An investigation on musculoskeletal injuries among users of locally-designed wheelchairs in a crafts workshop in the Philippines: focus on wheelchair design

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MASTER’S THESIS

2003:101 CIV

Christian Oliver A. Cruz

An Investigation on Musculoskeletal Injuries Among Users of Locally-Designed

Wheelchairs in a Crafts Workshop in the Philippines

Focus on Wheelchair Design

MASTER OF SCIENCE PROGRAMME M.Sc. Programme in Industrial Ergonomics

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AN INVESTIGATION ON MUSCULOSKELETAL INJURIES AMONG USERS OF LOCALLY-DESIGNED

WHEELCHAIRS IN A CRAFTS WORKSHOP IN THE PHILIPPINES: FOCUS ON WHEELCHAIR DESIGN

Christian Oliver A. Cruz

A project report submitted in partial fulfillment of the requirements for the Master of Science degree in Ergonomics

Division of Industrial Ergonomics Department of Human Work Sciences

Luleå University of Technology SE-971 87 Luleå

Sweden February 2003

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AN INVESTIGATION ON MUSCULOSKELETAL INJURIES AMONG USERS OF LOCALLY- DESIGNED WHEELCHAIRS IN A CRAFTS WORKSHOP IN THE PHILIPPINES: FOCUS ON

WHEELCHAIR DESIGN

Christian Oliver A. Cruz

Industrial Ergonomics

Department of Human Work Sciences, Luleå University of Technology

ABSTRACT

In the Philippines, locally-designed wheelchairs are modified from the US and European designs. Persons with disabilities usually receive wheelchairs from international and national charity organizations as donations for use in their daily lives, including work. However, no assurance is provided whether the locally- designed wheelchairs or the donated wheelchairs are proper (in terms of design and fit) for their intended users. No study investigating such has yet been done in the Philippines. It is therefore the aim of this research to identify musculoskeletal injuries among users, general design characteristics and attributes of locally-designed wheelchairs, identify perceptions of users of locally-designed wheelchairs towards their wheelchairs; and determine match-mismatch among the users and their wheelchairs and locally-designed wheelchairs and relationships between these factors. Twenty-five wheelchair-bound workers and their wheelchairs from a crafts workshop in the Philippines were studied. A modified Nordic Musculoskeletal Questionnaire, a Wheelchair Evaluation Form and observation methods were used. Mismatch analysis of anthropometric and wheelchair measurements and cross-tabulation analysis were performed. Results show a high prevalence of trouble in the low back and hip/thigh area, followed by the shoulder area. Results also show that most of the subjects are not fit for their wheelchairs or in standard locally-designed wheelchairs. No relevant statistically significant relationship was found between musculoskeletal trouble and wheelchair factors. Recommendations are then provided based on the results of the research.

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Acknowledgements

Thanks to my supervisors Prof. Houshang Shahnavaz and Prof. Emma-Christin Lönnroth for their valuable support and guidance during my study period in Luleå, Sweden and their unselfish sharing of knowledge

Thanks to my colleagues in the University of the Philippines, College of Allied Medical Professions, especially Prof. Catherine Capio, for her guidance and sharing of materials relevant to this research

Thanks to Mr. Jess Docot and the staff of Tahanang Walang Hagdanan (House With No Stairs) for allowing me to conduct my research freely in their institution.

Thanks to the class of MSc Ergonomics 2001-2003 for the joys, sorrows, frustrations and laughters that we shared.

Special thanks to Rupesh Kumar of Msc Ergonomics 1999-2001, for his unending provision of smiles

Finally, I would like to extend my gratitude to my parents, Tirso and Solita Cruz and my girlfriend, Jessere Guillergan for their unending support in all my endeavors.

Christian Oliver A. Cruz Philippines, 2003

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Introduction

Mobility is fundamental to health, social integration and well-being of the human being. Henceforth mobility is viewed as fundamental to the outcome of the rehabilitation process of wheelchair dependent persons. Lower limb disabled subjects depend upon a wheelchair for their mobility. Both the quality of the wheelchair, individual work capacity and the functionality of the wheelchair/user combination determine the freedom of mobility and quality of performance. (Pedotti, 1991)

Wheelchairs are probably the most commonly used type of assistive device to enhance mobility of people with a motor impairment. To date, the design of manually- propelled wheelchairs has accommodated a wide variety of user requirements. Under certain circumstances, such as in the home, maneuverability is of prime importance; in the outdoor environment, however, stability is equally important, and for sports, speed of movement is a crucial factor (Spaepen and Vanlandewijck, 1996).

Due to the enhanced mobility and performance provided by the wheelchairs to its users, people with motor impairments are afforded with greater and more varied opportunities for leisure/sports and work which enable them to self-explore and feel success (Adams, 1991). However, wheelchair users, like able-bodied persons, are not prone to musculoskeletal disorders and health risks precipitated by their work

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already exposed to due to ill-designed wheelchairs. Lack of good fit in the wheelchairs may lead to numerous injuries, among which is impingement symptoms, musculoskeletal disorders and skin damage. (Schmeler and Buning, 2000)

In the Philippines, locally-designed wheelchairs are modified from the US and European designs. Persons with disabilities usually receive wheelchairs from international and national charity organizations as donations for their use in their daily lives, including work. However, no assurance is provided whether the locally-designed wheelchairs or the donated wheelchairs are proper (in terms of design and fit) for their intended users (Docot, 2003).

Transfer of Wheelchair Technology?

