Comparison of Kinematics, Kinetics, and EMG ...

Sarah R. Dubowsky Postdoctoral Fellow Rehabilitation Engineering Analysis Laboratory, Human Performance and Movement Analysis Laboratory, Kessler Medical Rehabilitation Research and Education Center, 1199 Pleasant Valley Way, West Orange, NJ 07052

Sue Ann Sisto Professor, Physical Therapy Research Director, Division of Rehabilitation Sciences School of Health Technology & Management, Health Sciences Center, Level 2, Room 439, Stony Brook University, Stony Brook, NY 11794-8201

Noshir A. Langrana Chair, Professor Biomedical Engineering, Professor, Mechanical & Aerospace Engineering, Rutgers, The State University of New Jersey, 599 Taylor Road, Piscataway, NJ 08854

Comparison of Kinematics, Kinetics, and EMG Throughout Wheelchair Propulsion in Able-Bodied and Persons With Paraplegia: An Integrative Approach A systematic integrated data collection and analysis of kinematic, kinetic, and electromyography (EMG) data allow for the comparison of differences in wheelchair propulsion between able-bodied individuals and persons with paraplegia. Kinematic data from a motion analysis system, kinetic data from force-sensing push rims, and electromyography data from four upper-limb muscles were collected for ten push strokes. Results are as follows: Individuals with paraplegia use a greater percentage of their posterior deltoids, biceps, and triceps in relation to maximal voluntary contraction. These persons also reached peak anterior deltoid firing nearly 10 deg earlier on the push rim, while reaching peak posterior deltoid nearly 10 deg later on the push rim. Able-bodied individuals had no triceps activity in the initial stages of propulsion while their paraplegic groups had activity throughout. Able-bodied participants also had, on average, peak resultant, tangential, and radial forces occurring later on the push rim (in degrees). There are two main conclusions that can be drawn from this integrative investigation: (1) A greater “muscle energy,” as measured by the area under the curve of the percentage of EMG throughout propulsion, results in a greater resultant joint force in the shoulder and elbow, thus potentially resulting in shoulder pathology. (2) Similarly, a greater muscle energy may result in fatigue and play a factor in the development of shoulder pain and pathology over time; fatigue may compromise an effective propulsive stroke placing undue stresses on the joint capsule. Muscle activity differences may be responsible for the observed kinematic and kinetic differences between the two groups. The high incidence of shoulder pain in manual wheelchair users as compared to the general population may be the result of such differences, although the results from this biomedical investigation should be examined with caution. Future research into joint forces may shed light on this. Further investigation needs to focus on whether the pattern of kinematics, kinetics, and muscle activity during wheelchair propulsion is compensatory or evolutionary by tracking individuals longitudinally. 关DOI: 10.1115/1.2900726兴 Keywords: electromyography, kinematics, kinetics, paraplegia, rehabilitation, shoulder, spinal cord injury, wheelchair propulsion

Introduction In the United States alone, more than 10,000 spinal cord injuries 共SCI兲 are reported each year. This population depends on their upper limbs to provide a means of locomotion during completion of their activities of daily living. During the rehabilitation process following a SCI, an individual is prescribed a wheelchair. However, during rehabilitation minimal time is spent instructing the patient on proper propulsion techniques. Most patients are left on their own to discover how to propel the chair. As a result of greater than normal usage of the upper limbs, proper propulsion mechanics are paramount in preventing injuries and maintaining comfort during locomotion. Over time though, the upper limbs of a manual wheelchair user 共MWU兲 inevitably break down resulting Contributed by the Bioengineering Division of ASME for publication in the JOURBIOMECHANICAL ENGINEERING. Manuscript received November 30, 2006; final manuscript received September 14, 2007; published online December 18, 2008. Review conducted by Avinash Patwardhan.

