Squat test performance and execution in children with and without cerebral palsy.

BACKGROUND: Knowledge on lower extremity strength is imperative to informed decision making for children with cerebral palsy (CP) with mobility problems. However, a functional and clinically feasible test is not available. We aimed to determine whether the squat test is suitable for this purpose by investigating test performance and execution in children with cerebral palsy and typically developing (TD) peers. METHODS: Squat test performance, defined by the number of two-legged squats until fatigue (max 20), was assessed in twenty children with bilateral CP (6-19years; gross motor function classification system I-III) and sixteen TD children (7-16years). Muscle fatigue was assessed from changes in electromyography (EMG). Joint range-of-motion and net torque were calculated for each single squat, to investigate differences between groups and between the 2nd and last squat. FINDINGS: Fifteen children with CP performed <20 squats (median=13, IQR=7-19), while all TD children performed the maximum of 20 squats. Median EMG frequency decreased and amplitude increased in mm. quadriceps of both groups. Ankle and knee range-of-motion were reduced in children with CP during a single squat by 10 to 15°. No differences between 2nd and last squat were observed, except for knee range-of-motion which increased in TD children and decreased in children with CP. INTERPRETATION: Squat test performance was reduced in children with CP, especially in those with more severe CP. Muscle fatigue was present in both children with CP and TD peers, confirming that endurance of the lower extremity was tested. Minor execution differences between groups suggest that standardized execution is important to avoid compensation strategies. It is concluded that the squat test is feasible to test lower extremity strength in children with CP in a clinically meaningful way. Further clinimetric evaluation is needed before clinical implementation.

Energy cost of walking in children with spastic cerebral palsy: relationship with age, body composition and mobility capacity.

The energy cost (EC) of walking is different for typically developing (TD) and children with cerebral palsy (CP). The associated factors of EC are not fully understood in children with CP. We assessed the relationship between EC and age, body surface area (BSA), and gross motor function measure (GMFM). We retrospectively examined data collected between 2003 and 2011 on 276 children aged 4-18 years who were classified as Gross Motor Function Classification System level I, n=79; II, n=123; and III, n=74. Energy cost was assessed while children walked 6-8 min at a comfortable, self-selected speed using their typical walking aids and/or orthoses as part of a clinical gait analysis. During the test, participants wore a breath-by-breath portable gas analysis system, measuring oxygen consumption. To calculate EC (J/kg/m), oxygen consumption was converted to J/kg/min and divided by walking speed. Data were analyzed using linear regression model. Energy cost correlated inversely with age (ß=-0.16, R2=0.02, P=0.01), BSA (ß=-3.35, R2=0.11, P<0.0001), and GMFM (ß=-0.12, R2=0.42, P<0.0001). In the multiple linear regression model, GMFM was the most potent correlate of EC, BSA explained another 10% of the variance (R2=0.53), and age was a marginally significant correlate of EC (P=0.08). In summary, in children with CP in our study, EC decreased as GMFM and BSA increased, and GMFM was the most potent correlate of EC.

Cognitive precursors of arithmetic development in primary school children with cerebral palsy.

The aim of this study was to examine the development of arithmetic performance and its cognitive precursors in children with CP from 7 till 9 years of age. Previous research has shown that children with CP are generally delayed in arithmetic performance compared to their typically developing peers. In children with CP, the developmental trajectory of the ability to solve addition- and subtraction tasks has, however, rarely been studied, as well as the cognitive factors affecting this trajectory. Sixty children (M=7.2 years, SD=.23 months at study entry) with CP participated in this study. Standardized tests were administered to assess arithmetic performance, word decoding skills, non-verbal intelligence, and working memory. The results showed that the ability to solve addition- and subtraction tasks increased over a two year period. Word decoding skills were positively related to the initial status of arithmetic performance. In addition, non-verbal intelligence and working memory were associated with the initial status and growth rate of arithmetic performance from 7 till 9 years of age. The current study highlights the importance of non-verbal intelligence and working memory to the development of arithmetic performance of children with CP.

Feasibility and test-retest reliability of measuring lower­limb strength in young children with cerebral palsy.