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No study has yet been done in the Philippines investigating the occurrence of musculoskeletal injuries, the characteristics/attributes of these wheelchairs, potential match-mismatch and relationship of these factors among users of locally-designed wheelchairs. It is therefore the aim of this research to identify musculoskeletal injuries among users, general design characteristics and attributes of locally- designed wheelchairs, identify perceptions of users of locally-designed wheelchairs towards their wheelchairs; and determine match-mismatch among the users and their wheelchairs and locally-designed wheelchairs and relationships between these factors. Recommendations are then provided based on the results.

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Aims

This segment of the paper outlines the theoretical framework of the research. It presents the problem, the significance of this research, its objectives, scope and delimitations.

Statement of the Problem

Locally-designed wheelchairs are modified from the US and European designs. Persons with disabilities usually receive wheelchairs from international and national charity organizations as donations for their use in their daily lives, including work. However, no assurance is provided whether locally-designed wheelchairs or the donated wheelchairs are proper (in terms of design and fit) for their intended users.

No study has yet been done in the Philippines investigating the occurrence of musculoskeletal injuries, the characteristics/attributes of these wheelchairs, potential match-mismatch and relationship of these factors among users of locally-designed wheelchairs.

Significance of the Study

In general, wheelchair users will greatly benefit from this research. Knowledge of injuries occurring with the use of a locally-designed wheelchair may prompt them to use measures to avoid them, thus reducing incidence of further injury, save medical costs for attending to these injuries, and improve quality of life.

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Wheelchair manufacturers will also benefit from this research. Knowledge of injuries occurring with the use of their wheelchairs may prompt them to reconsider the continuation of such designs and modify the design in response to the results of the research.

No study has yet been done in the Philippines. From the standpoint of contribution to knowledge, the research of this research may prompt other similar researches focusing on the causality of the type of injuries, design issues and user characteristic issues. In general, this will contribute to the knowledge in the field of wheelchair ergonomics.

Objectives

It is therefore the objectives of this research to:

1. Identify musculoskeletal injuries among users of locally-designed wheelchairs 2. Determine relevant anthropometric measurements of users of locally-designed

wheelchairs

3. Identify general design characteristics and attributes and determine relevant dimensions of locally-designed wheelchairs

4. Identify perceptions of users of locally-designed wheelchairs towards their wheelchairs

5. Determine match-mismatch among the users and their wheelchairs and locally- designed wheelchairs

6. Determine relationships between general design characteristics/attributes, injuries and perceptions of users of locally-designed wheelchairs

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Scope and Delimitations

The study did not include wheelchair bound individuals not affiliated with TWH. All results from this research will only relate to the subjects included in this subject and may not necessarily extend to all wheelchair users, but may only provide insights and inferences regarding relationships established in this research.

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Background

Ergonomic Model in the Wheelchair System

It has been explained that mobility is fundamental to health, social integration and well-being of the human being. Henceforth mobility is viewed as essential to the outcome of the rehabilitation process of wheelchair dependent persons. Lower limb disabled subjects rely upon a wheelchair for their mobility. The characteristics and attributes of a wheelchair greatly determine the freedom of mobility. (Pedotti, 1991)

Ergonomic principles in design to enhance the effectiveness with which work and other human activities are carried out and to maintain or enhance certain desirable human values in the process can be applied in wheelchair design. The wheelchair system can be seen as interplay of elements between the wheelchair, the user and the environment. The model can be represented by the wheelchair-user interface, the wheelchair-environment interface and the person-environment interface (Figure 1).

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Figure 3.1. Ergonomic Model in the Wheelchair System

The wheelchair-user interaction takes into account the characteristic of the user against the characteristics of the wheelchair and its components. This can be viewed in many possible perspectives, but among these would include anthropometric considerations — whether the body structure of the user can be accommodated by the wheelchair without causing discomfort or diminished performance. Another perspective would be physiologic considerations — whether the task of propelling the wheelchair is fit to the abilities of the user. Psychosocial or cognitive considerations takes into account for example whether the control system of the wheelchair is too complex for the user, and whether it enhances the user’s self-image. The wheelchair-user system can already be considered as a unified system when interfaced with the environment, since factors affecting the loaded wheelchair will inadvertently affect the user also, and factors affecting the user directly by the environment will carry over to the limitations of the wheelchair. This interface will consider performance — whether the user will be safely and effectively arrive at the destination with the obstacles presented. It will also consider functional considerations which inquires whether the user can achieve various tasks within the wheelchair. Lastly, it considers how the environment is set-up around

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the wheelchair user — whether it affords the wheelchair user or presents as an obstacle.

This model in summary includes consideration of the person, the wheelchair and the immediate environment between the person and the wheelchair, the intermediate environment of the home and work, and the community environment (Minkel, 2000).

Ergonomics in wheelchair thus aims to optimize performance, defined by the task involving the wheelchair, while preventing potential injuries and promoting comfort.

General Considerations

Because wheelchair technology is still growing, there is no universally accepted terminology to describe the various kinds of wheelchair and its components. The following are the main components of the standard wheelchair:

1. Frame

The two common types of frame are the rigid frame chairs, which is characterized by being a unified unit and the standard cross-brace frame, which is characterized by its ability to fold for transport or storage. The advantages and disadvantages of each can be summarized by the Table 3.1.

