e.g., headphone jack) that will allow for private listening. The product should provide the ability to interrupt, pause, and restart the audio at any time. Voice output in public areas should have a volume control. When products deliver voice output in a public area, incremental volume control should be provided with output amplification up to a level of at least 65 dB. Where the ambient noise level of the environment is above 45 dB, a volume gain of at least 20 dB above the ambient level should be user selectable. A headset can be viewed as both an essential medium for cases where sound needs to be private and not interruptive of others, and as an assistive technology where there is a desire for noise filtering or personalized control of volume.

Motivated by: §1194.25(e and f), Section 508 Standards, Subpart B – Technical Standards, http://www.section508.gov/index.cfm?FuseAction=Content&ID=12#Self.

1.1.21 Provide volume controls for audio channels that do not require users to perform twisting movements, where readily achievable for the user population.

Volume controls should be provided with audio channels. Section 508 requires that volume should reach at least 65 dB or at least 20 dB above anticipated background noise, whichever is higher. The variation in volume should be at least 20 dB. Additionally, the volume should not be allowed too get too low; while there is no direct guidance on this, it should not go below 45 dB. For instance, many devices are used by multiple people (e.g., nurses) who may have different hearing capabilities. The volume also should reset to a default value (e.g., 65 dB) when turned back on. The preferred interface for volume is not a knob that requires twisting movements but rather a knob with a wedge shaped pointer that can be turned by linear motion against the side of the knob or with a finger or even a dowel. (Note that anything that requires twisting of the wrist is specifically disallowed by ADAAG and Sections 508 and 255, as are pinching and tight grasping.) An anticipated future interface is a speech-controlled volume capability.

Motivated by: ADAAG 4.13.9 (http://www.access-board.gov/adaag/html/adaag.htm), Section 508 1194.23(k2) (http://www.section508.gov/index.cfm?FuseAction=Content&ID=12#Telecommunications), and Section 255 1193.51(b) (http://www.access-board.gov/telecomm/rule.htm#41).

1.1.22 Speech recognition should be considered as a control input mode, where readily achievable for the user population.

Speech recognition systems continue to improve each year, both in terms of speed and flexibility of performance and recognition errors. There are two primary approaches that can be integrated with medical devices, including: general recognition systems that are intended for any user and usually support a small recognition language, and personalized recognition systems that involve interactive training so that the system is tuned to the voice of a specific user.

Personalized systems are worth noting, as many individuals with disabilities are willing to put in the time necessary for interactive training if it enhances access and performance. While the majority of recent performance-based research studies confirm that performance with speech systems is not as high as it is for standard keyboard and mouse operation, this appears to be highly dependent on user training and the types of tasks performed. It should be recognized that this approach may not be suitable in noisy environments, with multiple users, or when users may speak multiple languages. Consideration of speech-controlled systems is nonetheless encouraged as this can provide access for individuals who may otherwise lack it. Furthermore, such systems continue to improve and grow in importance, especially as intelligent agents mature.

1.1.23 Tactile features should be integrated into the interface surface, where readily achievable for the user population.

Many individuals with disabilities, especially persons who have visual limitations or are blind, make more use of controls and surface features than designers might realize. Tactile variation can provide effective cues that can facilitate use of the interface. Tactile contact senses include touch, pressure, pain and temperature. Sensitivity varies with local density of sensor nerve endings (e.g., especially high density in fingertips and tongue) and skin thickness. For the present purposes, the main role of pain and temperature sensing is safety, and these will not be considered other than to note that some individuals with disabilities lack these capabilities (e.g., individuals with spinal cord injuries, stroke). Touch provides both temporal and spatial (skin surface) sensitivity. The latency of touch sensing and the subsequent reflex response is similar to that for auditory sensing and shorter than for visual sensing; however, it is slightly slower than muscle stretch reflexes. The spatial sensitivity for tactile displays is considerably less than for visual displays, because like many sensors, tactile/pressure nerve endings are much more sensitive to rate-of-change of pressure, and consequently sensation diminishes with a sustained constant pressure. While often of value (e.g., forgetting that one is sitting in a chair or wearing a hat, or being prepared to recognize a shear-induced slip of an object in a hand), this can be a disadvantage for certain alternative/augmentative applications. Tactile sensing shares a similarity to visual sensing in that both depend on active motor exploration to sample their environments. Both also can be used a sensory mode for direct transmission of information or as an important component of manual operation.

