Philip S. Requejo, PhD, Lisa Lighthall Haubert, MPT, Craig J. Newsam, DPT, Sara J. Mulroy, PhD
ABSTRACT
Pushrim-activated power-assisted wheelchairs (PAPAWs) offer marginal users an alternative mode of manual propulsion. The purpose of this study was to evaluate the energy expenditure and propulsion characteristics in a user with tetraplegia propelling a standard wheelchair (WC) and 3 commercially available PAPAW designs ( i-GLIDE, Xtender and e.motion). We hypothesized that propulsion velocity and reduction in energy expenditure during PAPAW propulsion would depend upon the particular method of power-assist delivery. While all PAPAWs reduced oxygen consumption, only the Xtender demonstrated increased velocity which resulted in the greatest reduction in oxygen cost, likely by providing a stable and coordinated response to variations in force applied to each pushrim.
Keywords: wheelchair propulsion, push-rim activated power-assisted wheelchair, spinal cord injury, energy expenditure.
BACKGROUND
Wheelchair propulsion places an added burden on the upper extremities. Pushrim-activated power-assisted WCs (PAPAW) offer an alternative mode of manual propulsion. In particular, users with upper extremity weakness and/or pain, particularly those with tetraplegia, may benefit from PAPAW use. Metabolic and biomechanical evaluation of PAPAWs has shown decreased energy demands [1; 2], decreased muscular demand [3], and decreased range of motion [4] compared to standard lightweight WC propulsion.
Currently, there are 3 PAPAW models available, each with its own method of assistance via pushrim activation. The iGLIDE ™ (Independence Technology) is a variable-ratio power-assist unit that uses a microprocessor controller to monitor the torque signal from the pushrim and the feedback from the speed sensors in the transmission to adjust the motor output appropriately [3]. The Xtender® (Quickie, Sunrise Medical) uses a set of linear compression springs to regulate the pushrim force while a potentiometer senses the relative motion between the pushrim and hub [5]. The potentiometer signals from both wheels are interfaced to a microcontroller that coordinates control of each wheel’s direct current (DC) motor attached to a transmission within the hub. The microcontroller compensates for discrepancies between the wheels to help keep the chair traveling straight. The e .motion® (Albers) allows users to choose the settings [3]. The settings are varied in how much power is delivered after the pushrim is released, how quickly the motor responds, and how long the motor continues to deliver power after the pushrim is pushed. The sensitivity of the pushrim can be adjusted via a thumbscrew. After the setting is chosen, the same amount of power assistance is delivered with each cycle.
Differences in pushrim sensitivity and power assistance may influence the user’s ability to safely and effectively maneuver the PAPAW. For the marginal WC user, whose upper extremity strength, grasp function, and balance are impaired, these differences may be amplified. While comparisons of the energy demands and propulsion characteristics between a standard WC and a PAPAW have been performed on an ergometer [1; 2; 6], at various resistances [2], assessment in a more realistic environment and at sustained intervals of use is necessary. The purpose of this study was to evaluate the energy expenditure and propulsion characteristics of a standard manual WC and three commercially available PAPAW designs by a user with tetraplegia. We hypothesized that power assistance would decrease energy expenditure and the most stable delivery of power-assist would provide the greatest benefit to marginal users.
METHODOLOGY:
Subject
A 53-year old male with C6 complete tetraplegia for 33 years participated in the study. The subject weighed 195 pounds and his height was 6 feet. He was recruited from the outpatient clinic at Rancho Los Amigos National Rehabilitation Center. Prior to data collection, the subject was provided with a copy of the Bill of Rights of Human Subjects and was asked to read and sign an informed consent form that had been approved by the Rancho Los Amigos Institutional Review Board. All testing was performed at the Pathokinesiology Laboratory in Rancho Los Amigos National Rehabilitation Center.
Equipment and Procedures
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Metabolic energy expenditure (O 2 rate, CO 2 rate, heart rate, respiration rate) was measured during 20-minutes of WC propulsion at a self-selected pace over a 126-meter outdoor track using the COSMED Portable Gas Exchange system (Cosmed K4b 2). A calibration procedure was performed prior to each test according to manufacturer’s specifications. A Polar heart rate monitor was placed on the chest to monitor heart rate. The harness containing the gas analyses/data logger and the battery pack was placed on the subject (Photo 1). A facemask was placed over the subject’s mouth and nose to ensure that all expired gases were captured. Breath-by-breath samples of expired air were measured for volume and analyzed for O 2 and C O 2 concentration and subsequently telemetered to a laptop for real-time monitoring.
The subject propelled a standard manual WC (subject’s own Quickie 2) and three commercially available PAPAWs: i-GLIDE (Photo 2A), Xtender with a 1.5X power-assist (Photo 2B), and e.motion with the settings adjusted to mid sensitivity and power-assist (Photo 2C). The Xtender and e-motion were mounted on a Quickie 2 chair, similar to the subject’s own WC.
