Effects of a Dynamic Seating System on Wheelchair Propulsion in Individuals with SCI
Aditya Chawla, BA
Rehabilitation Institute of Chicago
ABSTRACT
An innovative wheelchair seat design that relieves ischial support can play a pivotal role in pressure ulcer prevention. However, changes in sitting posture may affect an individual’s ability to propel a wheelchair. To investigate this possibility, ten participants with spinal cord injury were asked to propel a wheelchair using a normal and the pressure-relief posture (WO-BPS) while unilateral muscle activity of the rhomboid major, upper trapezius, posterior deltoid, triceps brachii, long head of biceps brachii, anterior deltoid, sternal pectoralis major, and sternocleidomastoid was measured using electromyography. Additionally, torques and forces applied to the push rim were obtained using a SmartWheel® and kinematics of the upper and lower arm were acquired with a Vicon motion capture system. There was no significant difference between the Normal and WO-BPS postures with respect to wheelchair propulsion. Therefore, it is concluded that the WO-BPS posture does not reduce efficiency during performance of the tested tasks.
Key words:
posture, seating, muscle activation, wheelchair propulsion
INTRODUCTION
There are approximately 1.4 million wheelchair users in the United States of which 75% use manual wheelchairs. Wheelchair design can affect user independence, accessibility, and ability to propel. As the population of wheelchair users continues to grow, a strong understanding of propulsion becomes imperative. Specifically, wheelchair users need to maintain efficient motion without excessively exposing themselves to the risks of repetitive stress injury. The repetitive nature of propelling a wheelchair places significant stress on the hands and joints of the upper extremity creating notable risk of pain and muscular overload. The presence of SCI with its associated neuromuscular changes and compensatory muscle activity increases the mechanical strain placed on the upper extremities. Moreover, SCI related muscle paralysis can produce poor posture, increasing the strain placed on the neck and the spine. Finally, poor propulsion technique, especially with respect to initial hand placement, can increase both the load and frequency of stress exerted on the shoulder threatening joint integrity. Specifically, there is a relationship between pushrim forces and shoulder injury progression (1).
The Biomechanics Occupational Safety, and Sports Medicine lab at Northwestern University designed an alternating wheelchair seating system that intermittently transfers weight between the buttocks and thighs. Weight transfer is achieved by tilting the back part of the seat (BPS). Lowering the BPS shifts weight off of the buttocks towards the thighs. BPS lowering is accompanied by inflation of a lumbar support. These two adjustments alter the body’s posture by lowering the ischial support and accentuating lumbar lordosis. This was defined as the WO-BPS posture. WO-BPS posture helps to both relieve the sitting pressure from the buttocks and improve sitting posture. Nevertheless, it was of concern that the changes in posture may affect user ability to propel the wheelchair. The objective of this study was to compare the propulsion of a wheelchair with the seat in a standard and WO-BPS configuration.
METHODS
Ten volunteers with SCI were tested (42.7±11.8yrs; 84.4±27.1kg; 170.2±11.3cm; BMI 27.0±6.0; with injury levels from C5 to L1). None of the participants had a history of shoulder problems or presented with any medical contraindications that would have prevented their inclusion in this study.
Figure 1: The wheelchair equipped with a dynamic seating
system used in this study. (A) The normal seating position. (B) WO-BPS
configuration. (Click image for larger view) A wheelchair equipped with a dynamic seating system was used for all tests. Shown in Figure 1, the seating system consisted of a split seat and a backrest with an enhanced lumbar support. The split seat had a movable BPS that could be tilted downward (20°) to reduce contact between the user’s buttocks and the seat. The seating system can be set statically to a regular upright sitting posture (defined as a Normal posture), and a sitting posture with the BPS tilted down (defined as a WO-BPS posture). In addition, the seating system can also be set in a dynamic way to cyclically alter the sitting configuration of the wheelchair between the two aforementioned postures. In this way, the back part of the seat was cyclically tilted downward to reduce the sitting load exerted to the buttocks.
Figure
2: Experimental
Setup (Click image for larger view) Wheelchair propulsion was evaluated by measuring kinematics (Eight Camera MCam2, Vicon Peak, Centennial, CO), kinetics (SmartWheel®, Three Rivers Holdings LLC, Mesa, AZ), and muscle activities (Delsys Inc, Boston, Maryland) (Figure 2, experimental setup) during propulsion tasks.