Kinesthetic and muscle sensors are embedded in musculotendinous and joint tissues. Most prevalent are sensor nerve endings within muscle spindles, including the primary afferents which are most sensitivity to velocity and have larger axons enabling signal transmission to the spinal cord as fast as 120 m/s, the secondary afferents which are most sensitive to relative muscle length, and the Golgi Tendon organs which are most sensitive to muscle force and its rate of change. There are also various sensors within joint tissues such as ligaments. Collectively, these sensors provide kinesthetic sense that supports maintenance of body posture, positioning, supporting skilled movement production, controlling forces, judging weight, etc. They help people control their voluntary muscular activities without the aid of vision, but normally tend to work closely with vision when it is available. Typically visually-guided hand positioning requires more time to accomplish than kinesthetic sense, but vision increases spatial accuracy of endpoints like the hand. If high accuracy is not an issue, designs should expedite the shifting of dependence from visual cues to kinesthetic cues as quickly as possible when new skills are being learned. Two common impairments are de-innervation (as with spinal cord injury) or tremor.

The following guidance is provided:

a) Controls and keys need to be tactilely discernible without activating the controls or keys.

b) Consider providing textural transitions to a surface that can help orient a user or may organize related content into groups. This can involve surface textures, changes in plane, or raised ridges.

c) Avoid exposure to sharp edges or hot surfaces that could injure a person engaged in tactile exploration.

d) Consider use of tactile vibration as a redundant mode for transmitting information such as an attention-getting signal.

e) Consider making functions of the device achievable from controls that are tactilely discernable, which can be explored without activating the controls or keys.

f) Place touch controls so they are not accidentally activated.

g) Consider using controls or keys that differ in their tactile representation; for instance, vary the control size or use raised lettering or symbols.

h) In high-noise areas or in cases where visual and auditory detection may be impaired (e.g., early stages of hypoxia, dark rooms), tactile features (e.g., surface texture, vibration) can offer significant advantages.

Examples of tactile displays that have been used include:

Motivated by: § 1194.23k1, Section 508, http://www.section508.gov/index.cfm?FuseAction=Content&ID=12#Telecommunications

1.1.24 Designs should consider user reach abilities for seated persons and users of wheelchairs or scooters, where readily achievable for the user population.

Human reach is normally performed by a torso-shoulder-arm system that has considerably more kinematic degrees of freedom that there are degrees of freedom at the endpoint. In addition to this kinematic redundancy, there is also actuator redundancy in that there are many more muscles than are mathematically necessary to control these joints. Thus there are many different reaching variations that can be used to position the hand for use as an endpoint interface, except as the individual stretches to positions near the end of the workspace. Furthermore, because muscles change both their intrinsic stiffness and because of neuromuscular systems can have different reflex gains, the hand endpoint stiffness that encounters the environment can also change. There are three key implications related to hand reach that are of importance to the designer:

  1. The personal workspace for persons with disabilities is often smaller than normal for a range of reasons including neuromotor control (e.g., stroke), neuromechanical stamina (e.g., muscular dystrophy), and biomechanical constraints (e.g., amputation)).
  2. Except for near the ends of their personal workspace, individuals with functional impairments often find creative (and often remarkable) ways to move the torso, shoulder complex and arm to position the hand (relative to the pelvis). From a design perspective the key strategy is providing open space for such movements and assuring that the required reach is well within the personal workspace as such creative solutions disappear and performance tends to degrade when space and movement are constrained.
  3. The “stiffness” (or mechanical impedance) expressed at the hand endpoint can vary dramatically (by over a factor of ten), whether voluntarily (which is very advantageous for tool use) or involuntarily (e.g., due to spasticity).

This guidance specifically applies to people sitting as well as using wheelchairs or scooters, and it is important for reasons of both access and user performance, although it is important to note that the ADAAG reach ranges presented below relate to basic spatial access, not necessarily optimal performance. For medical devices, the following recommendations should be taken as a minimum requirement; but for reasons of performance it is recommended that designers try to stay well within these limits unless usability testing indicates otherwise.

