Corrective Actions Necessary for Maintaining a Wheelchair Wheelie
ABSTRACT
The purpose of this study was to examine the corrective actions necessary to balance a wheelchair in a wheelie position. Five manual wheelchair users with a spinal cord injury were asked to ‘pop' a wheelie and hold the wheelie position for approximately twenty seconds while pushrim forces, trunk angle, wheelchair displacement, and force plate data were collected. Balancing in a wheelie mostly involved corrective actions of the hands and to a lesser degree the torso. Proficient wheelie performers may be more apt to make force corrections of higher frequency and lower magnitude rather than lower frequency and higher magnitude. Understanding the strategies used to balance in a wheelie may help new wheelchair users learn this valuable skill as well as help clinicians who teach this skill.
KEYWORDS
Wheelie, balance, kinetics, kinematics, manual wheelchair
BACKGROUND
Being able to ‘pop' a wheelie and keep the front castors lifted off the ground can help with negotiating difficult terrain and obstacles. Understanding the strategies used to balance in a wheelie may help new wheelchair users learn this valuable skill as well as help clinicians who teach this skill. Bonaparte et al. studied proactive and reactive balance strategies of an unimpaired group of wheelie performers (1). Using video recordings, they determined pitch angle and displacement and studied the interaction and timing between the two to determine the type of strategy used to successfully perform and maintain a wheelie. Lin et al. (2) observed the effects of seat position on wheelie performance using an instrumented wheel that recorded pushrim forces and motion analysis of the body and wheelchair in a group of unimpaired volunteers. A seat that was more rearward relative to the wheel axle resulted in smaller pitch angles and lower tangential forces to initiate a wheelie.
To balance on the rear wheels, the body/wheelchair center of mass (COM) must lie within the base of support, in this case the contact area between the wheels and the ground (1). This base of support is small in comparison to a person standing upright where the base of support is the area outlined by the feet. As such, keeping the COM in alignment within the base of support in a wheelie task is more challenging and will require corrective actions of the hands and possibly the torso to maintain balance. Thus the purpose of this study was to determine directional changes in applied force and magnitude of force required to balance in a wheelie. In addition, we also looked at trunk angular position, pitch angle and wheel displacement changes that occurred while balancing in a wheelie. As wheelchair setup has a direct effect on the ability to perform a wheelie (2), the results of this analysis could provide insight into how to best configure a wheelchair to enhance mobility.
METHODS
Subjects:
Four men and one woman with a spinal cord injury provided informed consent to participate in the study. Descriptive information about the subjects is shown in Table 1. All subjects were full-time manual wheelchair users and had to be able to pop a wheelie and balance on the rear wheels for one minute without assistance to be included in the study. All but one subject (Subject #5) mentioned that they perform wheelies on a regular basis.
Subject |
Age (years) |
Height (m) |
Weight (kg) |
Wheelchair Use (years) |
Gender |
SCI level |
|---|---|---|---|---|---|---|
1 |
35 |
1.86 |
77.3 |
17 |
M |
C7 |
2 |
28 |
1.77 |
59.1 |
10 |
M |
C7/C8 |
3 |
43 |
1.83 |
59.1 |
23 |
M |
T7/8 |
4 |
37 |
1.83 |
77.3 |
17 |
M |
T12-L1 |
5 |
45 |
1.58 |
54.6 |
17 |
F |
T4 |
Totals |
37.6 (6.8) |
1.8 (0.1) |
65.5 (11.0) |
16.7 (4.6) |
- |
- |
Experimental Protocol:
SMART Wheels (Three Rivers Holdings, Mesa, AZ), force and torque sensing pushrims, were installed on the subject's own wheelchair. Two infrared markers of the OPTOTRAK (Northern Digital, Inc., Waterloo, CA) 3-D motion analysis system were placed along the wheelchair seat frame and a single marker was placed on both the acromion bony prominence of the shoulder and the hub. Subjects's wheelchairs were centered over two force plates (AMTI, Watertown, MA). Subjects' were asked to practice performing a wheelie first, to become acclimated to the experimental setup. Marker coordinates (sampled at 60 Hz), pushrim forces (sampled at 240 Hz) and force plate data (sampled at 360 Hz) were recorded for twenty seconds during which time the subject popped a wheelie and balanced in a wheelie position over the force plates until given a verbal cue after 18 seconds to land the front casters.