BACKGROUND: Quantifying leg muscle strength in young children with cerebral palsy (CP) is essential for identifying muscle groups for treatment and for monitoring progress. AIM: To study the feasibility, intratester reliability and the optimal test design (number of test occasions and repetitions) of measuring lower-limb strength with handheld dynamometry (HHD) and dynamic ankle plantar flexor strength with the standing heel-rise (SH) test in 3-10 year aged children with CP. DESIGN: Test-retest design. SETTING: Rehabilitation centre, special needs school for children with disabilities, and university medical centre. METHODS: Knee extensor, hip abductor and calf muscle strength was assessed in 20 ambulatory children with spastic CP (3-5 years [N.=10] and 6-10 years [N.=10]) on two test occasions. Intraclass correlation coefficients (ICC) and Smallest Detectable Differences (SDD) were calculated to determine the optimal test design for detecting changes in strength. RESULTS: All isometric strength tests had acceptable SDDs (9-30%), when taking the mean values of 2-3 test occasions (separate days) and 2-3 repetitions. The one-leg SH test had large SDDs (40-128% for younger group, 23-48% for older group). CONCLUSION: Isometric strength (improvements) can only be measured reliably with HHD in young children with CP when the average values over at least 2 test occasions are taken. Reliability of the SH test is not sufficient for measuring individual changes in dynamic muscle strength in the younger children. CLINICAL REHABILITATION IMPACT: Results of this study can be used to determine the optimal number of test occasions and repetitions for reliable HHD measurements depending on expected changes, muscle group and age in 3-10 year old children with CP.

Muscle fatigue during repetitive voluntary contractions: a comparison between children with cerebral palsy, typically developing children and young healthy adults.

AIM: To combine peak torque and EMG analyses to investigate the hypothesis that 1) children with cerebral palsy (CP) have lower muscle fatigability than typically developing children (TD) and whether 2) muscle fatigue correlates with muscle strength. METHODS: Seven CP children, eight TD children and ten young healthy adults (YHA) performed an all-out fatigue test of 35 maximal concentric knee extension and flexion contractions on an isokinetic dynamometer. Angular velocity was set at 60°/s. Peak torque (PT) was determined per repetition and either normalized to bodyweight or maximum voluntary torque. Surface-EMG of quadriceps and hamstring muscles was measured to obtain changes in median frequency (EMG-mf) and smooth rectified EMG amplitude per contraction. RESULTS: Decline in PT differed between all groups for extensors and flexors, where YHA showed the largest decline and CP children the smallest decline over the course of the test. YHA showed a larger decline in EMG-mf of all quadriceps and hamstrings than TD and CP children, while TD children showed a larger decline in EMG-mf of m.rectus femoris and m.vastus lateralis than CP children. INTERPRETATION: Results confirm that children with CP show lower fatigability than TD children and that the lower fatigability coincides with lower maximal muscle strength.

Physical fitness, everyday physical activity, and fatigue in ambulatory adults with bilateral spastic cerebral palsy.

This study assessed physical fitness and its relationships with everyday physical activity (PA) and fatigue in cerebral palsy (CP). Participants were 42 adults with ambulatory bilateral spastic CP (mean age 36.4 ± 5.8 years; 69% males; 81% with good gross motor functioning). Progressive maximal aerobic cycle tests determined VO(2peak) (L/min). Objective levels of everyday PA were measured with accelerometry and self-reported levels of everyday PA with the Physical Activity Scale for Individuals with Physical Disabilities. Fatigue was assessed with the Fatigue Severity Scale. The average aerobic capacity of adults with CP was 77% of Dutch reference values. Participants were physically active during 124 min/day (85% of Dutch reference values), and half experienced fatigue. In women, lower physical fitness was related to lower self-reported levels of PA (R(p)=0.61, P=0.03), and in men to higher levels of fatigue (R(p)=-0.37, P=0.05). Other relationships were not significant. Results suggest that ambulatory adults with CP have low levels of physical fitness, are less physically active than able-bodied age mates and often experience fatigue. We found little evidence for relationships between the level of physical fitness and everyday PA or fatigue.

Association between isometric muscle strength and gait joint kinetics in adolescents and young adults with cerebral palsy.