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Journal of Biomechanical Engineering

in pain and pathology in the upper limb. The development of these symptoms correlates to the duration since injury; in the first five year postinjury, 12% of patients experienced shoulder pain, especially during transfers, and this percentage grows to 100% sixteen year postinjury 关1兴. In comparison, the able-bodied population ranks shoulder pain as third in debilitating musculoskeletal pain— behind low back and neck pain—with a self-reported prevalence in the general population of 16–26% 共however, only approximately 1% of the adult populations is expected to visit a health care provider each year for these complaints兲关2–4兴. These numbers still, however, are dwarfed by the fact that the prevalence of shoulder pain in individuals with SCI has been reported to be between 42% and 73% 关5,6兴; this may be due to their reliance on the upper limbs for locomotion. Researchers have studied the biomechanical factors and underlying musculature involved during standard wheelchair propulsion 关7–9兴, and it is thought that prolonged wheelchair use and transfers may cause the high frequency of upper-limb cumulative trauma and strain injuries in SCI 关10兴. These studies illustrate the

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Table 1 Participant data Participant

Sex

Age 共yr兲

Wt 共kg兲

Injury level

Duration of injury 共yr兲

AB-1 AB-2 AB-3 AB-4 AB-5 AB-6

M M M F M F

26 23 25 29 27 25

84.4 83.5 104.3 50.8 65.8 53.8

n/a n/a n/a n/a n/a n/a

n/a n/a n/a n/a n/a n/a

P-1 P-2 P-3 P-4 P-5

M M F M M

51 29 31 43 47

78.9 61.2 70.3 98.2 102.3

T3 T6 T10 T6 L1/L2

15.0 4.5 3.5 5.5 4.5

importance of good muscular strength, muscular endurance, proper biomechanics, and suitable wheelchair prescriptions in maintaining the integrity of the musculoskeletal system of wheelchair users as they perform their activities of daily living 关5–15兴. By minimizing the stress placed on the upper body with a streamlined push stroke, an individual may prevent trauma, such as glenohumeral joint impingement 关11–13兴, and prolong a pain-free, independent way of life. The kinematics, kinetics, and EMG characteristics throughout wheelchair propulsion have each been investigated. Review of the literature tends to show that studies either examine individually 关16,17兴 or couple two of the aforementioned parameters together for analysis 关10,18–27兴. An investigation by Kulig et al., which focused on shoulder joint kinetics and kinematics during the push phase of wheelchair propulsion, concluded that to determine the true demands on the shoulder during wheelchair propulsion, the effects of kinematics, kinetics, and EMG need to be considered together 关23兴. While there exists multiple studies that compare a combination of participant kinematics, kinetics, and electromyography, to our knowledge there are few that compare all three parameters together 关8兴. The primary purpose of this biomechanical study is to simultaneously quantify and compare the push rim forces, upper-limb kinematics, and shoulder EMG during wheelchair propulsion between able-bodied participants and individuals with paraplegia. We hypothesize that an integrated, simultaneous data collection and interpretation will establish differences between kinematics, kinetics, and EMG profiles of these two groups. These data will serve as an important next step in the calculation of joint forces via patient-specific modeling. Ultimately, this study serves as the first step in the determination of whether differences between groups exist, and thus potentially lead to an associated intervention prescription that may aide in altering subject kinetics, kinematics, and EMG to potentially prevent the shoulder pain that so many MWU’s will experience.

Methods Participants. Approximately 80% of new spinal cord injuries occur among males; a representative sample of this population— four men and one woman with paraplegia—in addition to a matching sample of able-bodied participants 共with one extra female兲, gave informed consent to participate in this study. Subject data are summarized in Table 1. The Kessler Medical Rehabilitation and Research Education Center Institutional Review Board approved all experiments and each participant signed an informed consent form before participating in the study. Kinematics. Each participant was outfitted with 14 mm reflective markers so that a seven-camera motion capture system 共Vicon Peak, Lake Forest, CA兲 could record in real time the 3D trajectory data of the participant’s upper body during each propulsive stroke. 021015-2 / Vol. 131, FEBRUARY 2009

Fig. 1 Participant setup. Subject outfitted with reflective markers and surface electrodes on the upper arm, with SmartWheel’s attached to the subject’s wheelchair, for kinematics, EMG, and kinetics data collection, respectively. Wheelchair is mounted on a dynamometer and secured down for safety. The coordinate system of the SmartWheel’s is drawn on the setup.