Data Analysis:
SMARTwheel and force plate data were linearly interpolated for synchronization with the kinematic data collection rate of 60 Hz. The pitch angle was defined as the angle between a line connecting the two markers on the seat frame during the wheelie with respect to a line connecting these same markers at the beginning of the trial when all four wheels were on the ground. Trunk angle was defined as the angle between a line through the hub and seat frame and a line between the shoulder and hub. Linear movement of the rear wheels (displacement) was determined from the horizontal hub position coordinate.
The resultant force from the SMART Wheel was computed by taking the vector sum of the individual force components (Fx, Fy, and Fz). Prior investigations [1,2] on wheelie activity assumed the wheelchair moves along a Cartesian axis. However, in practice this may not be the case. Thus, resultant force was projected onto a vector that lies along the direction of wheelchair motion using Equation 1.
 |
|---|
The number of corrections was determined by counting the number of local maximum force values. Correction frequency was defined as the number of directional changes/second. The average difference in force magnitude (corrective force) between each local maximum value was also determined (Figure 1). The same variables were also determined for trunk angle, pitch angle and rear wheel displacement.
 |
|---|
Resultant force plots were visually inspected to discern the initial lift off of the casters from the wheelie balance phase. Using the middle five seconds of the balance phase, mean resultant force, pitch angle, trunk angle displacement were computed. In addition, the average number of corrections, correction frequency, and corrective values of force, trunk angle, pitch angle and displacement were determined over the same five seconds.
RESULTS
Subjects applied 11.2 N of force on average and 2.6 N of corrective force to balance in a wheelie (Tables 2 and 3). Pitch angles for the group ranged from 6 degrees to 11.6 degrees, trunk angle from 84.4 degrees to 90.4 degrees and displacement from 29.8 cm to 51.8 cm (Table 2). Corrective changes in force ranged from 0.2 to 10.6 N, pitch angle from 0.1 to 7.6 degrees, trunk angle from 0.1 to 3.5 degrees and displacement from 0.3 cm to 3.2 cm (Table 3). Corrective frequencies for force ranged from 9 to 22.8, pitch angle from 6.2 to 17.0, trunk angle from 3.2 to 52, and displacement from 3.2 to 32 directional changes per second (Table 4). Number of corrections and correction frequency were highest for the forces, followed by the pitch angle, trunk angle and displacement, respectively. Average corrective forces for Subject #5 were the highest at 32 N and her force correction frequency (9 directional changes per second) was the lowest of the group.
Subject |
Force (N) |
Pitch Angle (deg) |
Trunk angle (deg) |
Displacement (cm) |
|---|---|---|---|---|
1 |
5.4 |
6.1 |
87.4 |
46.7 |
4 |
7.7 |
11.0 |
90.0 |
32.8 |
2 |
7.0 |
10.7 |
90.4 |
29.8 |
3 |
4.0 |
11.6 |
89.4 |
45.0 |
5 |
32.0 |
8.0 |
84.4 |
51.8 |
Totals |
11.2 (11.7) |
9.5 (2.3) |
88.3 (2.5) |
41.2 (9.5) |
Subject |
Force (N) |
Pitch Angle (deg) |
Trunk Angle (deg) |
Displacement (cm) |
|---|---|---|---|---|
1 |
0.2 |
7.6 |
3.0 |
0.6 |
4 |
0.8 |
0.1 |
2.4 |
3.2 |
2 |
1.0 |
0.2 |
0.1 |
3.2 |
3 |
0.7 |
0.3 |
3.5 |
0.3 |
5 |
10.6 |
1.4 |
1.9 |
2.1 |
Totals |
2.6 (4.5) |
1.9 (3.2) |
2.2 (1.3) |
1.9 (1.4) |
Subject |
Force |
Pitch Angle |
Trunk Angle |
Displacement |
||||
|---|---|---|---|---|---|---|---|---|
|
No. of Max |
Frequency |
No. of Max |
Frequency |
No. of Max |
Frequency |
No. of Max |
Frequency |
1 |
114 |
22.8 |
48 |
9.6 |
30 |
6.0 |
7 |
1.4 |
4 |
85 |
17.0 |
85 |
17.0 |
39 |
7.8 |
11 |
2.2 |
2 |
60 |
12.0 |
32 |
6.4 |
26 |
52.0 |
11 |
2.2 |
3 |
66 |
13.2 |
31 |
6.2 |
26 |
52.0 |
12 |
2.4 |
5 |
45 |
9.0 |
34 |
6.8 |
16 |
3.2 |
10 |
2.0 |
Totals |
74 (27) |
14.8 (5.3) |
