Biomechanic evaluation of upper-extremity symmetry during manual wheelchair propulsion over varied terrain.

Abstract

To evaluate upper-extremity symmetry during wheelchair propulsion across multiple terrain surfaces.
Case series.
A biomechanics laboratory and the general community.
Manual wheelchair users (N=12).
Not applicable.
Symmetry indexes for the propulsion moment, total force, tangential force, fractional effective force, time-to-peak propulsion moment, work, length of push cycle, and power during wheelchair propulsion over outdoor and indoor community conditions, and in laboratory conditions.
Upper-extremity asymmetry was present within each condition. There were no differences in the magnitude of asymmetry when comparing laboratory with indoor community conditions. Outdoor community wheelchair propulsion asymmetry was significantly greater than asymmetry measured during laboratory conditions.
Investigators should be aware that manual wheelchair propulsion is an asymmetrical act, which may influence interpretation when data is collected from a single limb or averaged for both limbs. The greater asymmetry identified during outdoor versus laboratory conditions emphasizes the need to evaluate wheelchair biomechanics in the user's natural environment.

Full text

Page 1

ORIGINAL ARTICLE
Biomechanic Evaluation of Upper-Extremity Symmetry During
Manual Wheelchair Propulsion Over Varied Terrain
Wendy J. Hurd, PhD, Melissa M. Morrow, MS, Kenton R. Kaufman, PhD, Kai-Nan An, PhD
ABSTRACT. Hurd WJ, Morrow MM, Kaufman KR, An
K-N. Biomechanic evaluation of upper-extremity symmetry
during manual wheelchair propulsion over varied terrain. Arch
Phys Med Rehabil 2008;89:1996-2002.
Objective: To evaluate upper-extremity symmetry during
wheelchair propulsion across multiple terrain surfaces.
Design: Case series.
Setting: A biomechanics laboratory and the general
community.
Participants: Manual wheelchair users (N?12).
Interventions: Not applicable.
Main Outcome Measures: Symmetry indexes for the pro-
pulsion moment, total force, tangential force, fractional effec-
tive force, time-to-peak propulsion moment, work, length of
push cycle, and power during wheelchair propulsion over out-
door and indoor community conditions, and in laboratory
conditions.
Results: Upper-extremity asymmetry was present within
each condition. There were no differences in the magnitude of
asymmetry when comparing laboratory with indoor community
conditions. Outdoor community wheelchair propulsion asym-
metry was significantly greater than asymmetry measured dur-
ing laboratory conditions.
Conclusions: Investigators should be aware that manual
wheelchair propulsion is an asymmetrical act, which may in-
fluence interpretation when data is collected from a single limb
or averaged for both limbs. The greater asymmetry identified
during outdoor versus laboratory conditions emphasizes the
need to evaluate wheelchair biomechanics in the user’s natural
environment.
Key Words: Biomechanics; Rehabilitation; Upper extrem-
ity; Wheelchairs.
© 2008 by the American Congress of Rehabilitation Medi-
cine and the American Academy of Physical Medicine and
Rehabilitation
T
Upper-extremity pain1-3 and overuse injury4-6 are common in
manual wheelchair users. Limb pain is frequently associated
with activities of daily living2,3,7 and is hypothesized to be a
consequence of repetitive wheeling (eg, manually propelling a
wheelchair) and upper-extremity weight bearing activities. The
novel mode of ambulation and potential deleterious impact on
HE BILATERAL NATURE of wheelchair propulsion
places both upper extremities at risk for overuse injury.