Markers were placed unilaterally 共right side兲 on the following bony landmarks: temperomandibular joints, lateral-superior border of the acromion, lateral epicondyle, olecranon, radial styloid, prominent tuberosity of the ulna, third metacarpal, greater trochanter, hub, axle, as well as the C7 and T3 spinous processes, and the sternum 共Fig. 1兲. Kinematics data for Paraplegia Participant 1 were collected at 60 Hz, while all other participants were collected at 120 Hz. The collections of kinetic and kinematics data were synchronized in time. Kinematics data were reviewed immediately after testing to ensure proper data collection with minimal marker dropout. Shoulder and elbow flexion and extension, for contact and release points, were derived from Vicon motion analysis and anthropometrics data 共Fig. 2兲. Kinetics. Previously tested and validated force- and torquesensing push rims 共SmartWheel-Three Rivers Holdings, Inc., Mesa, AZ兲 were used to collect the kinetic data of each participant as they propelled their wheelchair 关28兴. Each participant with paraplegia, in their own wheelchair, swapped SmartWheel for their own wheels, while able-bodied participants pushed a Kuschall Competitor 共Kuschall, Longmont, CO兲, also outfitted with SmartWheel 共Fig. 1兲. Each participant was given ample time to become acclimated to pushing the chair on a dynamometer prior to data collection 共Fig. 1兲. Push rim kinetic data from the right side were collected wirelessly at a sampling frequency of 240 Hz. Push rim force data were reviewed immediately after testing to ensure good signal quality. Resultant 共F兲, tangential 共Ft兲, and radial 共Fr兲 forces were analyzed from SmartWheel x-, y-, and z-output forces 共Fig. 1兲. The conversion equations, as calculated by Robertson et al. 关26兴, are shown below: F = 冑F2x + F2y + Fz2

Ft =

Mz R

共1兲

共2兲

Transactions of the ASME


Fig. 2 Participant contact and release angles. Contact „top diagram… and release „bottom diagram… angles for able-bodied and persons with paraplegia. Anterior axle positioning „relative to the shoulder… was obtained from Vicon. Anthropometrics data—upper and lower arm lengths—were collected as part of the testing protocol. Most mechanically efficient „left… to least mechanically efficient „right…, based on initial positioning. Figures drawn to scale. Twodimensional motion assumed.

Fr = 冑共F2x + F2y 兲 − Ft

共3兲

Wheel

force outputs, M z is where Fx, Fy, and Fz are the raw Smart the moment about the z-axis 共from raw SmartWheel output兲, R is the radius of the push rim 共0.2667 m兲, F is the calculated resultant force, Ft is the calculated tangential force, and Fr the calculated radial force. Both the percentage of the propulsion phase and the corresponding degrees on the push rim at which point the peaks occurred are reported. Electromyography. Shoulder muscle activities were documented with four bipolar preamplified surface electrodes 共MA300 EMG System, Motion Lab Systems, Inc., Baton Rouge, LA兲 with a single ground electrode, and were placed unilaterally 共right side兲 on the anterior and posterior portions of the deltoid, and the long heads of the triceps and biceps brachii. These muscles were chosen for their role in shoulder and elbow flexion and extension. The skin surface was prepared by first scrubbing the area with a scouring pad to remove dead skin cells, and then cleaning the area with an alcohol preparatory pad. Once the electrodes were properly placed, as described in the Anatomical Guide for the Electromyographer 关29兴, they were taped down tightly with Blenderm™ hypoallergenic surgical tape 共3 M, St. Paul, MN兲. After the electrodes were secured, the preamplifiers were plugged into the system backpack, which is connected to the computer via a coaxial cable. The data were collected at an antialiasing bandwidth of 750 Hz, with gain settings ranging from 350 to 4000 times the input signal. The data were sampled and digitized on a computer at a rate of 1560 Hz. Prior to propulsion collection, electromyographic activity was obtained during maximal voluntary contraction 共MVC兲. Testing was modified to allow all muscles to be assessed with the participant seated in his wheelchair in four standardized positions. The aforementioned muscles were tested as described by Mulroy et al., in the following manner 关25兴: 共a兲 Anterior deltoid: 45 deg shoulder flexion, downward force applied to the elbow 共b兲 Posterior deltoid: 90 deg shoulder abduction with forward force applied to elbow 共c兲 Biceps: 90 deg elbow flexion, full supination, downward force applied to the wrist 共d兲 Triceps: 90 deg shoulder abduction, full internal rotation, 45° elbow flexion, downward force applied to wrist During testing, the participant’s trunk and wheelchair were stabilized by the investigator. Electromyography data were reviewed immediately after testing to ensure proper gain settings and signal quality. The timing of muscle onset and cessation 共and the resulting burst duration兲 and the percent of muscle effort related to each Journal of Biomechanical Engineering