Subjects were given the following tasks: propelling in a straight line for 5 meters, propelling five meters with a 45o turn, propelling five meters with a 90o turn, performing a push-up from a seated position, propelling up a 10º ramp, propelling over carpet, propelling over an asphalt-like surface, and propelling over an uneven surface.
The subject was asked to perform each of the propulsion tasks 3 times. All tasks were performed in both postures: Normal, and WO-BPS. The starting posture was randomized. The subject was instructed to perform each task as consistently as possible.
Muscle activity was monitored by unilateral surface EMGs that were placed on rhomboid major, upper trapezius, posterior deltoid, triceps brachii, long head of biceps brachii, anterior deltoid, sternal pectoralis major, and sternocleidomastoid. Vicon motion capture system was used to collect kinematic data of the upper body (elbow flexion angle), and a SmartWheel was used to measure pushrim forces. Collected data was processed in Matlab and analyzed in Microsoft Excel.
The elbow flexion angle, the tangential force and axial moment applied to hand rim (Mz: which rolls the wheel forward or backward), muscle activity of shoulder and arm muscles, and the stroke for each propulsion cycle were averaged between the two trials for each propulsion task. Recorded data was normalized to the average Mz of the trial.
The propulsion force on the push-rim has 3 components, tangential (FT), radial (FR), and axial (FA). A Propulsion Force Efficiency (PFE) was calculated as the fraction of the FT to the total force Ftotal (Equation 1). This PFE was used to quantify the efficiency of the propulsion as how much of the applied pushing force contributes to the movement of the wheelchair.
PFE = FT/Square_root(FT2+FR2+FA2)
A paired t-test was used to determine any significant differences in the above mentioned data between the Normal and WO-BPS sitting postures. All statistical analysis was performed using SAS software package (SAS 9.1.3, SAS Institute Inc., Cary, NC) with the significant level set to 0.05.
RESULTS
Table 1 shows the muscle activity differences (mv, mean±SE) between Normal and WO-BPS postures as measured from shoulder and arm muscles during wheelchair propulsion tasks (a straight line, a line with a cut of 45˚ or 90˚ turn midway, propelling over carpet, over an asphalt-like surface, uneven surface and propelling up a 10° ramp). During the straight propulsion task, which is arguably representative of the most basic form of propulsion, the differences in muscle activity were: rhomboid major 0.001±0.007mv; upper trapezius -0.001±0.006 mv; posterior deltoid 0.004±0.007mv, triceps 0.005±0.008mv; long head of biceps brachii -0.002±0.007mv; anterior deltoid 0.001±0.008mv; sternal pectoralis major 0.003±0.006mv; and sternocleidomastoid 0.001±0.008mv. There were no significant (P<0.05) differences between the Normal and WO-BPS postures for any of the wheelchair propulsion tasks.
| Paraplegic (N=10) |
Rhom-mj | Trapz-up | Delt-pos | Tricp | Bic | Delt-ant | Pec-mj | SCM |
|---|---|---|---|---|---|---|---|---|
| Straight |
0.001±0.007 | -0.001±0.006 | 0.004±0.007 | 0.005±0.008 | -0.002±0.007 | 0.001±0.008 | 0.003±0.006 | 0.001±0.008 |
| Cut 90° | 0.038±0.040 | -0.008±0.006 | -0.005±0.007 | -0.006±0.007 | -0.007±0.006 | -0.014±0.009 | 0.037±0.040 | -0.009±0.006 |
| Cut 45° | 0.012±0.014 | -0.001±0.003 | -0.002±0.003 | 0.000±0.003 | -0.001±0.004 | -0.002±0.005 | 0.048±0.041 | -0.002±0.003 |
| Carpet | 0.019±0.026 | -0.004±0.004 | -0.007±0.006 | -0.002±0.003 | 0.009±0.021 | -0.003±0.004 | 0.016±0.016 | -0.005±0.005 |
| Asphalt | 0.001±0.015 | -0.008±0.007 | 0.000±0.013 | -0.007±0.007 | -0.014±0.008 | 0.000±0.007 | -0.004±0.011 | -0.008±0.007 |
| Bumpy | -0.005±0.005 | -0.006±0.005 | -0.007±0.005 | -0.003±0.002 | -0.006±0.005 | -0.003±0.002 | 0.015±0.019 | -0.006±0.005 |
| Ramp | 0.000±0.005 | -0.002±0.003 | 0.000±0.003 | -0.001±0.004 | -0.051±0.049 | -0.007±0.009 | 0.004±0.010 | -0.003±0.003 |
The efficiency data are shown in Table 2 for Normal and WO-BPS postures during wheelchair propulsion tasks. For each task performed the two postures produced similar levels of efficiency. For example, for the straight task PFE was measured as Normal: 0.66±0.17 and WO-BPS: 0.63±0.12. Here the difference in PFE, calculated as W-N, was -0.03±0.12. Among all the trials performed PFE varied from 0.54±0.12 to 0.71±0.15. The differences in efficiency ranged from as low as -0.07±0.05to 0.02±0.06. Nevertheless, there were no significant (P>0.05) differences between the Normal and WO-BPS postures for any of the wheelchair propulsion tasks.
