Ultralight Research, Part II
Manual Chair Propulsion: New Insight Into Maneuvering Efficiency
Manual wheelchair propulsion is a topic so frequently talked about that it’s easy to forget how complex it is. Certain facets of propulsion seem obvious: Pushing yourself in a wheelchair is work. Some wheelchairs are easier to propel than others, and some environments are easier to manage than others. But what causes those differences? What forces are at play when clients propel themselves?
In Part II of this series, Dr. Stephen Sprigle addresses that question. (For clarity, we’ve reprinted a portion of Part I as a transition to Part II. ) — Ed.
To illustrate the influences of wheelchair design and configuration on the effort to maneuver a wheelchair, a robot was programmed to perform both straight and turning maneuvers. These maneuvers were designed to highlight the varying inertial and frictional influences that govern propulsion effort. The straight maneuver consisted of accelerating the chair to 0.7 meters/second (m/s) in 2.5 seconds and then continuing at that speed for another 2.5 seconds. The turning maneuver was a fixedwheel turn in which one wheel is driven forward while the other wheel is fixed in place. This maneuver accelerated the wheelchair to 0.5 m/s in 2.5 seconds and then maintained the trajectory for another 2.5 seconds. Therefore, both maneuvers had an acceleration phase and a steady-state phase. Furthermore, these maneuvers were performed on tile and low-pile carpet surfaces.
We selected two wheelchairs that represent disparate designs and configurations.
The standard folding-frame wheelchair with 7" front casters, 24" drive wheels and 1" solid tires had a mass of 17.6 kg (i.e., about 39 lbs.), and when loaded with the robot, had about 55 percent of the system mass loaded onto the drive wheels.
The second chair was an ultralightweight rigid-frame wheelchair with 5" casters and 24" wheels with 1 3/8"-wide pneumatic tires. It had a mass of 12.1 kg/26-plus lbs. with 70 percent of the system mass on the drive wheels. The specific configurations, including the type of tires and mass distribution, are very important to consider when comparing systems. Our methods include complete measurement of the inertias of all components as well as the frictional parameters as measured using a simple coastdown test. These parameters will be discussed as we review the results of the study.
The results are shown in Figures 1 and 2 (graphs on page 18). The values in the figures are the torques required to either accelerate the chair or to maintain the respective speed during the maneuver. Each graph depicts the torques for both phases on tile and carpet. One graph illustrates results from the straight maneuver, while the other is for turning. Torque is reported in N-m and reflects measured work.
Recording the Results
One important result is that the torque required to accelerate the chair is much greater than that required to maintain a steady speed. When going straight, acceleration torques are at least two times greater and are 70 to 110 percent greater during turning compared to steady-state speeds. This clearly illustrates the influence of inertia, since work is required to overcome inertia to impart speed onto a stationary wheelchair.
The forces required to keep a chair moving at a constant speed are quite low. Using the torque data, we can calculate the forces applied to the handrims. On tile, less than 2 lbs. of force were applied to both wheels to maintain the straight speed of the standard and ultralightweight wheelchairs, and about 3 lbs. were needed to maintain a turn. These forces are quite low. As a reference, the weight of the arm of a 150-pound person is about 8 lbs.
A second finding is that propelling on carpet requires much greater effort than propelling on tile. The torques applied during acceleration were 33 percent greater on carpet, while during the steady-state phases, rolling on carpet required 60 percent greater torque. This shows the influence of friction. As mentioned, friction is dependent on the weight of the occupied wheelchair and the interaction between the tires and the rolling surface.
Neither of these results is particularly surprising because they simply reflect the influences of inertia (during acceleration) and friction (carpet) on propulsion effort. However, the results are useful because they provide a guideline for predicting propulsion effort. If you want to see how a client maneuvers in a particular wheelchair, have them move about while accelerating and turning on a variety of surfaces.
A lot of interesting information, however, comes from comparing these two wheelchairs during these four maneuvers. The results offer new information about how wheelchair propulsion torque is directly influenced by wheelchair configurations. Just as importantly, the results provide a sense of the magnitude of differences, thereby permitting users and clinicians to gauge the importance of specific influences.
The straight maneuver shows a consistent influence of system mass on propulsion torque. The mass of an occupied chair is a measure of inertia, as we discussed. However, the weight of the chair also influences friction because friction is directly related to the force on the tires.
During acceleration, the standard wheelchair requires less than 10 percent more torque, whether on tile or carpet. Differences in torque during steady-state speed were more pronounced. Steady-state torque on tile was a whopping 50 percent greater for the standard chair and 18 percent greater on carpet. This shows that the influences of inertia (i.e., mass or weight) were more pronounced under a lower-friction condition.
Torque & Turns
Turning offers a different picture….
The differences between torques during turning are vast while on tile, but small when turning on carpet.
On a tile surface, the friction is relatively lower, so the greater mass and turning inertia of the standard wheelchair resulted in greater torque.
However, the wheelchairs required nearly identical torque to turn on carpet. This is because two factors, weight distribution and the tires, had a large impact on friction. The greater weight on the drive wheels increases the tire scrub during turning. Using a coast-down test, our data suggests that the standard wheelchair had greater frictional loss when going straight, but the ultralightweight wheelchair had greater frictional loss when turning. So, despite the lower overall weight and pneumatic tires, the ultralightweight chair has a greater frictional loss during turns.
How can this be?
The ultralightweight was configured with 1 3/8"-wide tires with noticeable tread. This is a general-purpose tire that tries to be adequate on a lot of surfaces. These tires are not, however, lowfriction tires when turning, especially on low pile carpet.