subject’s maximal voluntary contraction were investigated. The results are reported as percentage of propulsion phase. Data Processing and Analysis. This study focused on data derived from the right upper limb during the propulsion 共contact兲 phase of the push stroke only. Participants were asked to propel their wheelchair at a self-selected pace for 20 s. Kinematics, kinetics, and EMG from ten successive push strokes were collected; once data collection began, the initial two push strokes were neglected, and the next ten consecutive push strokes were saved for analysis. The remaining push strokes were neglected, so as to not have fatigue play a role in propulsion characteristics. Kinematics and Kinetics. Kinematics data were pipelined from the Vicon Workstation 共Vicon Peak, Lake Forest, CA兲 to MICROSOFT EXCEL for processing in MATLAB. Contact and release angles were calculated by coupling the trajectory between the wheelchair hub and the third metacarpal marker 共from Vicon兲, with the contact and release points from force-sensing push rims 共described below兲. All grab angles are referenced from horizontal. Resulting shoulder and elbow flexion and extension angles at these contact and release angles are reported in Fig. 2. Raw kinetics data from the SmartWheel was filtered by a fourth order, 20 Hz low pass Butterworth filter, and the resulting forces and moments are converted from volts to Newtons 共or N m兲 by trigonometry from SmartWheel angle data. The propulsive phase of ten push strokes, as defined by palm strike to palm off, was then calculated The onset of propulsion was visually defined as the point of divergence of the Fx and Fy components, and the end of propulsion was visually defined as the point of convergence of the Fx and Fy components 关10兴. This method of defining propulsion has been found to be comparable to the method utilizing the moment divergence from zero 关30兴. SmartWheel outputs 共Fx, Fy, Fz, M z兲, along with the radius R, of the push rim, were also used to calculate associated resultant 共Eq. 共1兲兲, tangential 共Eq. 共2兲兲, and radial forces 共Eq. 共3兲兲关26兴. Electromyography. EMG data were analyzed as previously described 关31兴, where the onset of an EMG burst was defined as the time when the signal amplitude remained above a threshold, defined by the mean of the base line plus three standard deviations, for 30 ms. The end of the EMG burst was defined as the time when the signal amplitude remained below the threshold level for 50 ms. Burst duration was then calculated as the difference in time between the onset and the end of the EMG burst. Significant EMG was defined as continuous activity for a duration of at least 5% of the propulsion cycle 关16兴. Wheelchair Parameters and Mechanical Advantage. A novel method was used to rank each participant’s mechanical advantage FEBRUARY 2009, Vol. 131 / 021015-3


Table 2 Percentage of propulsion and corresponding degrees where peak forces occur Resultant F

