INTRODUCTION

In low and middle-income countries (LMIC) the ease of pushing a wheelchair has a large impact on quality of life or wheelchair users and their assistants [1,2]. Power wheelchairs are not easily available; terrain is often rough, and many people do not have automobiles [1,2]. Reducing the physiological cost of pushing a wheelchair improves the quality of life of the wheelchair user and of the assistant [2].

Computer modeling can give insight into the morphological characteristics of a wheelchair which are predicted to impact the ease of rolling. However, computer models are not always able to predict human behaviors that impact parameters, and existing modeling has almost exclusively addressed models involving self-propelled users.

Rolling resistance is inversely related to wheel diameter [4], and for self-propelling users lifting the smaller casters off the ground by tipping the wheelchair into a wheelie position is known to reduce rolling resistance.

The Aspects of Wheelchair Mobility Test (AWMT) protocol has been used to provide data on the ease of rolling for those able to self-propel as well as for assistants pushing wheelchairs [2,5]. This mixed-methods test includes distance traveled in a four-minute period, a visual-analogue scale response, and a qualitative explanatory comment.

Our hypothesis was that a simple two-dimensional model of wheelchair rolling resistance could be used in conjunction with data collected using the AWMT to shed light on wheelchair structures and human behaviors that impact the ease of pushing a wheelchair. Significant differences in AWMT data would confirm the predictions of the model.

METHODS

A two-dimensional model of rolling resistance was constructed to predict the variation of energy expenditure as a function of wheelbase length (the separation between the rear and forward wheels). Energy expenditure for a wheelchair rolling on a level surface is directly proportional to the work required to overcome friction. The coefficient of rolling friction is a function of surface roughness, deformation of the wheel, and in mud, capillary attraction forces [3].  The coefficient of rolling friction is lower for larger diameter wheels [4], being approximately inversely proportional to the square root of the radius [4]. Distributions of weight that place the center of mass closer to the rear wheel axle will typically minimize the friction force opposing motion. However, these penalties can be mitigated by the variation of the angle of applied force to direct it partially downward, lifting the smaller front wheels off the ground.

Participants were a convenience sample of able-bodied university student volunteers (mean age 21.0, SD 6.1, 5M, 17F). BeeLine wheelchairs are widely adaptable and could be adjusted with a long wheelbase and a shorter wheelbase. Four BeeLine wheelchairs were utilized, two had longer wheelbases (43 cm) and two had shorter wheelbases (33 cm). Each pair included one chair on which the push handles has been hinged so that no downward pressure could be exerted. All wheelchairs were fitted with (50 Kg) ISO standard test dummies. The AWMT protocol was utilized in which participants pushed each wheelchair condition for four minutes over a rough terrain track built to roughness equivalent to that of a parking lot [2]. Distance traveled was measured; a response question was completed that included a visual analogue scale question and an explanatory qualitative comment.

Ethics approval was provided by the authors’ university and consent forms were completed by participants.

Analysis was completed using the MiniTab statistical analysis program to perform within-subjects ANOVA for distance traveled and visual analogue scale rating. Simultaneous comparison of means was also completed. Qualitative comments were categorized based on content and counted.

RESULTS

As described above, the model predicted that a long wheelbase would enable ease of pushing, especially on rough surfaces, and that downward pressure on rigid handles would reduce rolling resistance by lifting or partially lifting casters.

Results for participants pushing wheelchair are shown in Figure 2.  Within subjects ANOVA indicated that meters traveled differed significantly between rigid and flexible handles (F(1,55)=12.63, P<0.001). It also indicated that score of the visual analogue scale question regarding perceived ease of travel differed significantly between long and short wheelbase conditions (F(1,55)=5.03, P=0.03). ANOVA results for comparing the scores for flexible and stiff handles were not quite significant due to high variation.  For the comparison between long and short wheelbases while using flexible handles, ten participants commented that the long wheelbase was easier to control, and five commented that it rolled more easily. 

DISCUSSION

Our results support our hypothesis that wheelbase and push handle condition impact rolling resistance for wheelchair pushers. It seems that rolling resistance for wheelchair pushers can be reduced by utilizing downward pressure on push handles.  However, this is only possible when the assistant pushing the chair has learned to exert downward pressure. Because our participants were naïve wheelchair pushers, some seemed not to figure this out, and this likely increased the variation in our data. Future work with experienced wheelchair assistants would be beneficial.  

In real-world conditions, the benefit of downward pressure on the push handles is only available if the wheelchair pusher is tall and strong enough to exert that pressure. In LMIC settings, children with disabilities often attend boarding schools. The authors have often observed a child with cerebral palsy using a friend’s wheelchair as a walker, or a small child pushing another child.  Any difficulty in pushing the chair reduces the mobility of two children. The pushers often do not have the strength or weight to provide enough downward pressure to lift the casters. In this case, the benefit of long wheelbase is of even more concern. If push handles were designed to provide leverage so that shorter or weaker people could lift the casters, that could also potentially be of significant benefit.

CONCLUSION

As our model predicted, downward pressure on push handles and a longer wheelbase resulted in decreased rolling resistance.  This was evident in increased distance traveled and higher visual analogue scale responses for perceived ease of travel. This strongly suggests that wheelchair designs for LMIC should take into consideration push-handle and wheelbase characteristics to benefit the ease of pushing.