function has consequently made wheelchair propulsion the
focus of many biomechanic investigations.
Measurement of propulsion force is now possible with
instrumented wheelchair rims. The technology is, however,
expensive. Often investigators are able to purchase only 1
instrumented rim, limiting studies to evaluation of 1 extremity.
Investigators who have collected bilateral upper-extremity
kinetic data during wheelchair propulsion subsequently aver-
aged the data for both limbs8-10 or have selected only 1 limb for
analysis.11 This suggests that side-to-side differences during
wheelchair propulsion are not meaningful. However, in a study
of pushrim propulsion patterns, Boninger et al12 analyzed left
and right upper extremities separately and reported that several
subjects had different propulsion patterns between extremities.
They stated “assuming that the left and right sides are identical
may lead to errors.” Conditions, however, were limited to
evaluation of propulsion while the study participant’s wheel-
chair was secured to a dynamometer. We have found no studies
that have evaluated side-to-side differences in propulsion bio-
mechanics across varied terrain.
Wheelchair propulsion asymmetry may have clinical conse-
quences. In this population, both upper extremities are at high
risk for pain and injury as a consequence of the bilateral
demands of propulsion and weight relief. Curtis et al3 studied
the prevalence and intensity of shoulder pain during functional
activities in manual wheelchair users, and reported that the
majority of manual wheelchair users with paraplegia (34%)
experienced bilateral upper-extremity pain. A large number of
subjects, however, stated that they had pain in only 1 arm
(24%).3 Risk factors for unilateral upper-extremity pain in the
manual wheelchair user have not been identified. Perhaps the
presence of upper-extremity propulsion asymmetry may be a
contributing factor to the development of injury in the manual
wheelchair user.
Studies evaluating manual wheelchair propulsion kinet-
ics,13-19 technique,12,18,20-23 and the impact of propulsion bio-
mechanics on injury8,9,11 have traditionally been performed in
laboratory conditions. Laboratory terrain surfaces have con-
sisted of level tile floor surfaces or wheelchair dynamometers.
Few studies have investigated wheelchair propulsion tasks that
capture indoor and outdoor community ambulation conditions.
The type of terrain surface and surface inclination angle have,
however, been shown to impact propulsion kinetics, velocity,
and stroke pattern.16,17,23,24 These results emphasize the impor-
tance of evaluating wheelchair propulsion over a range of
conditions.
Evaluating both extremities during wheelchair propulsion in
a variety of conditions is critical to gaining a comprehensive
understanding of the biomechanics of wheeling, and the impact
manual wheelchair use may have on upper-extremity injury.
Therefore, the purpose of this study was to evaluate upper-
From the College of Medicine, Department of Orthopedic Research, Mayo Clinic,
Rochester, MN.
Supported by the National Institutes of Health (grant no. R01HD48781).
No commercial party having a direct financial interest in the results of the research
supporting this article has or will confer a benefit on the authors or on any organi-
zation with which the authors are associated.
Reprint requests to Kai-Nan An, PhD, Guggenheim Bldg 1-28, Rochester, MN
55905, e-mail: an.kainan@mayo.edu.
0003-9993/08/8910-00166$34.00/0
doi:10.1016/j.apmr.2008.03.020
List of Abbreviations
SCIspinal cord injury
1996
Arch Phys Med Rehabil Vol 89, October 2008