Ft

Fr

Participant

% Propulsion

Degrees

% Propulsion

Degrees

% Propulsion

Degrees

AB-1 AB-2 AB-3 AB-4 AB-5 AB-6

60.6⫾ 8.2 61.7⫾ 5.6 50.7⫾ 3.0 54.0⫾ 5.6 51.3⫾ 5.2 52.5⫾ 6.1

116.2⫾ 7.2 121.1⫾ 5.5 113.1⫾ 2.9 107.1⫾ 3.1 93.3⫾ 3.4 105.6⫾ 5.0

62.8⫾ 7.3 62.8⫾ 5.1 52.0⫾ 1.8 54.8⫾ 4.9 55.3⫾ 4.0 53.8⫾ 4.6

118.3⫾ 6.9 122.1⫾ 5.3 114.0⫾ 2.3 107.5⫾ 3.0 95.6⫾ 2.5 106.3⫾ 4.5

55.3⫾ 8.4 60.9⫾ 5.3 47.2⫾ 3.7 33.0⫾ 10.2 42.3⫾ 4.7 37.2⫾ 13.2

111.0⫾ 7.8 120.3⫾ 5.4 110.6⫾ 3.7 95.4⫾ 6.1 88.0⫾ 3.4 97.0⫾ 8.1

P-1 P-2 P-3 P-4 P-5

40.2⫾ 8.1 64.3⫾ 4.4 66.3⫾ 4.5 42.9⫾ 14.2 31.9⫾ 9.6

99.2⫾ 6.1 107.8⫾ 4.9 119.3⫾ 3.7 98.6⫾ 11.4 91.4⫾ 8.3

52.8⫾ 3.9 64.4⫾ 4.5 65.5⫾ 4.5 49.5⫾ 4.8 57.8⫾ 2.3

105.1⫾ 4.5 108.0⫾ 5.0 118.7⫾ 3.7 104.0⫾ 4.9 114.3⫾ 2.7

35.1⫾ 7.8 50.1⫾ 20.7 38.8⫾ 21.9 33.8⫾ 15.3 27.2⫾ 3.7

96.2⫾ 4.7 94.2⫾ 19.3 96.9⫾ 17.9 91.3⫾ 13.0 87.2⫾ 3.3

based on clinical guidelines from the Paralyzed Veterans of America 共PVA兲. Shoulder positioning, relative to the hub, was obtained from Vicon 3D motion analysis data for contact and release positions. The PVA guidelines recommend an inferior seat height to facilitate greater upper-limb motion and hand contact angles, lower stroke frequency, and higher mechanical efficiency 关32兴. The PVA guidelines also recommend an anterior axle position 共without compromising the stability of the user兲 to assist in the following: increased hand contact time, decreased muscle effort, lower stroke frequency, lower peak forces, less rapid loading of the push rim, and fewer strokes to go the same speed 关32兴. Each participant’s anterior-posterior posture was ranked according to the PVA guidelines; the participant whose axle was the most anterior relative to their shoulder was ranked to have the greatest mechanical advantage. The inferior-superior position was not assessed as participant height directly affected the ranking in the able-bodied group and would have skewed results 共as the WC height was never adjusted to each subject兲. Rank is demonstrated in Fig. 2, with the most mechanically efficient to the least mechanically efficient reading left to right.

Results Statistics. Participants were asked to propel at a self-selected speed for 20 s intervals during which kinematics, kinetics, and electromyography data were simultaneously collected. Data were individually and collectively analyzed. The fact that there is a small sample size makes it difficult to do statistical analyses; however, even with a small sample size trends between groups can be observed; the aim is to determine whether there are differences between the groups that will provide insight into the development of a patient-specific model. Results are reported as mean⫾ standard error of measure 共SEM兲. Kinematics and Mechanical Advantage. Shoulder and elbow flexion-extension angles, at contact and release, are shown in Fig. 2. Based on these postures, as previously discussed, a novel method was used to rank each participant’s mechanical advantage at contact time. Results are as follows 共ranked from greatest mechanical advantage to least mechanical advantage and shown in Fig. 2兲: AB-3, P-2, AB-2, AB-5, AB-6, P-5, AB-4, AB-1, P-3, P-1, and P-4. Trunk angles throughout propulsion, as calculated trigonometrically by the anterior axle position 共relative to the shoulder兲 and the total arm length, was 2.5⫾ 0.4 deg and 4.1⫾ 1.0 deg for participants with paraplegia and able-bodied individuals, respectively. In addition, participants with paraplegia had a greater contact time 共Fig. 2兲, as measured by degrees 共though not statistically significant兲. Contact angles for the paraplegic and able-bodied individuals are 63.2⫾ 4.9 deg and 69.7⫾ 3.4 deg, respectively, and release angles are 142.9⫾ 3.8 deg and 140.5⫾ 5.2 deg, respectively. Propulsion speed varied between the groups as well; the 021015-4 / Vol. 131, FEBRUARY 2009