The purpose of this study was to determine the association between isometric muscle strength of the lower limbs and gait joint kinetics in adolescents and young adults with cerebral palsy (CP). Twenty-five participants (11 males) with bilateral spastic CP, aged 14-22 years (mean: 18.9, sd: 2.0 yr) and Gross Motor Function Classification System (GMFCS) level II (n=19) and III (n=6) were tested. Hand held dynamometry was used to measure isometric strength (expressed in Nm/kg) of the hip, knee, and ankle muscles using standardized testing positions and procedures. 3D gait analysis was performed with a VICON system to calculate joint kinetics in the hip, knee and ankle during gait. Ankle peak moments exceeded by far the levels of isometric strength of the plantar flexors, while the knee and hip peak moments were just at or below maximal isometric strength of knee and hip muscles. Isometric muscle strength showed weak to moderate correlations with peak ankle and hip extension moment and power during walking. Despite considerable muscle weakness, joint moment curves were similar to norm values. Results suggest that passive stretch of the muscle-tendon complex of the triceps surae contributes to the ankle moment during walking and that muscle strength assessment may provide additional information to gait kinetics.

Development and validation of prognostic models designed to predict wheelchair skills at discharge from spinal cord injury rehabilitation.

OBJECTIVE: To develop and validate a statistical model to predict wheelchair skills at discharge (t(2)) from personal and lesion characteristics and wheelchair skills at the start of spinal cord injury inpatient rehabilitation (t(1)). DESIGN: Prospective cohort study. SETTING: Eight Dutch rehabilitation centres. SUBJECTS: One hundred and forty-two patients with a spinal cord injury. MAIN MEASURES: Models were developed with the performance time and ability score at t(2) as dependent variables and t(1) scores of performance time and ability score, age, gender, body mass index, level and completeness of the lesion as independent variables. The statistical models were evaluated by comparing individual estimated scores with actual measured scores. RESULTS: The main independent variables to predict wheelchair skills at discharge were the t(1) performance time and ability score, age, gender and lesion level. The intraclass correlation coefficient between the estimated and actual ability score was 0.79 and for the performance time 0.86. However, the 95% limits of agreement and their confidence intervals were relatively wide for both ability score (-2.3 to 3.4, range 0-8) and performance time (-12.5 to 8.2, range 11-40 seconds). CONCLUSION: The prognostic models developed in this study to predict future wheelchair skills might help planning the course of rehabilitation. The models should be used with caution in daily clinical practice, but may add useful information to clinical expertise and knowledge of the individual patient.

The longitudinal relationship between lipid profile and physical capacity in persons with a recent spinal cord injury.

STUDY DESIGN: A multicenter prospective cohort study. OBJECTIVE: To determine the longitudinal relationship between physical capacity and lipid profile in persons with spinal cord injury (SCI) during and 1 year after rehabilitation. SETTING: Eight Dutch rehabilitation centers with a specialized SCI unit. METHODS: A total of 206 subjects with SCI (78 with tetraplegia) participated. The longitudinal relationship between lipid profiles (total cholesterol (TC), high- (HDL) and low-density lipoprotein (LDL) and triglycerides (TG) and physical capacity (peak power output (POpeak), peak oxygen uptake (VO2peak), and muscle strength) was investigated during inpatient SCI rehabilitation (start, 3 months later, discharge) and 1 year after discharge. A correction was made for the possible confounding variables age, body mass index, gender, time since injury, lesion level and completeness. RESULTS: HDL and the ratios LDL/HDL and TC/HDL showed a significant and favorable relationship with VO2peak, POpeak and muscle strength. TG was positively related to POpeak and muscle strength. CONCLUSIONS: More favorable lipid profiles were seen in people with a higher physical capacity after correction for personal and lesion characteristics. Therefore, improving the physical capacity by being active during daily life or in sport may further improve the lipid profile and thus reduce the risk for coronary heart disease.

Physical fitness in people with a spinal cord injury: the association with complications and duration of rehabilitation.

OBJECTIVE: To assess the association between physical fitness and its recovery over time on the one hand, and complications and duration of phases of rehabilitation on the other. DESIGN AND SETTING: Prospective cohort study at eight rehabilitation centres. SUBJECTS: People with a spinal cord injury were assessed four times: at the start of active rehabilitation (n = 110), three months later (n = 92), at discharge (n = 137) and a year after discharge from inpatient rehabilitation (n = 91). MAIN MEASURES: Physical fitness was defined as aerobic capacity, determined at each occasion by the peak oxygen uptake (peak Vo(2); L/min) and the peak power output (peak PO; W) during a maximal exercise test. On these occasions, spasticity, musculoskeletal and neurogenic pain were determined (1 = present; 0 = absent). During inpatient rehabilitation, complications (urinary tract infection, pulmonary infection or pressure sore) and bed rest were registered (1 = complication; 0 = no complications, and 1 = bed rest; 0 = no bed rest). Complications and bed rest occurring during the year after discharge were registered similarly. RESULTS: Multilevel random coefficient analyses revealed associations in multivariate models (P

Motor outcome at the age of one after perinatal hypoxic-ischemic encephalopathy.