Page 2

extremity symmetry during wheelchair propulsion across typ-
ical laboratory, indoor, and outdoor terrain surfaces. Based on
the high incidence of unilateral shoulder pain in manual wheel-
chair users, and Boninger et al’s previous description12of
side-to-side differences in propulsion, we hypothesized that
propulsion within all conditions would be an asymmetrical
task. We also hypothesized that upper-extremity symmetry
measured during laboratory conditions would not be represen-
tative of community wheelchair ambulation. Specifically, we
predicted propulsion symmetry would be greater during labo-
ratory conditions compared with indoor and outdoor commu-
nity conditions. The rationale in this instance was that, as
wheeling conditions became more challenging, side-to-side
differences would become magnified.
METHODS
Participants
Subjects were recruited using public advertisements and
admission records from a large outpatient SCI clinic. Study
inclusion criteria included subject age between 18 and 65
years, a minimum of 1 year of experience as a manual wheel-
chair user, and an occupation that did not involve repetitive
overhead activities. Before testing all subjects underwent a
physical examination by a licensed physical therapist to iden-
tify the presence of upper-extremity impairments and/or pa-
thology. Study exclusion criteria included findings of incom-
plete upper-extremity range of motion, muscle weakness,
current or chronic upper-extremity pain, or a history of signif-
icant upper-extremity injury (eg, rotator cuff tear, dislocation,
fracture). Study participants self-reported their dominant upper
extremity. The study protocol was approved by the Mayo
Clinic Institutional Review Board and informed consent was
obtained from all research participants before initiating test
procedures.
Data Collection
Prior to each data collection, 2 instrumented SmartWheel
rimsawere attached to the subject’s wheelchair. The rims were
subsequently used during testing to measure bilateral upper-
extremity kinetic and temporospatial data for each stroke. The
SmartWheel is a commercially available, wireless, force- and
torque-sensing pushrim that may be used to examine 3-dimen-
sional forces (Fx, Fy, Fz), moments (Mx, My, Mz), and tempo-
rospatial (eg, contact time, velocity) characteristics of manual
wheelchair propulsion. The SmartWheel coordinate system is
defined with x representing forward progression, y representing
the axis perpendicular to the floor pointed superiorly, and z
pointing out of the wheel along the axle. The precision (2N)
and resolution (0.2N) of the SmartWheel rims have previously
been established.25Application of the rims did not alter any of
the wheelchair settings.
The testing procedure encompassed manual wheelchair pro-
pulsion during 8 conditions, including outdoor and indoor
community terrain, and laboratory terrain. All propulsion tasks
were performed at the subject’s self-selected pace. The outdoor
community terrain was a single, continuous, 500-m concrete
sidewalk course comprising 4 conditions performed in the
following order: (1) 2° right cross-slope (right-side lower), (2)
smooth level, (3) aggregate (ie, textured surface) level, and (4)
3° ramp (1:19 rise to run) with a smooth surface. Indoor
community terrain testing included, in order, 2 separate 10-m
conditions composed of a level low-pile carpet surface and a
4.8° ramp (1:12 rise to run) with a low-pile carpet surface.
Laboratory terrain included a 10-m long smooth level tile
surface and a dynamometer with a level surface. The outdoor
sidewalk course was completed once. Three trials each were
performed for all 10-m indoor conditions, and 1 trial 30 sec-
onds in length was performed for the dynamometer condition.
The outdoor sidewalk course and indoor carpet conditions were
chosen because these tasks are representative of terrain sur-
faces encountered during community wheelchair ambulation.
The tile surface and dynamometer were chosen because these
conditions are most commonly used during laboratory investi-
gations of wheelchair propulsion. In all instances the testing
terrain required the subject to propel the chair in a straight-
ahead direction (ie, there were no turns or curves).
Data Management
All kinetic data were sampled at 240Hz and low-pass filtered
at 30Hz with an eighth-order, zero-lag digital Butterworth
filter.26Force data were normalized to subject body mass. The
interval of interest was the push phase of wheelchair propul-
sion, with the onset of push defined as Mzgreater than 0 and off
as Mzis equal to 0.27Three consecutive, representative push
cycles from the steady propulsion state within each condition
were then identified. Using the propulsion moment (Mz), a
custom computer algorithmbwith visual confirmation was used
to identify push cycles of the dominant extremity with the
smallest average absolute deviation from the median propul-
sion moment:
1
n?
i?1
n?xi?x ˜?
where xiis peak M value for single push cycle; x ˜ is median
peak Mzfor entire trial; and n equals 3.
Data for the 3 consecutive push cycles were averaged, and
the average for each extremity was used for analysis. Data from
the 3 push cycles in the 3 trials performed for the 10-m indoor
conditions were averaged for analysis.
Variables that captured propulsion timing, effort, and force
were chosen for analysis (table 1). All variable calculations
were derived from kinetic data obtained from the 2 Smart-
Wheel rims and custom MatLab programs. To evaluate upper-
extremity symmetry during propulsion, a symmetry index was
calculated for each variable28as follows:
??1??
D
ND???
where D denotes the dominant upper extremity and ND denotes
the nondominant upper extremity.
Table 1: Variable Calculation
VariableCalculation
Average propulsion moment (Mz)
(Nm)
Average total force (Ftot) (N)
Direct SmartWheel output
?Fx
Mz/r
(Ftan?Ftot
Mzonset peak
?Mzd?
Mzonset off
??Mz
2?Fy
2?Fz
2
Average tangential force (Ftan) (N)
Average fractional effective force (N)
Time-to-peak propulsion moment (s)
Average work (J)
Length of push cycle (s)
Instantaneous power (W)
?1)?100
Abbreviations: ?, angular velocity; d?, angular distance.
1997
UPPER-EXTREMITY SYMMETRY DURING WHEELCHAIR PROPULSION, Hurd
Arch Phys Med Rehabil Vol 89, October 2008