group with paraplegia, at a self-selected speed, propelled faster than the able-bodied group, 1.4⫾ 0.1 m / s versus 1.1⫾ 0.1 m / s, respectively. Kinetics. Both groups demonstrated peak resultant, tangential, and radial forces subsequent to top dead center 共90 deg兲 on the push rim during propulsion 共Table 2兲. While there was no significance between the two groups in this, there was a trend in that participants with paraplegia reached peak resultant, tangential, and radial forces earlier on the push rim 共in degrees兲 than their able-bodied counterparts 共Table 2兲. Electromyography Anterior Deltoid. Burst duration between the groups was similar; both participants with paraplegia and able-bodied individuals had one burst throughout propulsion of 4.9⫾ 3.8– 95.0⫾ 4.2% and 4.0⫾ 2.8– 88.7⫾ 4.5%, respectively. Peak EMG amplitude during propulsion was similar between groups as well. Participants with paraplegia used 39.1⫾ 9.0% of MVC at peak, and able-bodied participants used 38.5⫾ 7.8% 共Fig. 3兲. What did vary between groups was the angle at which persons with paraplegia and able-bodied individuals achieved peak anterior deltoid firing, 91.8⫾ 5.5 deg and 99.1⫾ 3.6 deg, respectively 共corresponding to 37.4⫾ 8.0% and 40.85⫾ 5.3% propulsions兲. Peak anterior deltoid firing coincided with the impact spike on the resultant, tangential, and radial force profiles, as defined by Robertson et al. 关26兴 for nearly all subjects 共Fig. 4兲. Posterior Deltoid. Two paraplegic participants had two posterior deltoid bursts throughout propulsion, 1.8⫾ 1.9– 20.9⫾ 7.2%

Fig. 3 Comparing peak muscle firing in relation to maximal voluntary contraction between groups. Mean „±SEM… peak muscle amplitude in relation to maximal voluntary contraction for the anterior and posterior deltoid, biceps, and triceps.



Fig. 4 Comparing triceps burst duration and peak firing throughout propulsion. Muscle activity normalized to propulsion with peak muscle activity denoted by the triangles on the graph. Contrary to able-bodied participants, persons with paraplegia have a longer burst duration, with activity early on in the propulsion cycle.