OBJECTIVE: The aim of this report is to describe the motor outcome in one year-old children who were born at full-term with perinatal hypoxic-ischemic encephalopathy (HIE). Relationships between motor ability tests and neurological examination at one year, and between these tests and neonatal brain magnetic resonance imaging (MRI) were investigated. PARTICIPANTS AND METHODS: 32 surviving children, born full-term with perinatal HIE, are included in this report. All children had a neonatal MRI. At one year, motor ability was assessed with the Alberta Infant Motor Scale and the Bayley Scales of Infant Development (2nd version). Neurological examinations included the neurological optimality score (NOS). RESULTS: At one year, 14 children (44%) had normal motor ability, nine (28%) had mildly delayed, and nine had significantly delayed motor ability. The NOS ranged from 14.6-27 points. All children with normal motor ability had (near) optimal NOS, however, not all children with high NOS had normal motor ability. Eleven children (34%) had normal neonatal MRI; at one year, six of them had normal, and five had mildly delayed motor ability. Eight children with normal motor ability showed abnormalities on neonatal MRI. CONCLUSION: Neonatal brain MRI does not predict motor outcome at one year. Motor ability tests and neurological examinations should be used in a complementary manner to describe outcome after HIE.

Mechanical efficiency and wheelchair performance during and after spinal cord injury rehabilitation.

The purpose of the present study was to investigate whether mechanical efficiency (ME) relates to wheelchair propulsion capacity and wheelchair performance tasks during and after rehabilitation of people with a spinal cord injury (SCI). Eighty participants with a SCI were tested during rehabilitation (3 x) and 1 year after discharge. Two 3-minute submaximal exercise blocks, a maximal wheelchair exercise test, and four wheelchair performance tasks were performed. ME, peak power output (PO (peak)), the sum of the performance times of a 15-m sprint and figure-of-eight, and the heart rate reserve (%HRR) during 10 s of wheelchair propulsion on a 3 % and 6 % slope were calculated. The relationship between ME and PO (peak), %HRR and performance time was tested with a multilevel regression analysis. ME showed a significant relationship with PO (peak) (p

The individual relationship between heart rate and oxygen uptake in people with a tetraplegia during exercise.

STUDY DESIGN: Descriptive study. OBJECTIVE: To examine the individual heart rate-oxygen uptake (HR-VO(2)) relationship during exercise in persons with tetraplegia (TP). SETTING: Rehabilitation Centre Heliomare, Wijk aan Zee, The Netherlands. METHODS: The HR-VO(2) relationship was determined in untrained subjects with motor complete TP (C5 or C6, n=10 and C7 or C8, n=10) during a discontinuous graded exercise hand cycle test. The mean HR and VO(2) of the final 60 s of 2-min exercise blocks were used for calculation of the individual correlation coefficient and the standard error of the estimate (SEE). RESULTS: Two subjects of the C5-C6 group were not able to complete the test. Individual Pearson's correlation coefficients (r) ranged from 0.68 to 0.97 and SEE from 2.6 to 22.4% VO(2)-Reserve (VO(2)R). The mean Pearson's r and SEE were 0.81+/-0.12 and 10.6+/-5.6% VO(2)R in the C5-C6 group and 0.91+/-0.07 and 7.0+/-3.2% VO(2)R in the C7-C8 group, respectively. Two subjects of the C5-C6 group and six subjects of the C7-C8 group attained a linear HR-VO(2) relationship with an acceptable SEE (< or =6.0%) and r (>0.90). CONCLUSIONS: The HR-VO(2) relationship appeared linear in only eight out of 18 subjects. An individual analysis of the HR-VO(2) relationship is necessary to determine whether HR can be used to quantify exercise intensity. The use of HR to prescribe training intensity should be reconsidered in persons with TP. SPONSORSHIP: This study is supported from a grant by ZON-MW.