1.1.24.1 Forward Reach. If the clear floor space only allows forward approach to an object (e.g., by a person in a wheelchair), the maximum high forward reach allowed should be 48 in (1220 mm). The minimum low forward reach should be15 in (380 mm). If the high forward reach is over an obstruction, reach and clearances should be as shown in Fig. 5(b).

Motivated by: § 4.2.5, ADAAG, http://www.access-board.gov/adaag/html/adaag.htm#4.2

 

Fig. 5(b) (ADAAG § 4.2.5)  

1.1.24.2 Side Reach. If the clear floor space allows parallel approach by a person in a wheelchair, the maximum high side reach allowed should be 54 in (1370 mm) and the low side reach should be no less than 9 in (230 mm) above the floor (Fig. 6(a) and (b)). If the side reach is over an obstruction, the reach and clearances should be as shown in Fig. 6(c).

Motivated by: § 4.2.6, ADAAG, http://www.access-board.gov/adaag/html/adaag.htm#4.2

Fig. 6(a) and (b)) Fig. 6(c) ADAAG § 4.2.6

1.1.25 Difficulties in using mechanically operated controls and keys should be anticipated and addressed, where readily achievable for the user population.

This is relevant for mechanically operated controls that have to be reached and manipulated to be operated. Manual operation involves integration of tactile sensing of surfaces with proprioception and control of musculoskeletal structures. Often this area is referred to as haptics, and because of the availability of virtual reality and human movement measurement technologies, there is considerable research in this area. There is also considerable data on hand strength at such interfaces, although much of the existing data for normal populations may not apply to persons with disabilities or older adults.

Manual interfaces can be classified into two categories: coupled and controlled interfaces. Manual interfaces involve physical contact, and are inherently two-way “coupled” interfaces unless the device impedance is either really high (e.g., pushing against a wall or button) or really low (“pushing” against air). Such interfaces have location(s) and orientation(s), and the body and arm must be able to reach the contact location. With experience and practice, the body can often “discover” such two-way interfaces to the point where they can become almost a subconscious extension of the body. This is the “extended physiological proprioception” (EPP) concept that was first proposed by Simpson, which while originally applied to body-powered upper extremity prostheses, also applies to use of products such as a tennis racquet, a pencil, or many of the types of medical devices that can be considered hand tools. It turns out that the key criteria for EPP are: a) that a pair of coupled signals (e.g., a force-velocity pair, whose product is mechanical power) cross the interface, b) that the mechanical behavior on the other side of the interface be predictable so that it can be discoverable by the neuromotor system; and c) that there be enough richness in the interaction so as to assist this discovery process. Accessibility guidance for manual interfaces suggests the force required across a manual interface ideally should not have to be greater than 5 lbs (22.2 N) per hand or 10 lbs (44.4 N) total for a bimanual lift. Development of lightweight tools of under 5 lbs (22.2 N). To improve performance, consider developing power-assist interfaces for degrees of freedom of medical equipment such as exam chairs or hospital beds that satisfy the concept of EEP, only with force amplification, as an alternative to standard velocity controls for such medical equipment.

The following guidance can be used to increase the accessibility of such controls for users with disabilities.

1.1.25.1 If key repeat is supported, the delay before repeat should be adjustable to at least 2 seconds, with key repeat rate of 2 seconds per character. This pertains to actions associated with configuring the device as well as those associated with operating the device.

Motivated by: §1194.23(k3), Section 508

1.1.25.2 The status of all locking or toggle controls or keys should be visually discernible, and discernible either through touch or sound.

Motivated by: Section 508

1.1.25.3 For products that are freestanding, non-portable, intended to be used in one location, and which have operable controls, the operable control area or workstation should be no more than 48 in. wide, and between 15-54 in. from the floor if the depth is under 10” or between 15-46” from the floor if the depth is 10-24 in.

Motivated by Section 508, §1194.25(j)

1.1.25.4 For products that are mobile, modes of operation that do not require bimanual operation should be considered.

1.1.25.5 Where a product utilizes touch screens or contact-sensitive controls, an input method should be provided for mechanically operated controls or keys that meets the following:

Motivated by Section 508, §1194.23 (k) (1) through (4).

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