Advantages Disadvantages Rigid

Frame

• Frame design requires fewer components and thus has more strength for a given amount of weight

• Usually a lighter weight chair than a similarly equipped folding chair

• Fewer movable parts

• Required to meet National Wheelchair Basketball Association Specifications

• Seat-to-back angle is often adjustable

• Requires removal of quick- release rear wheels for loading into car

• May feel bumpier on uneven surfaces

• Does not fold into a small package for stowing in car or airplane

Table 3.1. Advantages and Disadvantages of the Rigid and Folding Frame Manual

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Advantages Disadvantages Folding

frame

• Folds into compact package for stowing in car or airplane

• Flexes to enable all four wheels to stay on the ground when riding on uneven surfaces

• Can be folded and stowed without removing parts

• More moving, adjustable, and removable components

• May not meet rider’s sports or leisure activity needs

• Seat-to-back angle usually not adjustable

• Lateral stability can decrease as the chair flexes or starts to fold Table 3.1 (con’t). Advantages and Disadvantages of the Rigid and Folding Frame

Manual Wheelchair (Axelson, et al., 1994) 2. Seat

The seat is considered as the main interface between the user and the wheelchair.

3. Backrest

The backrest provides support for the trunk. The seat and the backrest combination is described in this paper as the wheelchair seating system. A section on the wheelchair seating system will discuss research developments and issues in this component of the wheelchair. A headrest may be attached in some models.

4. Wheels

Most standard wheelchairs possesses two large rear wheels which serves as the main propulsion system and two small front wheels (front casters) which is described as the caster system. The wheels are considered as the main interface between the immediate environment and the wheelchair.

5. Hand rim

The hand rims are attached closely to the wheel and serves as the point of control of the user for propulsion and directional changes.

6. Legrest/Footrest

The legrest/footrest provides support for the lower leg and the foot.

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7. Armrest

The armrest provides support for the arms and forearms.

The following diagram illustrates the general location of these components. (Figure )

Figure 3.1. Basic components of the wheelchair

Anthropometric Considerations

The ANSI/RESNA Wheelchair Standards are established to provide a standardized guideline for wheelchair manufacturers in USA and potential wheelchair-users. The importance of each particular anthropometric consideration will be discussed under each aspect. (Axelson, et al. , 1994)

Seat Width

Generally, to enhance accessibility, the width of a chair should be as narrow as possible without causing pressure on the user’s hips. An increase in seat width usually results in an increase in the overall width of the chair. The user might select a wider wheelchair if he or she wanted a chair that was more stable sideways. Another consideration is the

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jacket, he or she may want more allowance for tucking in the clothing on the sides.

The medical condition of the user also has to be considered especially if the user has impaired sensation of the pelvic areas and lower extremity as this increases the probability of pressure sore development. (Figure 3.2)

Figure 3.2. Seat Width Seat Depth

The seat should be long enough to provide adequate leg support, which creates better weight distribution. If the legs can support weight, a longer seat depth will spread the weight out over the thighs. This means that the amount of weight on the bony prominences (ischial tuberosities) will be decreased, thus decreasing the risk of pressure sores. If the seat is too long, however, the front edge will catch the back of the knees.

The effective seat depth of a chair with a fabric backrest will measure longer than one with a rigid back support surface. (Figure 3.3)

If the user will be adding a rigid back support to a wheelchair with sling upholstery, the seat depth of the chair may change. The seat depth is increased even more when a chair is equipped with leg rests that have calf supports. Calf supports hold the legs forward of the front edge of the seat (Figure 3.4). If this is not taken into consideration when fitting the wheelchair, this may prevent the user from positioning the buttocks against the backrest and would then cause the user to sit in a slumped sitting posture.

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Figure 3.3. Seat depth Figure 3.4. Calf Support Seat Surface Height

The wheelchair seat must be high enough to accommodate the length of the legs and yet low enough so that the legs will fit under tables. Some users prefer to sit up higher so they are positioned in a more eye-to-eye level with people sitting or standing next to them.

If the chair has a fabric seat, the seat surface height will measure a bit lower than one with a rigid seating surface. If the user will be using a seat cushion, the appropriate wheelchair seat height should be determined while sitting on that seat cushion. The user must be seated in a similar wheelchair and then measured to the bottom of the seat cushion. The seat cushion itself requires some considerable anthropometric consideration regarding contour aspects (See seat cushion design).

If the seat height is too low and the users prefer footrests, the footrests may not have enough ground clearance and may scrape the ground at curb cuts. A seat that is too high may make transferring into and out of the chair more difficult. Changing the seat height will also change the body’s relationship to the drive wheels and may affect the ability to push the chair. A higher seat will make it harder to reach the pushrims, while

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important for people with hemiplegia or others who propel their chairs using their feet. If the user propel the chair with the feet, it is important to consider a lower seat.

(Figure 3.5)

Figure 3.5. Seat Surface Height Backrest Height

The height of the backrest depends on the user. Some wheelchair-users prefer a low backrest for enhanced upper body movement or because they like the way it looks.

Higher backrests help users who have less upper body balance. Regardless of the backrest height, the back posts or push handles must not interfere with arm movements while wheeling. The backrest height is measured from the seat surface of the wheelchair. The measurements start from the surface on which the seat cushion is resting. Since this measurement is made from the wheelchair upholstery surface, the backrest height measurement will be slightly higher for a wheelchair with sling upholstery than for a chair with a rigid seat. (Figure 3.6)

Figure 3.6. Back rest height

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Footrest-to-Seat Distance

Footrest-to-seat distance is measured from the bottom of the shoes that the user normally wears to the front edge of the seating surface just beneath the cushion. If the footrest length is adjustable, the manufacturer will indicate the range available for a particular chair and footrest. If the range does not meet the user’s needs, remember that footrests are usually available in a variety of styles; a different footrest may provide the range of adjustment necessary to accommodate the leg length. Sometimes changing the footrests is not enough. If the user has very long or very short legs, he or she may need to look for a different frame style. Tall or short frames, for proportionately taller or shorter people, are available in some models. To accommodate long legs, the user might also need a higher seat or a greater seat-to-leg angle.

