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Section 2Population ServedCategories of Functional DisabilitiesThe human factors guidelines address a wide variety of impairments subdivided under the threeprincipal categories of functional limitations:Physical Impairment Physical impairments– Extreme muscle weakness or reduced endurance– Paralysis—loss of sensation or voluntary movement– Paraplegia—paralysis of both legs and the lower portion of the trunk– Quadriplegia—partial or total paralysis of all four extremities, usually frominjury to the spinal cord at the sixth cervical vertebra or higher– Limited hand/arm reach capability, sometimes due to wheelchair positioning– Impairment of dexterity/manipulative skills– Tremor—continuous, involuntary quivering– Contracture—permanent contraction of a muscle– Spasticity—stiff, awkward movements due to involuntary muscle contractions– Ataxia—impaired muscle coordination, especially when voluntary musclecontrol is attempted– Apraxia—inability to perform voluntary movements although there is nosensory or motor impairment– Rigidity—immovability– Use of braces and other prosthetics– Extremes of size or maturity Speech impairments– Dysarthria—slurred speech due to impairment of the tongue or other musclesessential to speech– Stuttering– Aphasia—impairment of ability to communicate through speech, writing, orsigns– Complete loss of speech2-1

Perceptual Impairment Hearing impairments– Partial hearing loss– Complete hearing loss– Listening impairment Vision impairments––––Legal blindnessColor blindnessComplete blindnessDiplopia—double visionCognitive Impairment Cognitive ility to interpret written languageConfusion or disorientationLoss of short-term memoryLoss of long-term memoryImpairment of learning and retentionImpairment of perception and attentionImpairment of intelligence and thinkingSevere mental retardationDisabling ConditionsThe functional limitations listed above are caused by a wide variety of disabling conditionsincluding diseases, accidents, aging and birth defects. The following are some of the morefrequently occuring conditions: Aging,Arthritis,Multiple sclerosis,Cerebral palsy,Stroke and head trauma,Spinal cord injury.Limited strength is common to nearly all disabling conditions and is a common reason for beingunable to operate household equipment. Figure 2-1 shows the hand functions (hand grip andlateral pinch) of the elderly and people with rheumatoid arthritis, multiple sclerosis, and spinalcord injuries (levels C4–C7). In this figure, the hand functions for the four groups are normalizedas the percentages of normal strength. The data for individuals with spinal cord injury are afterany surgical procedures (such as tendon transfers).2-2

% of Hand Grip and Lateral Pinch GripStrength with Respect to Normal100% of 055–5960–6465–6970–74No DisabilityHand Grip75 RAMSDisabilitySCILateral Pinch GripC920317-01Adapted from Stover (1986), Tourtellotte (1965), Helliwell (1987), and Mathiowetz (1985)Figure 2-1. Hand Grip and Lateral Pinch Strength for Subjects with Various Disabling Conditions(Rheumatoid Arthritis (RA), Multiple Sclerosis (MS), Spinal Cord Injury (SCI))AgingThe aging process often leads to vision loss, hearing loss, and osteoarthritis, as well as loss ofstrength.Age-Related Effects on Hand Strength—It is well documented that muscle strength declineswith increasing age for all muscle groups. Here, only the change of hand strength with respect toage will be presented (Mathiowetz, 1985). The actual strengths and percentage losses, as ageincreases, of hand grip, lateral pinch, tip pinch strength and palmar pinch are illustrated in thefollowing eight figures. The data were gathered from a sample of 310 males and 328 females,ages 20 to 94 years old. The percentage loss is calculated based on the data at the age of 20 to24. Figures 2-2 to 2-9 show that in most cases the peak of hand strength occurs between 30 and40 years of age, and no more than 10% decline in strength occurs before age 50 or 55.Age-Related Effects on Visual Functions—Like other parts of the body, the eyes undergo manychanges due to aging. The lens becomes yellowed, thickened, and less elastic, which causesreduced accommodative capacity, reduced light entering the eye, and the scattering of light raysbefore they reach the retina. Changes in the cornea also lead to increased scattering of light rays.The iris dilator atrophies and becomes more rigid, which reduces the size of the pupil and itsability to adjust its size according to the amount of light available. The reduction in the size ofthe pupil is known as senile miosis. In addition, there may be deterioration in the visualpathways of the central nervous system (Whitbourne, 1985). These changes in the physicalstructure of the eye affect several aspects of visual function, including refractive power,accommodation, acuity, dark adaptation, sensitivity to light, color vision, temporal resolution,depth perception, and visual field.2-3

