Linda van Roosmalen, PhD*; Matthew P. Reed, PhD**; Gina E. Bertocci PhD*
*University of Pittsburgh, Department of Rehabilitation Science and Technology
This paper describes usability issues related to wheelchair occupant restraint systems for individuals seated in wheelchairs who have upper-extremity impairments relating to strength, range of motion, and dexterity. In a laboratory study, 10 people who sit in wheelchairs in motor vehicles evaluated safety belt usability in three occupant restraint scenarios. A coordinate measurement instrument was used to assess the participants’ reach capabilities for purposes of establishing guidelines for safety belt positioning. The findings of the study address methods to improve independence, seat belt usage, belt comfort and safety among the wheelchair-seated population traveling in motor vehicles.
Wheelchair, safety belt, usability, restraint, transportation
People seated in wheelchairs in motor vehicles are protected in crashes and vehicle maneuvers by a two-part safety system. The wheelchair is secured to the vehicle using a four point tie-down or docking system and the wheelchair occupant is restrained to the wheelchair or to the vehicle by a safety belt consisting of a pelvic and shoulder belt .
Currently installed vehicle-mounted safety belts are often difficult to use independently and properly because of wide variance in occupant and wheelchair size, and interference of the wheelchair structure with the belt system [2, 3]. Poor belt fit may result in pelvic belts crossing the soft tissue of the abdomen, creating the potential for abdominal injury in a crash. Shoulder belts crossing the occupant too close to the neck or too far from the neck may cause poor upper body restraint resulting in occupant impact with the vehicle interior .
An impediment to designing safe belt systems that maximize the independence of wheelchair-seated individuals is a lack of data on functional abilities related to seat belt use and belt donning by the wheelchair using population. Of particular interest are usability issues related to belt buckles and latch plates and data on the zones within which wheelchair occupants can reach and grasp stowed safety belts. Findings of this study can guide manufacturers towards the development of safety belts for wheelchair users that provide appropriate restraint but are easy to use. Improved safety belt design can increase perceived user comfort of restraint systems and the frequency of occupant restraint usage and thus the safety of wheelchair users traveling in motor vehicles.
Ten study participants were recruited in the Pittsburgh area by word-of-mouth, email, and phone (IRB # 021034). Most individuals had impaired range of motion or strength in one or both upper extremities. Eight of the participants used power or power-assisted wheelchairs, one used a manual chair, and one used a scooter.
A safety belt system simulator with reconfigurable anchor points was developed to present various seat belt scenarios to people seated in their own wheelchairs. The seat belt simulator fits various individuals seated in a variety of wheelchairs. Three safety belt scenarios were evaluated for usability (Figure 1):
a) Pelvic and torso belt with all anchorages on the vehicle or on the tiedown system;
Video and interview data were collected from four men and six women using the above restraint scenarios.
Description for Photo 1: Image shows (1) a vehicle mounted pelvic belt and vehicle mounted torso belt, (2) a wheelchair-integrated pelvic belt and vehicle-mounted torso belt and (3) a complete wheelchair-integrated pelvic and torso belt.
To evaluate reach among the wheelchair seated individuals, a laterally adjustable, vertically oriented plane was placed adjacent to the left and right of the safety belt simulator. Measurements were made on the measurement surface directly adjacent to the wheelchair and also 200 mm outboard from that position. Participants demonstrated their reach capability by drawing with a marker on the vertical plane positioned at the side of the wheelchair at the highest, lowest, most-rearward, and most-forward positions that he or she could reach (Figure 2). Data were collected for both hands and for reaches to the ipsilateral and contralateral sides. A digital coordinate measuring instrument, the FARO Arm (FARO Technologies, Lake Mary, FL) was used to record the three-dimensional reach locations of each individual.
Description for Photo 2: Image shows a participant from the pilot study leaning sideways over the wheelchair to reach for a vertical plane.
Description for Photo 3: The figure shows the various methods of reaching and grasping the shoulder belt latch plate.
Description for Photo 4: Sample right-side reach data from one participant showing right-hand reach to zero-mm plane (thin solid line), left-hand reach to zero-mm plane (thick solid line), right-hand reach to 200-mm plane (long dashes) and left-hand reach to 200-mm plane (short dashes).
Right: Plot was calculated from reach zones of all participants on a plane positioned left and right 200mm from the wheelchair centerline. Data is plotted in the side view with respect to the seat to back rest location. The peak density location is indicated by the darkest zone.
Usability Issues: All study participants had one or more problems when using the various safety belt scenarios presented to them and they used adaptive tactics for utilizing the various belts scenarios:
Reach Evaluation: Figure 4 (left) shows sample data from one participant overlaid in side view. This participant was able to complete reaches to both the zero- and 200-mm lateral positions (measured from the most outward side of the wheelchair) and with all four hand/reach-direction combinations (right hand to right and left side, left hand to left and right side). Figure 4 (right) shows a density plot for all participants in the 200 mm plane. Data from reaches to the same side as the hand (right hand reaching right, left hand reaching left) are combined, as are opposite-side reaches. The plot origin is the seat to back rest intersection point. Peak density reach location is indicated by the darkest zone.
Based on the results the authors give the following recommendations for seat belt design for individuals with upper extremity limitations. (1) The stowed location of the latch plate should be located forward of the wheelchair occupant’s shoulder. (2) The latch plate should allow users to ‘hook on’ to the plate and don the belt without requiring a pinch or power grip. (3) Buckle design should allow for single-handed latch plate engagement and disengagement, and also allow flexibility to latch plate alignment during engagement. (5) The buckle should be visible to the user. The pilot data suggest that a point about 250 mm forward of the seat to back rest intersection and no higher than 750 mm above the seat to back rest intersection point would likely be optimal for wheelchair seated individuals with limited upper extremity function.
These recommendations represent substantial design challenges, because designing for usability may conflict with occupant restraint requirements unless innovative designs are developed. For example, the requirement that the buckle be visible to the user to facilitate latching the belt suggests a higher and more forward buckle on a rigid stalk. Yet, such a design might impair the dynamic performance of the lap belt by introducing slack into the system. Further research will be needed to develop restraint systems that combine improved ease of use with optimal crash protection.
This research was supported by the National Institute on Disability and Rehabilitation Research (Grant # 133E010302). The authors thank the Kinedyne Corporation for donating seat belt equipment. Opinions expressed in this paper are those of the authors and do not necessarily reflect those of the funding agency.
Linda van Roosmalen, PhD, IDSA. University of Pittsburgh - 5044 Forbes Tower - Pittsburgh PA 15260 - Phone (412) 3836794 - Email: Lvanroos@pitt.edu