Once the footrest is adjusted, the user should have at least two inches of clearance under the foot pedals to save from hitting the bottom of curb cuts with the foot pedals.

Footrest clearance and leg length must be considered before selecting a seat height.

Footrests are available in a wide variety of styles and variations. The type of footrest that is appropriate will depend on your size, needs, and preferences. (Figure 3.7)

Figure 3.7. Footrest-to-seat distance

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Armrest and Headrest

If the user requires armrests, several measurements in the test procedures may be of interest: armrest height, front of armrest to backrest distance, and the armrest length.

Armrest Height

The armrest height is an important dimension to consider. The measurement for a wheelchair with sling upholstery will be different than for one with a rigid seating surface. To determine the armrest height, the user must be seated on the cushion in a chair. Arms should be hung down at the side, with elbows bent to 90 degrees, and measured from the bottom of your elbow down to the seating surface of the wheelchair beneath the cushion.

The armrest height of a wheelchair with a fixed-height armrest is given as a single value. For wheelchairs with adjustable-height armrests, a range of heights is given.

Some adjustable armrests have infinite adjustments within the range, while others have a limited number of preset height adjustments.

Armrests that are too high can cause the shoulders to be elevated; armrests that are too low can contribute to a slumped posture or even shoulder subluxation in riders without good shoulder muscles. Make sure the armrest height is appropriate to prevent shoulder problems and further complications caused by poor posture. (Figure 3.8)

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Figure 3.8. Armrest height Front of Armrest-to-Backrest Distance

The distance from the backrest to the front of the armrest is important if the user requires the armrests to transfer into or out of your chair. If the armrests do not extend far enough forward, they may not provide the needed support. If the armrests are too far forward, they may prevent the user from getting close to a desk or table. This measurement will be slightly longer for a wheelchair with sling upholstery. (Figure 3.9)

Figure 3.9. Front of Armrest-to-backrest Distance Armrest Length

The length of the padded part of the armrest is the armrest length. When the user sits back in the chair, the armrest pad should reach far enough forward from the backrest to support your arm in a comfortable position. If the user prefers to use a lap tray, the length of the armrests should provide enough support for the tray. (Figure 3.10)

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Figure 3.10. Armrest Length Distance Between Armrests

The distance between the innermost edges of the armrests is only measured on wheelchairs with fixed armrests. Armrests welded directly to the frame of the chair tend to limit the maximum available seat width at the height of the armrest pads because of the width of the support pads.

Armrests are available in many styles and sizes. Armrest measurements may vary if the user changes the type of armrest on the wheelchair. The type of armrest that is best suited for the user depends on the size, needs, preferences of the user. (Figure 3.11)

Figure 3.11. Distance between Armrests Headrest Height

If a chair comes with a headrest, the standards require that the manufacturer disclose how high the center of the headrest is above the seat upholstery. If the headrest is adjustable, the test results will indicate the range of heights at which it can be positioned. To determine the headrest height, the user should be seated on the cushion

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in a chair and then measured from the back of the head down to the seating surface beneath your cushion.

Another measurement that may be of interest is the distance the headrest is in front of the backrest. This measurement will indicate if the headrest is directly in line with the backrest or if it can be positioned in front of or behind the backrest. It may be a single value or, if the headrest position is adjustable, a range of values. (Figure 3.12)

Figure 3.12. Headrest height Joint Flexibility

In addition to the size, the flexibility of the joints (how far the arms and legs bend and straighten) will influence the fit of the chair. The ability to maintain the sitting balance will also affect the selection process. The flexibility of the hips affects the seat-to- backrest angle you need. In the standards, the seat-plane angle refers to the slope of the seat. Some riders have found that wedged or squeeze frames (chairs with a rearward slope to the seat) help with balance and stability. If the user keep your backrest upright (not reclined) and increase the rearward slope of your seat, he or she will need to bend your hips more to fit into the chair. If the user does not have good hip flexion, too much squeeze can cause pressure problems because the body cannot bend enough to fit into the chair. If user is not very flexible, he or she may want to look for a chair

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between the seat and the backrest will more closely match the angle between the thigh and the trunk. This may concern users most especially if they are experiencing muscle tone imbalances.

Some wheelchairs are available with a power recline feature. This option may be necessary if the user must perform independent weight shifts and repositioning for increased sitting tolerance and cannot achieve weight shifting. A power recline feature can also eliminate the need for transfers to bed for rest or catheterization. Quick position changes can help reduce spasticity, the body’s response to low blood pressure, and dysreflexia.

It is also important to know the flexibility of the knee and ankle joints. Many wheelchair manufacturers offer chairs with the foot pedals closer to the front edge of the seat. These “tighter” footrests reduce the overall length of the chair and make it easier to get closer to things in your environment. To fit into these tighter wheelchairs, good knee flexion is required.

Leg-to-Seat Surface Angle

The smaller the leg-to-seat surface angle, the more flexion or bend the user will need at the knees. If they have limited knee movement, look for the angle that most closely matches the angle between the thigh and lower leg. When this angle is measured correctly, it will almost always be greater than 90 degrees.

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The angle of knee flexion itself would have an effect on the performance of wheelchair turning, however further studies must be done to investigate on increments of knee- angle. (Mac Phee et. Al., 2001)

Reach is also a very important measurement to consider especially in workspace considerations however will not be discussed further.

In this research only the measurements relating to the seating system will be considered.

Seating System Issues

The seating system is the main constant interface between the user and the wheelchair.