Demographics of the Dutch multicenter prospective cohort study 'Restoration of mobility in spinal cord injury rehabilitation'.

STUDY DESIGN: A multicenter prospective cohort study. OBJECTIVE: To compare the demographic data of the included population with other studied spinal cord injury (SCI) populations in the international literature. SETTING: Eight Dutch rehabilitation centers with a specialized SCI unit. METHODS: A total of 205 individuals with SCI participated in this study. Information about personal, lesion and rehabilitation characteristics were collected at the beginning of active rehabilitation by means of a questionnaire. RESULTS: The research group mainly consisted of men (74%), of individuals with a paraplegia (59%), and had a complete lesion (68%). The SCI was mainly caused by a trauma (75%), principally due to a traffic accident (42%). The length of clinical rehabilitation varied between 2 months and more than a year, which seemed to be dependent on the lesion characteristics and related comorbidity. CONCLUSIONS: The personal and lesion characteristics of the subjects of the multi-center study were comparable to data of other studies, although fewer older subjects and subjects with an incomplete lesion were included due to the inclusion criteria 'age' and 'wheelchair-dependent'. The length of stay in rehabilitation centers in The Netherlands was longer compared to Denmark but much longer than in eg Australia and the USA.

Upper extremity musculoskeletal pain during and after rehabilitation in wheelchair-using persons with a spinal cord injury.

STUDY DESIGN: Prospective cohort study. OBJECTIVES: To study upper extremity musculoskeletal pain during and after rehabilitation in wheelchair-using subjects with a spinal cord injury (SCI) and its relation with lesion characteristics, muscle strength and functional outcome. SETTING: Eight rehabilitation centers with an SCI unit in the Netherlands. METHODS: Using a questionnaire, number, frequency and seriousness of musculoskeletal pain complaints of the upper extremity were measured. A pain score for the wrist, elbow and shoulder joints was calculated by multiplying the seriousness by the frequency of pain complaints. An overall score was obtained by adding the scores of the three joints of both upper extremities. Muscle strength was determined by manual muscle testing. The motor score of the functional independence measure provided a functional outcome. All outcomes were obtained at four test occasions during and 1 year after rehabilitation. RESULTS: Upper extremity pain and shoulder pain decreased over time (30%) during the latter part of in-patient rehabilitation (P<0.001). Subjects with tetraplegia (TP) showed more musculoskeletal pain than subjects with paraplegia (PP) (P<0.001). Upper extremity pain and shoulder pain were significantly inversely related to functional outcome (P<0.001). Muscle strength was significantly inversely related to shoulder pain (P<0.001). Musculoskeletal pain at the beginning of rehabilitation and BMI were strong predictors for pain 1 year after in-patient rehabilitation (P<0.001). CONCLUSIONS: Subjects with TP are at a higher risk for upper extremity musculoskeletal pain and for shoulder pain than subjects with PP. Higher muscle strength and higher functional outcome are related to fewer upper extremity complaints.

A physiological comparison of synchronous and asynchronous hand cycling.

The purpose of this study was to compare submaximal physiological responses (oxygen uptake, ventilation, heart rate) and gross mechanical efficiency between synchronous and asynchronous hand cycling at different cadences. Thirteen non-disabled men (22.4 +/- 1.6 yr) performed two submaximal exercise tests on a treadmill, using synchronous and asynchronous crank settings in counter balanced order. Tests were performed using a commercially available hand cycle unit that was attached to a hand rim wheelchair. Each test consisted of five 5-min exercise bouts at 36, 47, 55, 65, and 84 rpm. ANOVA for repeated measures showed a significant effect of crank mode (p < 0.001) and cadence (p < 0.001), as well as an interaction effect between both (p < 0.01). Physiological responses were lower, and efficiency higher, in synchronous versus asynchronous hand cycling at all cadences. Post-hoc analysis of the (overall) effect of cadence showed significantly higher physiological responses and lower efficiency at the higher (84 vs. 65 rpm and 65 vs. 55 rpm) and lower (36 vs. 47 rpm) cadences. The interaction effect indicates that the effect of crank mode was dependent on cadence, showing a larger difference between synchronous and asynchronous hand cycling at 84 vs. 65 rpm and at 36 vs. 47 rpm. It is concluded that, in contrast to previous results in arm crank ergometry, synchronous hand cycling is less strenuous and more efficient than asynchronous hand cycling.

