Body Mass Index and its Limitation on Functional Wheelchair Propulsion

Jennifer L. Mercer BSE, Michael Boninger MD, Brad Dicianno MD, Alicia Koontz PhD, Rory Cooper PhD, Erik Wolfe MS, Shirley Fitzgerald, PhD
Human Engineering Research Laboratories,
VA Pittsburgh Healthcare System
Departments of Bioengineering and Rehabilitation Science and Technology,
University of Pittsburgh, Pittsburgh, PA 15206

ABSTRACT

Weight is known to increase rolling resistance however its effect on wheelchair propulsion ability and quality of life in wheelchair users is not known. The purpose of this study was to inspect the relationship between BMI and propulsion variables and quality of life measures. Subjects propelled an instrumented wheelchair while velocity data was recorded and also completed CES-D and CHART questionnaires. When controlling for velocity, no difference was observed between cadence and contact time for overweight individuals compared to normal weight wheelchair users. Overweight subjects who were known to have decreased function propulsion ability reported spending more time propelling their wheelchairs and had higher levels of depression.

Keywords :

wheelchairs; body weight; body mass index; functional propulsion; velocity

BACKGROUND

Often upper limb strength is a major criterion for determining who gets a manual wheelchair, while many other factors that affect functional wheelchair propulsion ability are not considered. Increased body weight has been shown to increase rolling resistance, as well as the risk of some repetitive strain injuries (1). To overcome this extra resistance, overweight individuals would need to use more force, increase cadence, or increase contact time with the pushrim. Nearly 25% of adults with disabilities are considered obese, compared to only 15% of the general population (2). Once a person reaches a certain weight, functional propulsion may not be possible, even with normal strength. Our previous study found overweight individuals propelled slower on thick carpet and up a ramp (3). The goal of this study was to add to these findings by investigating cadence and contact time, which have been linked to injury (1). In addition, lifestyle variables including time spent in a wheelchair, time spent propelling a wheelchair, the Center for Epidemiologic Studies Depression Scale (CES-D), and the Craig Handicap Assessment Recording Technique (CHART), a measure of social participation were evaluated (4,5). We hypothesized that overweight wheelchair users would have increased cadence and contact time per stroke to overcome rolling resistance. Additionally, we hypothesized they would have decreased quality of life secondary to decreased functional ability in a wheelchair.

METHODS

Subjects :

Twenty-one subjects were recruited at The 2003 National Veterans Wheelchair. Inclusion criteria included ownership of a manual wheelchair that subjects were capable of propelling and age between 18 and 65 years. Exclusion Criterion was self-reported history of heart or cardiovascular conditions. We specifically attempted to recruit subjects with a range of BMI.

Protocol :

After subjects provided informed consent, a SMART Wheel (Three Rivers Holding, Mesa, AZ) was secured to their manual wheelchairs or a manual wheelchair provided for them (6). Subjects in wheelchairs and the wheelchairs alone were weighed individually to calculate subject weight. Subjects were asked to propel at a self-selected comfortable speed, over an ADA ramp, thick carpet, and indoor tile while velocity data were recorded from the SMART Wheel . Cadence (strokes/second) and contact time, the time spent on the pushrim as opposed to recovery, were each calculated based on three strokes. Subjects were divided into two groups based on being overweight as defined by BMI (BMI- < 25 kg/m 2 and BMI+ > 25 kg/m 2 ) (7). One subject from each group did not complete the CES-D or CHART. The same subject in the BMI- group did not report time spent in his wheelc hair or time propelling. CES-D scores range from 0 to 60, where a score of 22 or above indicates clinical depression (4). CHART scores range from 0 to 100 where a score of 100 indicates no handicap in a particular area of participation (5).

Statistics :

Independent Samples t-tests were used to compare the two groups for differences based on age, height, time spent in a wheelchair, and time spent propelling a wheelchair. An ANCOVA controlling for velocity, was performed to evaluate associations between BMI and cadence and contact time. CES-D and CHART scores were examined using a Mann-Whitney Test.

RESULTS

BMI- had 9 subjects (8 male, 1 female), and BMI+ had 12 subjects (11 male, 1 female). One subject in Group 1 and two subjects in Group 2 had MS, while all others had SCI. Average height for Groups BMI- and BMI+ was not significantly different (72.39 + 0.61 in. and 70.29 + 0.99 in., respectively). However, BMI+ was significantly older with a mean age of 50.42 + 2.48 years compared to 42.56 + 2.18 years for BMI- (p=0.028). Groups BMI- and BMI+ had average BMI of 21.3 + 1.9 kg/m 2 and 29.9 + 4.4 kg/m 2 , respectively.

Table 1 provides a summary of the calculated results for the propulsion variables. Average velocity from our previous study is presented for convenience (3). When controlling for velocity, there were no differences in cadence and contact time between the two groups.