The wheelchair seating cushion serves two major functions: as a pressure-distributing device and as a support surface. Research on the wheelchair seating system has mainly focused on contour mapping, pressure distribution and wheelchair back support and cushion design.

Related Injuries

Pressure sores or ulcers among wheelchair users develop in the presence of the following factors: an excessive uniaxial downward pressure from the weight of the user against the seating system, friction and shear forces between the user and the seating system, heat and moisture, usually in the form of perspiration from the human body.

The wheelchair user may also increase the likelihood of the development of pressure sores through his potential inability to shift or redistribute body weight over the seating

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resistance to blood flow created by pressure on soft tissue by bony parts of the skeleton.

Also, the wheelchair user may lack sensation which causes the body not to be aware of the need to change positions. Due to the inherent impairment the wheelchair user might have, skin and soft tissue deterioration may be more imminent, coupled with a dampened ability of the circulatory system and urinary and poor hygiene problems Pressure sores especially when uncontrolled usually require hospitalization and requires high costs for treating (Schmeler and Buning, 2000). Krause et al. (2001) stated that pressure sores are least likely to occur among individuals who maintain normal weight, return to work and family role, and who do not have a history of tobacco use, suicidal behaviors, or self-reported incarcerations, or alcohol or drug abuse. In a survey among wheelchair users in Japan, a majority (85.7%) had previous pressure sores and 46.3%

had undergone multiple surgeries. Some subjects (17.9%) were still suffering from pressure sores which commonly developed at the ischial tuberosities, suggesting insufficiency of self-care practice during wheelchair activities (Sumiya et al., 1997). It has been found that relationship between pressure ulcer incidence and buttock-seat cushion interface pressure (Brienza et al., 2001). Several studies have also been done regarding the assessment of risks in developing pressure sores (Anthony et al., 1998, Boes, 2000; Bergquist and Frantz, 2001).

The occurrence of spinal deformities is also likely in poorly designed seating systems.

Kyphosis occurs when there is an exaggeration of the curves of the thoracic (chest) and cervical (neck spines). Lordosis occurs when there is an exaggeration of the curve at the lumbar spine. Scoliosis occurs when there is an atypical lateral or sideways curve introduced to the spine. Kyphoscoliosis, otherwise known as rotoscoliosis in some

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literature, occurs when kyphosis is combined with a scoliosis to create a deformity in 2 planes (Schmeler and Buning, 2000).

Falls is one of the most incident of the total accidents occurring among wheelchair users. Aside form the inherent characteristics of the environment to cause falls among wheelchair users and other components of the wheelchair such as caster wheel size and type, the seating surface also plays an important factor.

Seating Cushion Characteristics

An analysis of the characteristics of wheelchair cushions among the same sample in the Japanese study showed that 91% of seating cushions were ready-made while the rest were custom-made. Seventy-six percent of the ready-made cushions were made from polyurethane foam suggesting that insufficient consideration was taken in the selection of cushions. The custom-made cushions displayed unique modifications to relieve contact pressure or to stabilize sitting posture, which could have been systematically provided for all subjects. The variety of cusion types and the frequent dissatisfaction with cushions seen in the subjects with pressure sores suggested a strong demand for the effective prescription of cushions. Thirty percent of all cushions studied had an excessively prolonged use, indicating insufficient follow-up (Sumiya et al. 1997).

Schmeler and Buning (2000) suggest having 4 variables as a guideline in seating cushion selection. The cushion has to be easy to use and easy to move around. The cushion may provide excellent pressure relief but if it doesn’t supply stability then it may not be safe or effective for the user. A feeling of instability may be created when

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provide a sense of ease in weight shifting. This may lead to falls, twists and secondary injuries. It must take the resources of the individual and their environment into consideration. Cushions have varying levels of need for maintenance, inflation, care, installation, replacement, etc. Numerous factors affect the seating surface. These factors include body weight distribution, mechanical properties of the cushion, mechanical properties of the buttocks, the shape of the buttocks and the shape of the cushion. It was found that thin patients had higher pressures over bony prominences and greater frequency of the maximum pressure occurring in a bony location than did average- weight or obese subjects (Garber and Krouskop, 1982).

A variety of seating technology has been developed over the years. The most commonly used are the following: (Figure 3.13)

a. Solid base — the solid base seating system provides good stability and allows heat to be easily transferred from the body. However, it does not provide good insulation and usually leads to pressure distribution problems.

b. Flat foam base — the flat foam base system provides not only good stability but also some limited dynamic functions. It provides some insulation and heat transfer, however effectiveness is usually dependent on the quality and wearing- out of the foam.

c. Generically contoured foam base — these systems provides the good and bad points of a flat base system however the effectiveness of pressure distribution depends greatly on the fit of the user with the cushion. These systems provide greater shear.

Other types include: viscuous and solid gel cushion systems, air flotation systems, custom-contoured foam base systems, plastic honeycomb and other dynamic systems.

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A. B.

C. D. E.

Figure 3.13. Seat Cushions (A) Solid Base, (B) Flat Foam Base, (C, D, E) Generically Contoured Foam Base (Schmeler and Buning, 1999)

Considering the interface between the buttocks of the user and the wheelchair, the following factors play major roles in affecting the interface: the body weight and its distribution, the anthropometric characteristics of the user, the gender of the user, the material properties of the cushion and other biomechanical factors affecting the position of the pelvis on the seating surface. Numerous attempts have been made to determine the optimum interface between the buttocks of the user and the wheelchair seating system. Measuring the shape of the buttock-cushion interface has been used successfully in research to study tissue loading and as a means to fabricate custom contoured cushions. Seat contours are also able to provide useful clinical information on the weight-bearing surface of the cushion, which can be used to address posture. In a study by Yue et al. (2000), it was revealed that there were 4 distinct generic shapes primarily categorized by the lateral symmetry of the buttock shapes. This was done using an electronic shape-sensor to measure the body-seat interface of the chairs of 30 elderly subjects.

