[Excerpts from "Weight Limits for Foldaway Wheelchairs" by Anil Mital & Ursula
Wright, in ERGONOMICS IN DESIGN, Volume 4, Number 3, July 1996. Copyright 1996
by Human Factors and Ergonomics Society, P.O. Box 1369, Santa Monica, CA
90406-1369 USA; 310/394- 1811, fax 310/394-2410,
Wheelchairs provide mobility to individuals suffering from a wide variety of disabilities. As many as 1.5 million Americans who are not permanently wheelchair-bound use powered wheelchairs. Some of these users are able to walk short distances and may be classified in one of the following groups: (a) those who walk independently but may have low exercise tolerance, painful arthritis, or poor coordination, (b) those with limited ability to walk and only do so with assistance, and (c) those with little or no ability to walk because of lower limb amputation, respiratory disease, stroke, or severe heart problems.
A large percentage of wheelchair users are elderly (Hunter, 1985; Stamp & McLaurin, 1982), and although they can walk independently, they use a wheelchair because of low exercise tolerance resulting from cardiac or respiratory conditions.
Because most hand-driven and hybrid (combination of manual and power drives) wheelchairs require significant arm force, arm movement, and cardiovascular endurance, wheelchair users have difficulty maneuvering these wheelchairs in and out of car trunks, homes, or workplaces. In short, these wheelchairs lack portability.
Many researchers have provided design recommendations for wheelchairs (Hoiden, Fernie, & Lunau, 1988; Larkin & Martin 1988; Purdy, 1986) that have focused on seat height, and armrest, footrest, seat, and backrest size, but very little attention has been paid to designing for portability, particularly for foldaway powered wheelchairs. To help fill this research gap, we conducted a strength simulation study that presents data for the design of portable powered foldaway wheelchairs and examines the maximum weight of such wheelchairs to ensure their safe handling.
Individuals who use foldaway powered wheelchairs for travel between the house, car, and workplace make up a significant proportion of wheelchair users. Portability is a critical design consideration for these users (American Heart Association, 1990; Platts, 1974). It is important to study acceptable weights and collapsible designs of wheelchairs to accommodate this large population of wheelchair users.
The Experimental Simulations
In our study, experimental static and dynamic strength simulations were conducted to determine the upper weight limit of folded wheelchairs that can be comfortably loaded and unloaded from the trunk of a car. The limiting factor for handling folded wheelchairs in such situations would be an individual's body strength (Mital, Wang, & Fard, 1987).
We attempted to answer the question, "What is the maximum weight of a folded wheelchair that can be handled comfortably and without excessive exertion?" We performed both static and dynamic strength simulations and compared simulation types
In the study we collected the following measurements: static strength performed in two sequences to analyze the effect of height, holding capacity to analyze the effect of horizontal distances of the wheelchair from the trunk of a car, and dynamic strength to analyze the effect of two lifting techniques.
Outcome of the Simulation
Statistical analysis was used to determine (a) how lifting to various heights (vertically) effects one's static strength, (b) the handling capacity of lifting wheelchairs at various horizontal distances from the trunk of the car, and (c) the dynamic effort involved in the two lifting techniques.
The maximum acceptable weight of the folded wheelchair for straight, forward lifting was therefore the limiting weight for dynamic strength. This value (21.0 kilograms, or about 46 pounds) is substantially higher than the limiting value resulting from the static strength simulation (14.19 kg, or about 31 lb). Dynamic strengths had significant and positive correlations with several anthropometric variables, such as stature. body weight, arm reach, and chest weight.
Design Recommendations for Wheelchair Manufacturers
Findings from this study indicate that the weight of the folded wheelchair should not exceed 14.19 kg, despite the fact that participants in the study were able to handle heavier weights (on average 21.0 kg) when the activity was infrequent. The static simulations did not provide an accurate assessment of the individual's capability, though this is not surprising. Several previous studies have pointed out the limitations of static strength simulation to assess an individual's capability to perform frequent dynamic tasks (Mital et al., 1986, 1993). Our results indicate that in dynamic strength simulations, healthy individuals are capable of loading or unloading wheelchairs weighing up to 21.0 kg from the trunk of a car.
The weight found in the study (21.0 kg) is below that of commercially available foldaway wheelchairs (powered by a retrofitted motorized unit), which weigh approximately 40.0 kg (88 lbs), and manually powered foldaway wheelchairs, which weigh about 23.0 kg (50.6 lbs).
Ergonomics practitioners and wheelchair designers can use these findings when investigating alternative designs for foldaway wheelchairs that are lightweight, collapsible, portable, and easily loaded or unloaded by a user with limited exercise endurance.
Clearly, there must be a drastic reduction in the weight of conventional foldaway wheelchairs powered manually or by a retrofitted motorized unit. Because most wheelchair users are elderly with reduced strength, it is important to investigate strength tolerances that limit people's ability to comfortably load and unload wheelchairs.