Submaximal physical strain and peak performance in handcycling versus handrim wheelchair propulsion.

STUDY DESIGN: Experimental study in subjects with paraplegia and nondisabled subjects. OBJECTIVE: To compare submaximal physical strain and peak performance in handcycling and handrim wheelchair propulsion in wheelchair-dependent and nondisabled control subjects SETTING: Amsterdam, The Netherlands. METHODS: Nine male subjects with paraplegia and 10 nondisabled male subjects performed two exercise tests on a motor-driven treadmill using a handrim wheelchair and attach-unit handcycle system. The exercise protocol consisted of two 4-min submaximal exercise bouts at 25 and 35 W, followed by 1-min exercise bouts with increasing power output until exhaustion. RESULTS: Analysis of variance for repeated measures showed a significantly lower oxygen uptake (VO2), ventilation (Ve), heart rate (HR), rate of perceived exertion and a higher gross efficiency for handcycling at 35 W in both subject groups, while no significant differences were found at 25 W. Peak power output and peak VO2, Ve and HR were significantly higher during handcycling in both groups. The differences between handcycling and wheelchair propulsion were the same in subjects with paraplegia and the nondisabled subjects. CONCLUSIONS: Handcycling induces significantly less strain at a moderate submaximal level of 35 W, and shows noticeably higher maximal exercise responses than wheelchair propulsion, which is consistent in subjects with paraplegia and nondisabled controls. These results demonstrate that handcycling is beneficial for mobility in daily life of wheelchair users.

Health related functional status in men with spinal cord injury: relationship with lesion level and endurance capacity.

STUDY DESIGN: Cross-sectional study. OBJECTIVES: To determine the relationship of health related functional status with lesion level and endurance capacity in persons with spinal cord injury (SCI). METHODS: Thirty-seven men with SCI were divided in four lesion groups: high tetraplegia (motor complete; C5-C6, n=10), low tetraplegia (motor complete, C6/7-C8, n=9), motor incomplete tetraplegia (n=7), and paraplegia (n=11). Health related functional status was measured with the short version of the Sickness Impact Profile (SIP68), including a physical (SOM), psychological (PSY) and social subscore (SOC). Endurance capacity, defined as maximal power output (PO(max)) and peak oxygen uptake (VO(2peak)), was measured in a maximal exercise test on a wheelchair ergometer. RESULTS: Total SIP68-score and SOM were significantly different between lesion groups, showing higher values in the high- and low-tetraplegia group. There were no differences between lesion groups for PSY and SOC subscores. VO(2peak) and PO(max) were significantly higher in the paraplegia group, compared to the high and low tetraplegia groups. VO(2peak) was also higher in the motor incomplete versus other tetraplegia groups. Significant Spearman correlation coefficients were found for VO(2peak) and PO(max) with SIP68 and SOM (ranging from -0.68 to -0.79) and SOC (ranging from -0.39 to -0.51). No significant relationship was found with PSY. Hierarchical regression analysis showed that after correction for lesion level, 22% of the variance of SIP68, 8% of the variance of SOM, and 30% of the variance of SOC was explained by PO(max) or VO(2peak). CONCLUSIONS: Results indicate that there is an evident relationship between the physical dimensions of health related functional status and lesion level, but not for the psychological and social dimensions. After controlling for lesion level a significant amount of the variance of health related functional status can be explained by endurance capacity parameters. Although no causal relationships can be established in this cross-sectional study, these results suggest that functional status may be improved by increasing the endurance capacity.

Alternative modes of manual wheelchair ambulation: an overview.

An estimated 90% of all wheelchairs are hand-rim propelled, a physically straining form of ambulation that can lead to repetitive strain injuries in the arms and, eventually, to secondary impairments and disability. Further disability in wheelchair-dependent individuals can lead to a sedentary lifestyle and thereby create a greater risk for cardiovascular problems. Studies on lever-propelled and crank-propelled wheelchairs have shown that these propulsion mechanisms are less straining and more efficient than hand-rim-propelled wheelchairs. This article reviews these studies and substantiates that the frequent use of these alternative propulsion mechanisms may help prevent some of the secondary impairments that are seen among today's wheelchair-user population.