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Pressure mapping is not used extensively in seating practice, but it provides valuable data in the understanding of the body-seat interface by measuring uniaxial downward pressure (however it can not measure shear forces). The pressure mapping system involves a flat surface with sensors that theoretically measures pressure and is usually interfaced with a computer for visual representation of the readings (Figure 3.14).

Figure 3.14. An example of a pressure-mapping system. (Schmeler and Buning, 1999)

The following readings in the pressure mapping system usually corresponds a general intervention:

- A reading of 80 mm Hg or less requires no intervention

- A reading of 80-120 mm Hg with uneven pressure distribution usually requires the readjustment or changing of cushions, education of the user regarding pressure relieving techniques and readjustment of other wheelchair components.

- A reading of more than 120 mm Hg with high peaks of pressure usually warrants more detailed investigation, utilization of other cushion designs, education on pressure relieving techniques and utilization of other wheelchair styles (e.g. in terms of tilt of recline of the system). (Schmeler and Buning, 2000)

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Studies on general seat cushion designs have been also used as models and may provide valuable insight in developing designs for wheelchairs (Ebe and Griffin, 2001) (Figures 3.15 and 3.16).

Figure 3.15. Three-dimensional expression of pressure distribution beneath the buttocks. (Ebe and Griffin, 2001)

Figure 3.16. Two-dimensional expression of pressure distribution beneath the buttocks (Ebe and Griffin, 2001)

Aside from pressure mapping, the ultrasound mapping system has also been developed to measure contour and describe the geometry of soft tissues deformed under load.

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analytical modeling to predict internal tissue stresses during sitting among wheelchair users (Cadaba et al., 1984).

The inherent characteristics of the wheelchair cushion itself have been subject to numerous studies. Finite modeling of the wheelchair cushion was also attempted by Dionne et al. (1998) and cited the importance of the mechanical properties of the cushion to support and conform to the user’s buttocks (Figure 3.17).

Figure 3.17. Simulations of real pressure distribution applied on wheelchair seat cushion models (Dionne et al., 1998)

Studies have demonstrated the importance of fitting and maintaining waterproof coverings to wheelchair cushions as the water content of polyurethane foams affect the stiffness and thus the effectivity of wheelchair cushions. A similar study has been done on air-filled cushions on the possible effects of inflation pressure (Krouskop et al.

1986). Aissaoui et al. (2001) recently studied the effects of seat cushion on the dynamic stability in sitting during a reaching task among wheelchair users. The tested three types of seating cushion devices were an air flotation, a generic contoured and a flat polyurethane foam. It was found that center of pressure velocity, magnitude and asymmetry were significantly different among the cushions, citing the effect seat cushion has on the seating balance during reaching tasks. A similar non-distributive nature of pressure pattern was seen during wheelchair propulsion (Dabnichki and

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temperature, heat flux, and relative humidity on the basis that the characteristics of the cushion themselves may be inductive to the development of pressure sores (Fisher et al., 1978; Stewart et al., 1980). This warrants the use of thermistor systems (Ferrarin and Ludwig, 2000) and other temperature-mapping systems in the analysis of cushion designs.

Changing seating posture can extend the amount of time a person can safely remain seated without damaging tissue or becoming fatigued. (Cooper et al., 2000). A study by Pellow (1999) showed the importance of positioning and pressure-relieving techniques in combination with seat cushions in preventing the development of pressure sores. The study showed that tilting of the system and the assumption of reclining positions diminishes pressure over the seat cushions.

In some cases, wheelchair users who cannot transfer to vehicle seats must remain seated in their wheelchairs using them as motor vehicle seats during transportation. This adds a newer dimension in the analysis of the wheelchair seating system. Since vehicle seats have been considered as key to the protection of its occupants, wheelchair seats in this case should also be designed and constructed to provide support to the occupant under impact loading and rebound during transportation. The ANSI/RESNA WC-19 standard was devised to evaluate the complete wheelchair crashworthiness. In a study by Bertocci et al. (2000), some drop seats sold commercially are incapable of withstanding crash level loads. The extension of the use of the wheelchair seating system to other functions also provides complexities in its design.

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Back Support Characteristics

Three characteristics serve as important variables in back supports of wheelchairs.

Backrests of wheelchairs vary by height, shape and stiffness.

The low backrest on a wheelchair allows increased mobility in the spine permitting more rotation of the upper spine paired with good support of the lumbar region.

However, these kinds of backrests offers less stability making it easier for rearward tipping and provides less support for the upper spine. More active users who prefer these types of backrests, however requires good trunk strength from the user for long term success. (Axelson, et al., 1994) These types of backrests are often seen in wheelchairs used in various sports. The high backrest is more often found on a typical wheelchair and provides little room for adjustability. These backrests provides additional support throughout the spine, however unlike the low backrest, provides less mobility. The low backrest is often used by users who have suffered with higher level spinal cord injuries. However, the choice of the right height through customization and proper measurement must give ample stability with little or no loss of mobility. (Figure 3.18).