60MenWomenForce 70–7425–2935–3945–4955–5965–6975 Age (Years)C920317-02Figure 2-2. Change of Hand Grip Strength with Age120Men110WomenPercent ��6470–7425–2935–3945–4955–5965–6975 Age (years)2-4C920317-04

Figure 2-3. Percent Loss of Hand Grip Strength with Age16Men14WomenForce –7425–2935–3945–4955–5965–6975 Age (Years)C920317-03Figure 2-4. Change of Lateral Pinch Grip Strength with Age2-5

120MenWomen110Percent 70–7425–2935–3945–4955–5965–6975 Age (years)C920317-05Figure 2-5. Percent Loss of Lateral Pinch with Age16Men14WomenForce –7425–2935–3945–4955–5965–6975 Age (years)C920317-07Figure 2-6. Change of Tip Pinch Grip Strength with Age2-6

120MenWomen110Perce nt 70–7425–2935–3945–4955–5965–6975 Age (years)C92031 7-09Figure 2-7. Percent Loss of Palmar Pinch with Age16Men14WomenForce –7425–2935–3945–4955–5965–6975 Age (years)C920317-06Figure 2-8. Change of Palmar Pinch Strength with Age2-7

120MenWomen110Percent 70–7425–2935–3945–4955–5965–6975 Age (years)C920317-08Figure 2-9. Percent Loss of Tip Pinch Grip with AgeRefractive Power—From about age 30 to the mid-60s, the eye gradually becomes morehypermetropic (farsighted) as the lens flattens. After the mid-60s, this reverses and the eyebecomes more myopic (nearsighted) (Whitbourne, 1985). Roughly one-third of individuals overage 80 have vision of 20/50 or less (Lewis, 1985).Accommodation—Adaption is the eye’s ability to change its focus from far to near objects.The change in the refractive power of the lens in shifting focus from far to near objects becomesreduced from a peak of 14 diopters at 8 years to less than 2 diopters at age 50. By the age of 60,the lens is completely incapable of accommodating to focus on objects at close distance(Whitbourne, 1985). This is known as presbyopia (Figure 2-10).2-8

5455565C920317-10Source: Tuniras (1988)Figure 2-10. Changes in Visual Accommodation with AgeThe reduced elasticity of the lens with age makes the lens less able to accommodate for nearvision. The minimum distance between an object and the eye for a clear image, the “near point,”increases from 9 cm at 10 years to 10 cm at 20 years, 20 cm at 45 years, 84 cm at 60 years(Tuniras, 1988), and 100 cm by age 70 (Kenney, 1989).Acuity—Visual acuity is the ability to detect details on objects at varying distances. Scatter inthe cornea and lens serves to deflect light rays entering the eye and reduce the contrast necessaryfor detecting details (Tuniras, 1988; Whitbourne, 1985). Senile miosis, the decrease in pupil size,reduces the amount of light entering the eye by as much as one-third (Kenney, 1989; Tuniras,1988) and therefore also reduces the ability to detect details (Tuniras, 1988; Whitbourne, 1985).Visual acuity increases during the 20s–30s and remains stable for 10–20 years. It begins todecrease in the 40s, and by age 85, there is an 80% loss of acuity when compared to the acuity atage 40 (Figure 2-11). The loss of acuity is especially severe at low levels of illumination(Whitbourne, 1985) (Figure 2-11).Elimination of glare and improved illumination can help enhance visual acuity substantially inthe aged population (Tuniras, 1988).2-9

Visual rce: Pitts (1982)Figure 2-11. Changes in Visual Acuity Measured in Snellen Index with AgeDark Adaptation—Dark adaptation occurs when moving from a bright light to a dimenvironment. Decreased dark adaptation begins in the 20s and is especially pronounced after age60. This appears to be caused by reduced light transmission through the lens and the reduction inpupil size or senile miosis (Whitbourne, 1985). In a fully dark-adapted eye, the mean diameter ofthe pupil is 8 mm in the 20s and gradually decreases to a mean of 5 mm in the 90s (Tuniras,1988) (Figure 2-12).Age-Related Effects on Visual Functions85030205410Light-Adapted352Pupillary Diameter (mm)Dark-Adapted6Pupillary Area (mm )4072111020304050Age (years)607080C920317-12Source: Tuniras (1988); original data from Verriest (1971)Figure 2-12.2-10Changes with Age in Pupillary Diameter and Area, Measured in Light-Adapted