and 72.8⫾ 8.7– 95.6⫾ 3.1%, while the remaining three in the group had only one burst, 17.3⫾ 12.5– 98.4⫾ 2.1%. Three of the six able-bodied participants had onset of posterior deltoid muscle activity early on in the push stroke at 5.1⫾ 6.0% 共AB-2 had two bursts兲. Two participants had activity beginning at 62.0⫾ 6% of the push stroke 共including AB-2兲, while the last participant had activity beginning at both 37.6⫾ 3.0% and 64.9⫾ 1.2% of the push stroke. All six able-bodied participants show activity through to the end of the propulsive stroke 共96.3⫾ 3.0% 兲. Peak EMG amplitude between groups varied dramatically; participants with paraplegia used 45.0⫾ 12.0% of MVC at peak, while able-bodied participants used 29.7⫾ 6.6% 共Fig. 3兲. In tracing the average percent of MVC throughout propulsion and calculating the area under this curve, participants with paraplegia use a 36% greater posterior deltoid “muscle energy” than their able-bodied groups 共Table 3兲. The angle at which persons with paraplegia and ablebodied individuals achieved these peaks varied as well; participants with paraplegia had peak posterior deltoid firing at 126.5⫾ 5.4 deg on the push rim, while the able-bodied group had peak firing at 117.6⫾ 9.4 deg. This corresponds to 78.5⫾ 5.0% and 64.2⫾ 11.5%, respectively, of the propulsive stroke. Biceps Brachii. Biceps brachii burst duration in the paraplegic group occurs for 62.9⫾ 11.2% of propulsion; however, the location throughout propulsion varied for each participant. P-1, P-4, and P-5 had two biceps bursts, one early on 共2.3⫾ 1.5– 32.8⫾ 6.1兲 and one at the end of propulsion 共69.5⫾ 8.9– 91.7⫾ 10.5兲. P-2 and P-3 each had one burst throughout propulsion occurring at varying times, 4.7⫾ 5.2– 48.1⫾ 3.0 and 0.8⫾ 1.2– 97.7⫾ 1.9, respectively. Overall burst duration for able-bodied participants was Table 3 Muscle effort comparison % Difference in energy

Muscle

Population

“Energy”

Anterior deltoid

Able bodied Paraplegic

1862.2 1772.1

−5.1%

Posterior deltoid


1009.5 1372.6

+36.0%

Biceps brachii


764.5 1778.5

+132.0%

Long head of the triceps


1087.7 1817.1

+67.1%


62.0⫾ 7.7% of propulsion. Collectively, the able-bodied group had onset at 6.0⫾ 3.1% of propulsion; however, the range of cessation for participant’s with one burst was large, ranging from 60.9⫾ 15.2% to 78.6⫾ 4.6%. AB-3 and AB-6 had two biceps bursts of 2.3⫾ 1.9– 52.2⫾ 5.7 and 70.9⫾ 11.0– 89.0⫾ 11.7, and 7.8⫾ 9.8– 28.3⫾ 8.5 and 68.1⫾ 8.0– 81.7⫾ 17.2%, respectively. Peak EMG amplitude between groups varied dramatically; participants with paraplegia used 36.8⫾ 10.3% of MVC at peak, while able-bodied participants used 20.8⫾ 8.7% 共Fig. 3兲. The muscle energy differences between groups, as described above, are dramatic; participants with paraplegia use 132% greater biceps muscle energy 共Table 3兲. The angle at which both groups achieved these peaks was similar; participants with paraplegia had peak firing at 77.4⫾ 2.1 deg on the push rim, while the able-bodied group had peak firing at 80.5⫾ 3.1 deg. This corresponds to 15.5⫾ 5.2% and 15.9⫾ 2.6%, respectively, of the propulsive stroke. Peak biceps firing coincided with the impact spike 关26兴 on the force curve 共resultant, tangential, and radial兲 for the majority of all subjects. Long Head of the Triceps. Obvious differences in triceps muscle activity between able-bodied and paraplegic groups can be seen in Fig. 4. It is clear that all five participants with paraplegia had triceps activity early on in the push stroke while none of the able-bodied participants had any initial activity. As a result, the participants with paraplegia had a longer triceps burst duration in comparison with the able-bodied participants 共85.2⫾ 8.3% versus 58.4⫾ 6.5%兲. Peak EMG amplitude between groups varied dramatically as well; participants with paraplegia used 43.0⫾ 6.4% of MVC at peak, while able-bodied participants used 24.9⫾ 4.3% 共Fig. 3兲. Participants with paraplegia had greater 共67.1%兲 triceps muscle energy as well 共Table 3兲. The angle at which both groups achieved these peaks, however, was similar; participants with paraplegia had peak firing at 108.1⫾ 3.9 deg 共58.3⫾ 5.4% of propulsion兲 on the push rim, and the able-bodied group had peak firing at 112.0⫾ 2.7 deg 共60.7⫾ 4.1% of propulsion兲. Peak triceps activity coincided with peak resultant, tangential, and radial forces for the majority of participants. Shoulder Pain. As this pilot study was a continuation of another study on shoulder pain in manual wheelchair users 共Collaboration on Upper Limb Pain in SCI or CULP-SCI兲, there was an existing database on each participant with paraplegia including informational questionnaires. While the majority of the subjects 共three out of five兲 had a WUSPI 共Wheelchair Users Shoulder Pain Index兲关33,34兴 score of zero at the time of testing, every subject experienced “pain and/or numbness in both shoulders” in follow-up questionnaires at either or all of the 6, 12, 18, 24, and 36 month 共if applicable兲 follow-up calls. On a scale of 0–10 共zero being no pain at all, 10 being worst pain ever felt兲, four of the five participants felt the worst intensity of pain over the past month 共at the time of calling兲 to be at least 3 共on either or both the right and left sides兲, with three subjects rating their pain as a 4, and one subject rating their pain as high as a 7. The average pain in the past month across the board in this group of four participants ranged from 2 to 4. Two of the four subjects stated that shoulder pain affects their quality of sleep, though no one reported an “interference with daily life” as a result of their pain. One participant, P-3, reported no pain. Able-bodied subjects reported no pain before, throughout, or after the data collection.