| Paraplegic |
Normal |
WO-BPS |
Efficiency |
|---|---|---|---|
| Straight |
0.66±0.17 |
0.63±0.12 |
-0.03±0.12 |
| Cut 90° |
0.59±0.11 |
0.56±0.13 |
-0.03±0.05 |
| Cut 45° |
0.54±0.12 |
0.55±0.10 |
0.01±0.08 |
| Carpet |
0.61±0.11 |
0.61±0.12 |
0.00±0.04 |
| Asphalt |
0.56±0.14 |
0.57±0.11 |
0.02±0.06 |
| Bumpy |
0.64±0.15 |
0.71±0.15 |
-0.07±0.05 |
| Ramp |
0.62±0.12 |
0.64±0.11 |
0.02±0.06 |
Finally, there was no significant difference between the Normal and WO-BPS postures in elbow flexion angle data (P>0.05) when propelling the wheelchair.
DISCUSSION
This study aimed to find evidence supporting or negating the hypothesis that changes in posture will not significantly change muscle activity, elbow flexion angle or force efficiency during wheelchair propulsion. Changes in posture produced by the WO-BPS seating system have been proven beneficial to wheelchair users by decreasing pressure on the ischial tuberosities, as well as improving the spinal curvature, i.e. increasing the segmental and total lordosis, and rotating the pelvis anteriorly (2). Poor posture is a condition that individuals with disabilities may not be able to avoid, as previous studies have established. The strength of the chronically paralyzed muscles is reduced in individuals with SCI. The WO-BPS seating system was designed to support the spine, as well as decrease the risk of pressure ulcers. In sitting, postural correction tended to decrease level of muscle activation required in all the upper extremity muscles (3). The efficiency of propulsion with respect to the shoulder is based on the direction of forces. The relevant forces are radial, tangential and axial. The axial forces are negligible. The tangential forces cause the forward movement, and the radial forces, while inefficient, provide traction for gripping the wheel. Modified patterns of propulsion that give a better ratio of tangential to radial forces improve propulsive efficiency but also create a greater degree of biomechanical strain, particularly on the shoulder(4). Individuals who propel with a greater percentage of force directed towards the axle are at an increased risk of shoulder injury (Boninger, 2003). Our evidence shows that there were no significant changes in muscle activity, elbow flexion angle, and force efficiency between the two tested postures.
The lack of significant differences implies that the WO-BPS seating system can be used to reap the benefits it confers to user posture and pressure relief without presenting significant detriment to wheelchair propulsion. Moreover, users would not need to worry about manually adjusting the seating system’s configuration when confronted with all functional propulsion tasks; a similar level of propulsion efficiency can be achieved with either postural configuration.
REFERENCES
- Boninger ML et al. “Shoulder Magnetic Resonance Imaging Abnormalities, Wheelchair Propulsion and Gender” Archives of Physical Medicine and Rehabilitation. 2003 Nov;84(11):1615-20.
- Makhsous M, Lin F, Hendrix R W, Hepler M, Zhang L-Q. 2003. “Sitting with adjustable ischial and back supports: Biomechanical changes.” Spine, 28(11): 1113-1122. June 1, 2003
- McLean L. “The effect of postural correction on muscle activation amplitudes recorded from the cervicobrachial region” Journal of Electromyographical Kinesiology. 2005 Dec: 15(6):257-35.
- Rozendaal LA. Veeger HE. van der Woude LH. “The push force pattern in manual wheelchair propulsion as a balance between cost and effect.” Journal of Biomechanics. 36(2):239-47, 2003 Feb.
ACKNOWLEDGEMENT
The project was supported in part by Falk Medical Research Trust, Medical Technology Systems, and STTR NIH #R41HD047959-01.
Aditya Chawla, BA
Rehabilitation Institute of Chicago
Sensory Motor Performance Program
345 East Superior St., Suite 1408
Chicago, IL
60611
312-503-6801
a-chawla@northwestern.edu
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