A. B. C.

Figure 3.18. Differences in backrest seat height. Note variations in seat height. (A) A recliner wheelchair, (B) A standard lightweight wheelchair, (C) A tennis

wheelchair

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The shape or position of the backrest greatly affects the inclination of the trunk, which as can be seen in the next section as important in the performance of wheelchair propulsion. Similar test methodologies are employed to determine appropriate contour for the back rest. In a study by Parent et al. (2000) comparing an adjustable-tension backrest, a rigid-support back-rest with back cushion and a flexible contour backrest, it was found that pressure distribution, back profile accommodation and short-term comfort all varies depending on the type of back support. The installation of sling type mechanisms to improve stability of the user may also limit mobility of the users and lead to possible spinal deformities. However, in another study, it was determined that the flexible contour backrest which features such sling mechanisms improves back posture and better fit according to the morphology of the user (Parent et al., 1998).

Also, for obvious reasons, the shape and height of the backrest would greatly affect the range of motion of the upper extremity and the trunk, thus its repercussions to the function of the user. The relative positioning of the backrest to the wheelchair system can also affect the body-seat interface. Extrapolation of results in a study by Hobson (1992) suggests that full-body tilt to approximately 25 degrees reduces the surface shear force to near zero while a backrest-only recline of 20 degrees causes a 25% increase in the surface shear force. It should be recognized that posture and body orientation in space are additional variables that can have a profound effect on the interaction between a seated person and his or her supporting surface. Some manual wheelchairs have the feature of reclinable backrests.

Research on cushioning of the backrest is very similar to seat cushion development, as most seat cushion designs also include extension to the backrests. The material and

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inherent characteristics of the cushion used in backrests also determines the stiffness required by the user.

The extension of the function of the wheelchair to other functions, such as a vehicle seat in transportation, would require additional design considerations. In a study by Ha et al., (2000), all test back supports were unable to tolerated crash load conditions.

Leg Support Systems

Research on legrest systems mainly focus on its relation in affecting the posture assumed by the wheelchair user. A study was undertaken to investigate the effects of elevating legrest on posture and pressure distribution in a group of ten able-bodied subjects sitting in a manual wheelchair. Two types of legrest were tested: a conventional elevating legrest with a fixed axis of rotation, and a compensatory elevating legrest with a moving axis of rotation. A three-dimensional (3-D) kinematics analysis was carried out to assess body posture simultaneously with pressure measurement data collected at the back, seat, calf and foot supports. The compensatory legrest enables to lengthen foot support as the legrest proclines. This compensation at the knee joint level has a beneficial effect in minimizing pelvic and thigh motion as well as in reducing pressure distribution under seat and foot supports. In contrast, the use of a conventional legrest modifies significantly the subject's posture and induces a substantial increase of 40% on pressure data under ischial tuberosities in procline position. These findings are important for disabled and elderly people who need to elevate their lower leg frequently (Aissaoui et al., 2000).

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Since the positioning and features of the legrest not only greatly influences the trunk and pelvic posture assumed by the user, it can also have indirect effects on the function of the wheelchair as a whole through its direct influence of the knee position. In a study investigating the effect of knee-flexion angle on wheelchair turning, it was found that increasing knee flexion improves angular velocity and was perceived to be a lot easier than when the knee was extended. It was concluded that knee-flexion angle had a significant effect on wheelchair turning (MacPhee et al., 2001).

Wheelchair Propulsion Issues

Manual wheelchair propulsion is based on the use of the upper extremities, which are usually only capable of generating less force than the lower limbs, and with less mechanical efficiency. Further wheelchair design improvement therefore needs to be focused on optimizing the use of energy generated by the arms. Studies in wheelchair propulsion mainly aim at the optimization of performance, through a balance between speed production and biomechanical and physiologic costs, and prevention of injuries.

Related Injuries

An estimated 90% of all wheelchairs are hand-rim propelled, a physically straining form of ambulation that can lead to repetitive strain injuries in the arms and, eventually, to secondary impairments and disability. Further disability in wheelchair-dependent individuals can lead to a sedentary lifestyle and thereby create a greater risk for cardiovascular problems. (van der Woude, 2001). The incidence of upper extremity injury in wheelchair users has been reported to be between 31 and 73% risk for upper extremity injury due to increased upper extremity use with sport (Stankovits, 2000).

These injuries include repetitive strain injuries (RSI) of the soft tissues, including

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hazards for wheelchair athletes was revealed and included duration of impairment, muscle imbalance, awkward positioning, inadequate rest breaks, repetition of muscle use in daily activities and in sport participation, the degree of force needed for propulsion, and fatigue (Stankovits, 2000). In a study by Ide et al. (1997) evaluating muscle damage that may occur from wheelchair propulsion, it was found that plasma creatinase (CK), myoglobin (Mb) and lactatedehydrogenase (LDH) significantly increased after up-hill running wheelchair treadmill exercise among seven college-age men. This indicated that wheelchair propulsion causes muscle damage in certain situations such as up-hill running. Injuries also seem related to the certain kinematic motions occurring at the shoulder and elbow levels (Rao et al. 1996; Boninger et al., 1998). Since the upper extremity functions as weight-bearing and propulsion generating limbs for wheelchair-users, the hands and wrists are subjected to high force, high repetition stresses, increasing the likelihood of median nerve dysfunctions. Wrist splints and specialized gloves have been successfully designed to address this issue (Malone et al., 1998).

Biomechanical Efficiency

In manual wheelchair propulsion, only the hand force component tangential to the hand-rim contributes to propulsion therefore certain characteristics of the wheelchair itself may affect the biomechanical efficiency of wheelchair propulsion. Biomechanical efficiency is defined as the product of mechanical effective force (ratio between the tangential and the total applied force to the pushrim) and the mechanical use (ratio between total force generated during wheelchair propulsion and that generated during maximal isometric contraction). In an investigation of the effects of the wheelchair system tilt and back recline angles on the biomechanical efficiency of wheelchair

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propulsion, it was found that the tilt angle of the wheelchair system significantly affects biomechanical efficiency in wheelchair propulsion more than back recline angles (Aissaoui et. al., 2002). Seat position changes was seen to have a greater effect on joint motion ranges using hand-rim propulsion in a study utilizing a 2 x 3 matrix of randomized seat positions for evaluation (Hughes, et. al., 1992).