46 (23) |
9.2 (4.6) |
27 (8) |
24.2 (25.4) |
10 (2) |
2.0 (0.4) |
DISCUSSION
Applied forces by the hand were used more than gross movements of the trunk to compensate for perturbations in COM displacement. It was interesting to find that the two wheelchair users with tetraplegia had more hand corrections than the wheelchair users with paraplegia. In addition, directional changes of the trunk were less for users with tetraplegia compared to the two wheelchair users with paraplegia who frequently performed wheelies. The directional changes in applied forces were approximately 2 times higher than the directional changes in pitch angle and 7 times higher than directional changes in displacement of the wheelchair. So, even though the hand moves frequently on the pushrim, the amount of force that is exerted is small (<= 1.0 N in the users performing wheelies regularly) and may not necessary lead to a corresponding change in pitch angle or displacement. For the one subject who did not perform wheelie's regularly, the force corrective frequency was small and the magnitude of corrective force large. Thus, it may be that wheelchair users who are less proficient in performing wheelies are less familiar with their ‘balance point' and allow the COM to deviate further from the base of support before making a correction which in turn needs to be large to compensate for the large COM displacement. Persons who perform wheelies regularly and know their ‘balance point' move their hands more leisurely along the pushrim making subtle corrections to small deviations in COM displacement.
Directional changes in the pitch angle may not necessary lead to directional changes in wheel displacement indicating that tilting the wheelchair is used more often than linear movements of the wheelchair to keep the COM in alignment. In addition, the results provide an idea of how much effort is required to balance in a wheelie. Average forces necessary to maintain balance in a wheelie were approximately 6% of forces used to propel a wheelchair at 0.9 m/s (3). However, it's important to consider that the forces required to initially lift the castors off the ground are higher (Figure 1). In addition, pitch angle, which is a measure of how well the wheelchair is adjusted to the user, didn't seem to be related to the amount of correction needed to maintain balance. But degree of pitch does make a difference in the amount of force needed to lift the castors off the ground (2).
CONCLUSION
Balancing in a wheelie mostly involves corrective actions of the hands and to a lesser degree the torso. Proficient wheelie performers may be more apt to make force corrections of higher frequency and lower magnitude rather than lower frequency and higher magnitude. Future studies should look more closely at the interaction and timing between corrective force and torso actions and corresponding changes in wheelchair pitch and displacement. Future studies should also include more subjects with varying levels of SCI and wheelie experience.
REFERENCES
- Bonaparte JP, Kirby RL, & Macleod DA ( 2001 ). Proactive balance strategy while maintaining a stationary wheelie: Archives of physical Medicine & Rehabilitation . 82(4): 475-9.
- Lin Po-Chou, Chang Jyh-Jong, Su Fong-Chin. The effects of seat for-aft position in wheelie activity http://140.116.211.15/service/s-report/ci01.doc
- Boninger ML, Cooper RA, Baldwin MA, Shimada SD, and Koontz AM (1999). Wheelchair pushrim kinetics: body weight and median nerve function. Archives of Physical Medicine & Rehabilitation . 80:910-5.
ACKNOWLEDGEMENTS:
This study was supported by U.S. Department of VA Affairs, Eastern Paralyzed Veterans of America and National Institute on Disability and Rehabilitation Research (NIDRR H133A011107) .
Alicia Koontz,
Human Engineering Research Labs,
VA Pittsburgh Healthcare System,
Pittsburgh, PA 15206
Phone: (412) 365-4850,
Fax: (412) 365-4858,
Email:akoontz@pitt.edu.