Page 3

With this calculation technique smaller values represent
greater symmetry (perfect symmetry is 0). This symmetry
index was selected because it was designed to allow the data to
form a normative or Gaussian distribution. This index over-
comes problems of simpler indices that lack linearity and thus
are unacceptable for statistical purposes.29
Statistical Analysis
Within-condition wheelchair propulsion symmetry was eval-
uated with a 1-sample t test, comparing symmetry indexes with
a test value of 0 (perfect symmetry). Between-condition wheel-
chair propulsion symmetry was evaluated for each variable of
interest with a repeated-measures analysis of variance, includ-
ing 8 between factors (condition) and 1 repeated factor (sub-
ject). When significant main effects were found, post hoc
contrast statements were conducted to evaluate differences
between conditions. The first contrast statement compared
within-laboratory conditions to determine whether each task
(tile, dynamometer) should be considered as separate versus
combined laboratory variables. The concern was that wheel-
chair dynamometers may present users with the unique chal-
lenge of constant linear momentum generation demands (ie,
propulsion on a tiled floor results in a greater rolling distance
during push recovery, but there is no linear momentum during
dynamometer propulsion), thus making the dynamometer a
distinct laboratory condition. Subsequent contrast statement
analyses were consistent with our a priori hypotheses, includ-
ing comparison of (1) laboratory (tile, dynamometer) versus
indoor community (level carpet, indoor ramp) conditions, and
(2) laboratory versus outdoor community (cross-slope, smooth
concrete, aggregate concrete, outdoor ramp) conditions. For all
analyses statistical significance was established at P less than
.05. Statistical testing was performed using commercially
available software.c
RESULTS
The 12 subjects comprising the study sample included 11
men and 1 woman. Subjects were on average 43?6.4 years old
(range, 29–56y) with 18?9 years of experience as a manual
wheelchair user (range, 1–29y). Eleven of the subjects were
wheelchair users secondary to SCI (range, T4–L10), and 1
secondary to spina bifida. Ten subjects were right-hand dom-
inant; 2 were left-hand dominant.
Within each condition (cross-slope, smooth concrete, aggre-
gate concrete, outdoor ramp, carpet, indoor ramp, tile, dyna-
mometer), symmetry indexes were statistically significant for
all variables (table 2). Between conditions, symmetry indexes
were also significantly different for all variables (propulsion
moment, P?.001; total force, P?.004; tangential force,
P?.001; fractional effective force, P?.001; time-to-peak pro-
pulsion moment, P?.001; work, P?.001; contact, P?.001;
power, P?.001). Post hoc contrast statements indicated that
there were no differences in the symmetry index for within-lab
conditions (tile vs dynamometer) for any variables (fig 1).
Subsequent contrast statements were therefore performed with
tile and dynamometer tasks combined to represent the lab
condition. There were no significant differences in the symme-
try index between lab and indoor community conditions for any
variables (see fig 1). Comparison of lab and outdoor commu-
nity conditions resulted in significant differences in symmetry
indexes for all variables (outdoor ? lab) with the exception of
time-to-peak propulsion moment (P?.188) (see fig 1).
DISCUSSION
This study has shown that manual wheelchair users with no
pain or upper-extremity injury exhibit asymmetry during pro-
pulsion, with the magnitude of asymmetry impacted by the
wheeling environment. These findings were consistent for
variables that captured propulsion timing, effort, and force.
As we predicted, propulsion asymmetry was identified dur-
ing all laboratory, indoor community, and outdoor commu-
nity conditions. Our hypothesis that symmetry measured dur-
ing laboratory conditions would not be representative of
community wheelchair ambulation was partially supported by
the results. The magnitude of propulsion asymmetry was sig-
nificantly different during outdoor tasks compared with labo-
ratory tasks. There was no difference, however, between indoor
community and laboratory symmetry indexes.
Within all conditions, propulsion asymmetry was identified
for each variable of interest. Previously, Boninger et al12de-
scribed side-to-side differences in propulsion patterns (based
on kinematic-based categories) during evaluation of 38 manual
wheelchair users with SCI. The differences in propulsion pat-
terns described by Boninger,12however, occurred during the
recovery phase of propulsion. Thus, there were no side-to-side
kinetic differences. Additionally, Boninger’s analysis12of dif-
ferences in bilateral propulsion patterns was descriptive in
nature and not based on a statistical evaluation. The current
investigation is the first study known to directly evaluate the
symmetry of wheelchair propulsion. The subjects comprising
our cohort were free of upper-extremity injury, and testing was
performed over multiple terrain surfaces and conditions. Future
studies will be necessary to determine whether the magnitude
of asymmetry is a significant factor contributing to, or predict-
ing, the development of upper-extremity injury in manual
wheelchair users.
We attempted to determine the role of arm dominance in
propulsion asymmetry. Though there were not large differ-
ences in dominant and nondominant limb means for the group
across variables and conditions, these values do not capture
side-to-side differences among subjects. Therefore, we visually
inspected the symmetry indexes by limb for individual subjects
to determine if there was a consistent pattern in the magnitude
of dominant and nondominant extremity contribution to wheel-
chair propulsion. In the cross-slope condition the lower arm is
exposed to greater propulsion demands in an effort to resist the
downhill turning tendency. With 10 of 12 right-hand dominant
subjects, it was therefore expected for the dominant limb, the
lower limb on the cross-slope, to generate higher forces, power,
and work compared with the nondominant limb. For the re-
maining conditions the timing characteristics, forces, work, and
power were not consistently higher on either the dominant or
nondominant limb. The absence of a consistent pattern of
upper-extremity contribution to wheelchair propulsion limits
the ability to predict injury in a specific limb based on asym-
metry. Furthermore, investigators should be aware of variable
side-to-side difference during wheelchair propulsion, which
may influence interpretation when data are collected from a
single limb or averaged for both limbs.
The magnitude of propulsion asymmetry was dependent, in
part, on environment. The rationale for our hypothesis that
symmetry indexes measured during community wheeling
would be greater than those measured during laboratory con-
ditions was based on differences in task demands. That is,
reliance on 1 limb may become pronounced during more chal-
lenging propulsion conditions. Consistent with this hypothesis,
the greater asymmetry identified during outdoor community
conditions compared with the laboratory may be a reflection of
the varied surface textures and surface inclinations encountered
by subjects. It was therefore surprising that there were no
differences in the magnitude of propulsion symmetry between
indoor community and laboratory conditions. Both the ramp
1998
UPPER-EXTREMITY SYMMETRY DURING WHEELCHAIR PROPULSION, Hurd
Arch Phys Med Rehabil Vol 89, October 2008