The relationship between the mechanical effect of wheelchair propulsion and its musculoskeletal cost has also been found also to be a useful tool in analyzing and improving wheelchair design. Direction of forces produced in wheelchair propulsion were simulated using simplified biomechanical diagrams, where mechanical effect of the force was defined as the inner product of the force and a normalized tangential vector (n_t). Cost was defined as the weighted sum of the undesired force components, namely the shoulder-hand (n_sh) and elbow-hand (n_eh) vectors both of which are perpendicular to the normalized tangential vector (Fig. 3.19 A and B). The effect-cost ratios are then constructed to indicate the optimum force direction, described as the middle of the region (Fig. 3.19 C). This research presents the suggestion that the propulsion force can be predicted to a considerable extent from the combined geometry of the wheelchair and the user. When the user is seated and holds the rim at a certain point, the posture is determined to a large extent whereas there is some freedom in upper extremity positioning.

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Figure 3.19. (A) Arm posture with cost directions, n_sh and n_eh (B) The normalized tangential n_twith the cost directions (C) The effect-cost ratio contours (Rozendaal and

Veeger, 2000)

It is then suggested that the wheelchair geometry, thus the physical design of the wheelchair determines the posture of the user and in turn, the maximum effect-cost ratio. This may be a valuable tool for preliminary analysis of wheelchair design adaptations (Rozendaal and Veeger, 2000).

Aside from seat height causing alterations in the force vector applied to the handrim of the wheelchair from the upper extremity, the handrim position also influences torque development. In the study by Murata et al. (2001), handrim position influenced torque development for wheelchair propulsion at the start period (Figure 3.20). A front handrim position caused greater torque development at the start period of wheelchair movement. As a mechanism responsible for this result, the force vector applied to a handrim can be worked more efficiently as torque at the front handrim position by changing the direction of the force vector. For persons with weak muscular strength who find it difficult to propel the handrim at the start period, it is important to adjust the handrim at the front position. Thus, for a design of the wheelchair, it is therefore need to take handrim position into account.

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Figure 3.20. Handrim placements based on the angle of shoulder extension and the angle of elbow flexion. (Murata et al. 2001)

Recent development of wheelchair in sports has raised the issue of wheelchair propulsion to a greater extent. Numerous materials have been used in the construction of handrims to improve friction between the hands of the user and the handrim.

Gloves are alternatives to improve interaction. In the high-performance sport of wheelchair racing, it can be seen that the handrim diameter is greatly reduced as compared to the standard manual wheelchair (Figure 3.21). It was suggested that such arrangement was done to produce greater speed with shorter but stronger strokes as opposed to the greater rotational control that a bigger handrim diameter provides.

However this also raises new question regarding propulsion techniques.

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A. B. C.

Figure 3.21. Differences in wheelchair handrim diameters. (A) A wheelchair racer, (B) A basketball wheelchair and (C) a standard manual wheelchair

Propulsion Techniques

Relating energy expenditure and muscular activity patterns in wheelchair propulsion, numerous researches have indicated that human movement economy is dependent on velocity (Gaesser and Brooks, 1975; Seabury et al., 1977; Hagberg et al., 1981; Coast and Welsh, 1985; Coast et al., 1986; Veeger et al., 1992; Spaepen et al.; 1996).

Mechanical efficiency dropped as wheelchair velocity increased and takes place only at a particular speed. Wheelchair propulsion, as compared to ordinary cycling where optimum pedaling rates seem to be positively related to the level of training, is far more complex because other factors like push characteristics, segment excursion and the ability to incline the trunk are involved which emphasizes the need to test highly experienced wheelchair athletes. Muscular activity, as measured through integrated electromyography (IEMG) was reported to be linearly related to each other in steady- state in well-defined movement patterns (Bigland-Ritchie and Woods, 1976).

However, IEMG correlates poorly with overall energy expenditure in a study by Spaepen (1996). Concentric mechanical work (CMW), which is integration of each

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muscle separately as a function of corresponding concentric angular displacement, was instead found to have a closer relationship with metabolic energy expenditure and seems a valid standard for energy consumption in manual wheelchair propulsion.

However, it was suggested that further investigation be implemented to extend the formula for CMW not only to concentric but also to eccentric and isometric components of wheelchair propulsion. Optimization can be attempted if the actual technique and the desirable technique to wheelchair propulsion are known. Chow et al. (2001) compared two racing wheelchair propulsion techniques, particularly the conventional and the para-backhand techniques and found significant differences between push and recovery times, speed production and muscle activation where the latter technique showing to be more suitable for experienced athletes. However, in another study investigating the effect of push frequency on the economy of wheelchair users, it has been found that push frequency had an effect on pushing economy and that the wheelchair user’s freely chosen push frequency was the most economical (Goosey et al., 2000). Style of propulsion also has a great influence on the positioning of the user, its residual strength and performance (Sanderson and Sommer, 1985).

These suggest that the optimum technique may not necessarily be the natural technique of the user and may vary for each particular user. It seems therefore important to determine the optimum technique to achieve wheelchair propulsion, in the light of other determinants like comfort, perceived exertion, injury prevention or psychological profiles.

Wheelchairs utilizing lever and crank propulsion methods have been designed as alternatives to the more popular hand-rim-propelled wheelchairs, proposing that these