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Testing was performed in a single WC for each session 7 days apart. A 15-minute adaptation period for each PAPAW was allowed prior to data collection. Testing began with a 3-minute resting state. The subject then pushed at a free pace for 20-minutes. Propulsion speed, cadence and cycle length were determined during 4 time intervals during the propulsion phase: 3 to 5 minutes, 9 to 10 minutes, 14 to 15 minutes, and 19 to 20 minutes. Meter marks on the 126-meter track were noted at the beginning and the end of the 20-minute propulsion period to record the distance traveled. A 5-minute recovery phase was subsequently collected with the subject at rest. Average metabolic energy expenditure and propulsion characteristics were determined for the 4 time intervals. O2 Cost was determined from the ratio of O2 rate and propulsion velocity.
RESULTS
Propulsion speed averaged across the 4 time intervals was 78 m/min in the standard WC. Propulsion speed increased by 23% in the Xtender (96 m/min) with increased cycle length (1.4 vs. 1.7 meters). Propulsion speed decreased by 18% in the i-GLIDE (64m/min) and 1% in the emotion (76m/min) due to decreased cadence (Table 1). Total distance traveled increased by 22% in the Xtender, decreased by 10% in the i-GLIDE, and was similar in the e.motion. In all sessions propulsion speed and cadence were consistent across the recorded time interval.
| Velocity (m/min) | Cadence (cycles/min) | Total Distance (m) | |
|---|---|---|---|
| Standard | 78 | 57 | 1393 |
| i-GLIDE | 64 | 53 | 1219 |
| Xtender | 96 | 57 | 1703 |
| e.motion | 77 | 52 | 1385 |
During all PAPAW propulsion, average heart rate and O 2 rate across all time intervals decreased compared to standard WC propulsion (Table 2). O 2 cost was reduced by 36% (0.07 vs. 0.11 ml/kg-meter) in the Xtender and 27% (0.08 vs. 0.11 ml/kg-meter) in the e.motion. O 2 cost did not change in the i-GLIDE as a result of decreased propulsion speed. Respiratory rate was higher in all PAPAWs compared to the standard WC.
| Rate of Oxygen Consumption [ml/kg*min] | Oxygen Cost [ml/kg*m] | Heart Rate [beats/min] | Respiratory Rate [breath/min] | |
|---|---|---|---|---|
| Standard | 8.4 (1.3) |
0.11 (0.02) |
82(1) |
27(3) |
| i-GLIDE | 6.9(1.7) |
0.11 (0.03) |
72(1) |
34(7) |
| Xtender | 6.7(1.6) |
0.07 (0.02) |
75(1) |
38 (6) |
| e.motion | 6.2(1.4) |
0.08 (0.02) |
78(1) |
32(7) |
DISCUSSION
The current study compares three PAPAW designs, delineating the effect of differences in the type of power-assistance on efficiency of propulsion by a marginal user. For this subject the Xtender, compared to the iGLIDE and e.motion, had the most stable response to asymmetrical forces applied to the pushrims, subsequently reducing O 2 cost and increasing velocity. While the impact of the difference in method of power delivery and control may be diminished in WC users with less impaired upper extremities, these differences need to be considered in the selection of an appropriate PAPAW for the marginal user.
Our findings are consistent with previous studies demonstrating a reduction in metabolic demands with use of a single PAPAW (Xtender) compared to a standard manual WC [2]. Recently, Algood et al. (2004) reported a reduction of oxygen consumption, ventilation, and mean heart rate in users with tetraplegia. A significant increase in mean velocity during high resistance propulsion with the Xtender was found. Levy et al. (2004) reported a reduction of metabolic and muscular demands in the elderly during i-GLIDE propulsion. While we found a similar decrease in heart rate and O 2 rate, the O 2 cost did not change due to reduced velocity and cadence.
The current investigation was limited since a single subject was tested and the PAPAW adaptation period was short. Future studies should allow a longer adaptation period prior to testing. Additionally, a larger sample size incorporating subjects with various levels of strength, upper extremity pain and balance is necessary. Incorporation of an obstacle course to further evaluate PAPAW efficacy in a natural environment [5], as well as inclusion of a user satisfaction measure [3] would enhance future work.
REFERENCES
- Arva, J., et al. (2001). "Mechanical efficiency and user power requirement with a pushrim activated power assisted wheelchair." Med Eng Phys 23(10): 699-705.
- Algood, S. D., et al. (2004). "Impact of a pushrim-activated power-assisted wheelchair on the metabolic demands, stroke frequency, and range of motion among subjects with tetraplegia." Arch Phys Med Rehabil 85(11): 1865-71.
- Levy, C. E., et al. (2004). "Variable-ratio pushrim-activated power-assist wheelchair eases wheeling over a variety of terrains for elders." Arch Phys Med Rehabil 85(1): 104-12.
- Corfman, T. A., et al. (2003). "Range of motion and stroke frequency differences between manual wheelchair propulsion and pushrim-activated power-assisted wheelchair propulsion." J Spinal Cord Med 26(2): 135-40.
- Cooper, R. A., et al. (2001). "Evaluation of a pushrim-activated, power-assisted wheelchair." Arch Phys Med Rehabil 82(5): 702-8.