In addition, due to the differences in mass distribution, the ultralightweight wheelchair had 10 pounds more weight on its drive wheels. This extra weight appears to have had a significant influence on the tire scrub when turning, and this impacted the propulsion torque to be similar to that of a standard wheelchair. So, based upon these results, the ultralightweight wheelchair requires less torque to perform a maneuver compared to a standard wheelchair. This is not really a surprise to anyone, but these results are the first time that accurate comparisons can be made across maneuvers and rolling surfaces. Differences between these chairs are greater when moving in a straight direction and while traversing a tile floor. However, the ultralightweight loses its advantage when turning, especially on carpet. Its tires and weight distribution combine to result in a configuration with high frictional energy loss during turning.
Anticipating Configuration Trade-Offs
These results represent the third important point of this article: 3) Every wheelchair configuration has a trade-off in inertia and/or friction. Clinicians and wheelchair users should reflect upon these characteristics with respect to wheelchair use and propulsion effort.
As an example, moving an axle forward has several benefits. It moves the handrims in a better position to be propelled by the user (a biomechanical benefit) and decreases the turning inertia of the wheelchair. It also places more weight on the drive wheels, which results in lower rolling resistance when going straight. However, it increases friction when turning, especially on highfriction surfaces like carpet. Therefore, moving the axle forward has positive and negative impacts on wheelchair propulsion, and nearly every other configuration decision will as well.
These results are also useful in directing clinicians on how to evaluate different wheelchairs. If you want to assess how a client maneuvers in a particular wheelchair, have them move about while accelerating and turning, and do so on a variety of surfaces. Asking someone to push straight down a tile hallway is not very effortful, and most importantly, is not reflective of how people use wheelchairs. Going further, it is crucial to determine how a person uses their wheelchair. People who largely stay indoors on flat and hard surfaces may benefit from different tires and configurations more than individuals who travel outdoors over different grades and surfaces.
Our data from monitoring 68 wheelchair users indicate that maneuvering is the key functional aspect. In distinction, persons who accelerate rapidly to achieve a higher steady-state speed will be more impacted by friction and inertia. More active users should, in fact, be more concerned about the inertia and friction of their wheelchairs compared to less active users. More active users apply greater instantaneous effort to the handrims and do so more often throughout the day. One can rightfully ask if using a robot is an appropriate way to study propulsion effort. In general, the effort required to propel a wheelchair is a function of biomechanics and mechanical design, and the interaction between these two variables. While a robotic system cannot address issues related to biomechanics, we believe it is the only way to characterize the influences that wheelchair design and configuration have on propulsion torque. This is because the robot is highly repeatable, and never tires. Therefore, it can perform multiple maneuvers in the same manner over and over. In addition, the means by which torque is applied to the handrims does not change the inertia or friction of a manual wheelchair. In other words, whether a human or a robot applies torque to the handrim does not alter the inertia or friction of a wheelchair — given the same maneuver.
The last phrase in the previous sentence is important to remember. All these measurements are reflective of the maneuvers being performed. Remember, we defined wheelchair efficiency as the work required to perform a maneuver. The robot affords the ability to perform the same maneuver over and over again.
However, we defined the maneuver. Different speeds, different accelerations, or different surfaces would all result in different torques. For example, a fixed wheel turn is a tight turn that involves a lot of tire scrub; certainly wheelchair users perform tight turns, but they also perform more sweeping turns, which include different frictional losses. However, different maneuvers would still be beholden to the laws of physics and that fact that inertia and friction have the primary influence on propulsion effort.
With that said, we have more work to do in order to better inform users and clinicians about configuring wheelchairs. In my opinion, making binary comparisons is not as useful as defining influences as a continuum. We can use this approach to answer many questions:
- How should axle position be optimized to gain the most biomechanical and mechanical benefits?
- How much does propulsion torque vary across different ultralightweight wheelchairs?
- Which tires perform best on various surfaces (knowing that no single tire will perform best on all surfaces)?
- What is the difference in propulsion torque of folding frames versus rigid frames?
- How quickly must one accelerate to gain benefit from highend wheels that have low rotational inertia?
Two More Clinical Implications to Consider
Clinical implication #2
The data presented here reflects the differences in propulsion torque due to the friction and inertia of two wheelchairs. However, it does not reflect the differences that can occur in the biomechanics of the propulsion stroke. Good biomechanics are based upon a well-fitted wheelchair and proper training. A user seated in an ill-fitted ultralightweight wheelchair could very easily exert more effort than if seated in a well-fit standard wheelchair. Therefore, equal attention must be paid to positioning users in a properly fitted chair.
Clinical Implication #3
Both inertia and friction of an occupied wheelchair are reflective of its mass and mass distribution. Therefore, both mass and distribution are clinically relevant. However, the occupied wheelchair is the important consideration because the wheelchair user is, by far, the dominant mass. The chairs used in this study varied by 5.5 kg, which is an appreciable amount. The unoccupied standard wheelchair’s mass was 45 percent greater than that of the unoccupied ultra lightweight wheelchair. However, the occupied mass difference was 5 percent.
Editor’s Note: Stephen Sprigle, Ph.D., PT, is a professor of Applied Physiology, Bioengineering & Industrial Design at the Georgia Institute of Technology in Atlanta. For the first part of this series, check out the November 2014 issue of Mobility Management, or go directly to the story online.
This article originally appeared in the January 2015 issue of Mobility Management.