**Table 1: Comparison of Functional Wheelchair Propulsion Variables Between Two Groups**
Propulsion Variable	Surface Condition	BMI-	BMI+
Average Velocity (m/s) 1	Indoor Tile	1.41 + 0.19	1.33 + 0.23
	Thick Carpet	1.33 + 0.49**	1.05 + 0.24**
	Ramp	1.37 + 0.42*	0.96 + 0.27*
Cadence (1/s)	Indoor Tile	0.928 + 0.18	0.864 + 0.14
	Thick Carpet	1.05 + 0.13	0.932 + 0.16
	Ramp	1.18 + 0.20	0.969 + 0.20
Contact Time (s)	Indoor Tile	0.374 + 0.08	0.414 + 0.10
	Thick Carpet	0.424 + 0.09	0.529 + 0.12
	Ramp	0.379 + 0.16	0.502 + 0.08
results are significant (p<.05), * results approaching significance (.05=p<.10) 1 Previously reported (3)

Table 2 summarizes the differences in lifestyle variables between the two groups. The two groups spent the same amount of time in their wheelchairs, but the overweight individuals reported spending more time propelling. Overweight individuals also had significantly higher CES-D scores but no difference was observed in the CHART.

**Table 2: Comparison of Lifestyle Variables Between Two Groups**
Lifestyle Variable	BMI-	BMI+
Time spent in a wheelchair (hours/day)	10.25 + 6.39	12.71 + 3.36
Time spent propelling a wheelchair (hours/day)	4.50 + 3.59**	7.58 + 3.45**
CES-D score	4.00 + 2.65*	14.78 + 14.36*
Mobility CHART score	91.14 + 5.82	91.60 + 4.47
Social Participation CHART score	88.29 + 6.33	83.70 + 8.31
results are significant (p<.05), * results approaching significance (.05=p<.10)

DISCUSSION

Our previous analysis found that freely chosen speed differed based on BMI (3). Overweight individuals may select a lower propulsion speed to protect themselves against injuries resulting from increased cadence or higher forces needed to maintain a higher speed. Alternately, these individuals may simply experience difficulty propelling their wheelchair at a higher speed because of their increased weight. Contrary to our hypothesis, the BMI+ group did not propel with an increased cadence, they simply went slower on more challenging surface. Going slower has practical implications for socialization and function. On thick carpet and ramps the BMI+ group went much slower than average walking speed.

The slow self-selected propulsion speed likely impacted measures of quality of life. Although subjects in both groups reported spending the same amount of time in their wheelchair per day, the BMI+ group reported spending a higher proportion of that time actively propelling the wheelchair. The BMI+ group likely needed more time in order to accomplish activities of daily living involving mobility. While no differences were observed in CHART scores, overweight individuals had a significantly higher score on the CES-D questionnaire indicating higher levels of depression. Although many factors may influence depression, it is possible that the decreased propulsion ability may affect quality of life to the point of impacting depression. Although no differences were seen between the two groups based on the CHART, it is possible that overweight individuals get out less because of their limited ability to propel a wheelchair.

CONCLUSION

Our study found that aside from the slower velocity, no biomechanical differences were seen between overweight individual and non-overweight individuals. However, individuals with higher BMI were found to be more depressed and perceived themselves as spending more time propelling their wheelchairs than a low BMI group. These findings indicate that clinicians should consider multiple factors, including weight, when deciding on type of mobility device. It is possible that, if the overweight individuals were provided with power wheelchairs that improved their mobility, they may have scored higher on the quality of life measures. Further studies should investigate if this relationship is due to the decrease in functional propulsion that was also observed. Additionally, studies should examine if quality of life measures can be improved by changing the type of a person's mobility device.

REFERENCES

Boninger ML, Cooper RA, Baldwin MA, Shimada SD, and Koontz A. Wheelchair pushrim kinetics: body weight and median nerve function. Arch Phys Med Rehabil 1999 Aug.; 80(8): 910-5.
Weil E, Wachterman M, McCarthy EP, Davis RB, O'Day B, Iezzoni LI, Wee CC. Obesity among adults with disabling conditions. JAMA 2002 Sept.; 288(10): 1265-8.
Dicianno B, Boninger M, Cooper R, Mercer J, and Wolfe E. Weight and Manual Wheelchair Propulsion. Accepted to the 40 th Annual Association of Academic Physiatrists Educational Conference. Albuquerque, NM, Feb 24-29, 2004.
L. S. Radloff. The CES-D scale: A self-report depression scale for research in the general population. Appl Psychol Meas; 1:385-401, 1977.
Whiteneck G, Charlifue S, Gerhart K, Overholser J, and Richardson G (1992). Quantifying handicap: A new measure of long-term rehabilitation outcomes. Arch Phys Med & Rehab; 73(6): 519-26.
Cooper R, Robertson R, VanSickle D, Boninger M, and Shimada S. Methods for Determining Three-Dimensional Wheelchair Pushrim Forces and Moments - A Technical Note. J Rehabil Res Dev; 34(2):162-70, 1997.
Heiat A; National Institutes of Health (NIH: the NIH Consensus Conference on Health Implications of Obesity in 1985); United States Department of Agriculture (the 1990 Department of Agriculture's Dietary Guidelines for Americans); National Heart, Lung, and Blood Institute. Impact of age on definition of standards for ideal weight. Prev Cardiol. 2003 Spring; 6(2): 104-7.

ACKNOWLEDGEMENTS

This study was supported by the VA RR+D Center (F2181C).

Jennifer L. Mercer,
7180 Highland Drive,
Building 4, East Wing, 151R-1,
Pittsburgh, PA 15206.
Phone: (412) 365-4850,
email: jlmst147@pitt.edu