Page 4

Table 2: Within Condition Symmetry Indexes
Outdoor
Terrain
Cross SlopeSmooth ConcreteAggregate ConcreteOutdoor Ramp
D NDSID NDSID NDSID NDSI
Mz(Nm)
Ftot(N)
Ftan(N)
FEF (N)
Time peak (s)
Work (J)
Contact (s)
Power (W)
5.66?1.98
0.47?0.09
0.28?0.08
61.00?13.96
0.12?0.04
7.98?3.87
0.21?0.06
42.60?17.89
4.71?1.79
0.43?0.11
0.24?0.09
58.19?13.30
0.11?0.04
6.16?2.90
0.20?0.05
36.39?19.69
0.44?0.35†
0.22?0.16†
0.44?0.34†
0.28?0.27†
0.42?0.39†
0.76?0.56†
0.25?0.22†
0.45?0.42†
7.71?2.32
0.64?0.21
0.38?0.15
59.33?14.46
0.20?0.09
11.89?5.07
0.32?0.11
45.26?19.51
9.52?2.10
0.67?0.20
0.47?0.13
69.42?10.71
0.20?0.06
15.79?5.04
0.32?0.06
58.79?20.05
0.27?0.15†
0.13?0.13†
0.27?0.15†
0.24?0.16†
0.25?0.35*
0.38?0.23†
0.15?0.19*
0.31?0.19†
13.91?4.76
0.86?0.20
0.65?0.12
74.43?11.75
0.24?0.06
23.28?8.48
0.35?0.07
77.42?21.59
10.74?2.31
0.78?0.17
0.52?0.12
65.95?10.30
0.22?0.06
16.94?4.68
0.33?0.06
56.48?18.86
0.41?0.28†
0.23?0.21†
0.41?0.28†
0.20?0.16†
0.20?0.13†
0.59?0.48†
0.13?0.10†
0.43?0.45†
15.77?3.91
0.99?0.28
0.76?0.20
74.97?14.89
0.29?0.60
26.55?8.76
0.42?0.80
77.71?24.91
12.78?3.52
0.91?0.26
0.62?0.18
65.56?12.62
0.28?0.08
21.17?7.22
0.40?0.08
62.94?21.29
0.33?0.23†
0.17?0.14†
0.33?0.23†
0.17?0.08†
0.10?0.09†
0.36?0.22†
0.07?0.03†
0.33?0.22†
Indoor
Terrain
CarpetIndoor Ramp
DNDSID NDSI
Mz(Nm)
Ftot(N)
Ftan(N)
FEF (N)
Time peak (s)
Work (J)
Contact (s)
Power (W)
10.24?3.13
0.67?0.08
0.48?0.06
69.47?8.14
0.21?0.04
16.97?4.86
0.34?0.06
60.59?17.88 61.24?22.48 0.14?0.10†
10.21?3.14
0.65?0.12
0.48?0.11
72.45?12.06 0.11?0.08†
0.20?0.03
16.64?5.34
0.33?0.06
0.14?0.08†
0.11?0.09†
0.14?0.08†
22.14?5.92
1.26?0.28
1.07?0.27
83.06?20.32 84.72?20.12
0.44?0.11
37.83?12.29 40.09?11.73
0.57?0.15
90.93?39.81 90.47?38.71
22.08?5.52
1.24?0.27
1.07?0.28
0.05?0.04†
0.06?0.05†
0.05?0.04†
0.05?.04†
0.03?0.04*
0.09?0.07†
0.02?0.01†
0.05?0.04†
0.09?0.04†
0.17?0.14†
0.05?0.04†
0.45?0.12
0.57?0.14
Laboratory
Terrain
Tile Dynamometer
D ND SID NDSI
Mz(Nm)
Ftot(N)
Ftan(N)
FEF (N)
Time peak (s)
Work (J)
Contact (s)
Power (W)
6.80?2.20
0.51?0.10
0.33?0.11
63.50?14.76
0.20?0.06
10.91?4.49
0.32?0.07
41.19?19.60
6.97?2.48
0.50?0.12
0.34?0.13
65.99?14.58
0.19?0.05
10.55?4.46
0.31?0.07
42.60?21.99
0.13?0.15*
0.10?.12*
0.13?0.15*