and Dark-Adapted IndividualsSensitivity to Light—Older adults are less able to detect low levels of light because theyellowing and increased density of the lens, as well as senile miosis, reduces the amount of lightthat enters the eye (Whitbourne, 1985) (Figure 2-13). Therefore, the minimum amount of lightnecessary to see an object increases with age (Tuniras, 1988). Glare may also increase and createvisual difficulties (Whitbourne, 1985).8Threshold Intensity (Log 10 µµl)7658046040203210203040Time (minutes in darkness)C920317-13Source: Tuniras (1988); original data from McFarland (1960)Figure 2-13.Decline in Light Sensitivity with Age; Data Show Changes in Visual ThresholdAfter Dark Adaptation in Different Age GroupsColor Vision—In the 40s and 50s, color perception begins to decline and there is reduced abilityto discriminate between colors at the green-blue-violet end of the spectrum. This is due to thethickening and yellowing of the lens with age and is further affected by senile miosis, whichrestricts light entry to the thickest and most yellow central region of the lens (Tuniras, 1988;Whitbourne, 1985).Temporal Resolution—The critical fusion frequency is the least frequency of a flashing lightthat is perceived as an uninterrupted signal. This is also referred to as the critical flicker fusion.With increasing age, there is a decrease in the point at which a flickering stimulus fuses into acontinuous light. This is due to changes in the lens that reduce the amount of light entering theeye as well as aging effects in the central nervous system (Whitbourne, 1985).2-11

Depth Perception—Depth perception appears to remain unchanged through the mid-40s, butsome changes occur in later life (Whitbourne, 1985).Visual Field—The size of the visual field is reduced significantly due to loss of receptors, aswell as other factors (Kenney, 1989).Age-Related Effects on HearingHearing impairment is the third most prevalent chronic condition among the elderly population.About half of the population of hearing-impaired people is over 65. The prevalence of hearingimpairment is about 12% of all people between ages 45 and 64, about 24% of those between 65and 74, and about 39% of those age 75 and over (Schow, 1980). In nursing homes, theprevalence of hearing impairment is much higher than in the general population, ranging from 51to 97% (Schow, 1980).Hearing loss is classified as conductive, sensorineural, or mixed (Meyerhoff, 1984). Aconductive hearing loss is due to a malfunction in the gathering of sound by the external ear,secondary to obstruction of the external auditory canal, or incomplete or inefficient transfer ofsound energy to the cochlea by the eardrum or the ossicles of the middle ear. Sensorineuralhearing loss may be caused by a lesion of the sensory receptor elements in the cochlea or theauditory nerve or a lesion of the higher processing center. Mixed hearing losses are those thathave elements of both disorders (Meyerhoff, 1984).Presbycusis—The most common cause of sensorineural hearing loss is presbycusis, the gradualreduction in hearing loss with advancing age (Staloff, 1987). It has been shown that hearing lossincreases with age and is generally greater in the higher frequencies (Goldstein, 1984; Maurer,1979). The severity of loss is greater for men than women (Maurer, 1979). Studies have alsoshown that the elderly can have greater speech discrimination impairment than young adults,although hearing ability is the same for both groups (Kirikae, 1964). Most elements or phonemesof speech fall within the frequency range of 300 to 4000 Hz.Presbycusis is the most common communicative problem among the aging. It also is the mostcommon auditory disorder in the entire population (Mader, 1984). There are four specific classesof presbycusis, and they have different characteristics in hearing loss (Corso, 1984). Sensorypresbycusis produces a high-frequency loss (above 8000 Hz); neural presbycusis affects speechdiscrimination without a parallel loss in pure tone thresholds; metabolic presbycusis has a nearlyuniform threshold loss for all frequencies with loudness recruitment; mechanical presbycusisshows an increase in hearing loss values from low to high frequencies. The most common formis sensorineural presbycusis (Sataloff, 1987).Hearing Thresholds—Hearing tests are usually performed in the frequency range of 500 to 4000Hz (1000 to 2000 Hz in some cases) (Goldstein,1984), since this range is an estimate of theperson’s overall functional hearing sensitivity (Schow, 1980). The level of hearing impairmentthat is considered significant varies from different surveys and research projects and is still beingdebated.2-12

The level of impair

Human Factors Issues Human factors engineering is the study and practice of designing equipment, machines, and environments to accommodate human users. Human factors encompasses physical requirements of size and force as well as mathematical models of