0.07?0.05†
0.14?0.08†
0.18?0.20*
0.07?0.07†
0.14?.18*
6.21?2.32
0.45?0.09
0.29?0.10
63.48?18.29
0.20?0.08
9.78?4.56
0.43?0.09
27.06?13.29
6.27?2.26
0.45?0.11
0.29?0.09
63.72?15.69
0.21?0.06
9.89?4.91
0.44?0.10
28.14?15.06
0.10?0.11*
0.11?0.04†
0.10?0.11*
0.15?0.10†
0.22?.19*
0.20?0.17*
0.08?0.06‡
0.12?0.09†
NOTE. Values are mean ? SD.
Abbreviations: Contact, length of push cycle; D, dominant; FEF, average fractional effectiveness force; Ftan, average tangential force; Ftot, average total force; J, joule; Mz, average propulsion
moment; ND, nondominant; SI, symmetry index; Time peak, time-to-peak propulsion moment; W, watt.
*P?.05;†P?.001.
1999
UPPER-EXTREMITY SYMMETRY DURING WHEELCHAIR PROPULSION, Hurd
Arch Phys Med Rehabil Vol 89, October 2008

Page 5

and the rolling resistance encountered with the low-pile carpet
surface exposed subjects to more challenging propulsion con-
ditions than either level tile or dynamometer tasks in the
laboratory. Elucidation of factors contributing to or the absence
of propulsion symmetry is unclear at this time. However, it is
possible that fatigue may have been a factor during the outdoor
condition. The outdoor tasks were components of a single,
continuous course. In contrast, the indoor community and
laboratory tasks were performed as discrete trials, with rest
provided according to each subject’s needs. Future studies that
evaluate propulsion symmetry during fatigued and nonfatigued
states may provide further insight to this question.
Laboratory, indoor, and outdoor community conditions were
comprised of diverse tasks. The objective was to compare and
contrast representative terrain surfaces for each condition.
Ramps and side-slopes are routinely encountered during out-
door sidewalk wheeling. To have omitted these tasks would
have been an incomplete evaluation of community propulsion
Fig 1. Symmetry indexes for (A) propulsion moment, (B) total force, (C) tangential force, (D) fractional effective force, (E) time-to-peak
propulsion moment, (F) work, (G) contact, (H) power. *Significant differences between laboratory and outdoor community conditions.
2000
UPPER-EXTREMITY SYMMETRY DURING WHEELCHAIR PROPULSION, Hurd
Arch Phys Med Rehabil Vol 89, October 2008