Discussion It is well known that the shoulder muscle complex offers a wide range of movements and, as a result, has a large compensatory ability 关35兴. This may partially explain why upper-limb repetitive movements have been found to be so variable in able bodied and persons with paraplegia 关36兴. The functioning of muscle groups may be directly responsible for the observable differences between and within the studied groups. An integrated simultaneous FEBRUARY 2009, Vol. 131 / 021015-5


data collection and analysis allow for the comparison of differences in wheelchair propulsion between able-bodied individuals and participants with paraplegia. In this section, muscle activity with respect to kinetics, kinematics, and wheelchair propulsion characteristics are discussed and evaluated with other findings in the literature. Propulsion speed plays a role in force generation and muscle activity; therefore, one can conclude that varying the propulsion speed will vary these parameters. Self-selected propulsion speed differed between groups 共though not significantly兲; however the purpose of asking subjects to propel their chair at a self-selected speed was multifold. First, the importance of studying wheelchair technique under freely chosen conditions has been validated 关37兴; individuals have been shown to have a maximum mechanical efficiency, and a minimum ventilation, oxygen uptake, and heart rate when they propel at a freely chosen cycle frequency 关37兴. Secondly, trunk or lean angle has been shown to demonstrate a relatively small range of motion at self-selected speeds 关7兴; Rao et al. reported the difference between the minimum and maximum angles throughout propulsion to be less than 5 deg 共coincident with our findings兲. Further analysis and discussion of trunk angle were ignored; as a result of the minimum impact, we believe it to have played in other areas of our investigation. Examination of the anterior deltoid burst durations for both investigated groups mimic previous results 关8,15,35兴; there was anterior deltoid EMG activity in both the initial stage and throughout propulsion. However, there are confounding results regarding the posterior deltoid. Mulroy and Rodgers both report posterior deltoid activity onset 2 / 3 of the way through propulsion with activity well into recovery 关8,16,25兴. Schantz, however, reports posterior deltoid activity for two of four participants in the initial stages of propulsion 共one of the remaining subjects showed activity halfway throughout propulsion, while the last showed no activity at all兲关35兴. McLaurin and Brubaker reported posterior deltoid activity of a participant to be consistent with Schantz’s findings as well 关15兴. Our results mimic both scenarios. Posterior deltoid onset for the majority of participants with paraplegia 共four of the five兲 mimics the early activity findings of Schantz 关35兴. The parity between our study and these reported findings may be explained by the similar testing scenarios; both studies investigated self-selected speeds, and all tests were done in the subject’s own wheelchair. This may further explain why half of the able-bodied participants did not follow this trend; these participants did not demonstrate posterior deltoid activity in the initial stages of propulsion and, in fact, had results similar to those reported by Mulroy and Rodgers’ 关8,16,25兴共coincidentally, all setups were similar in the fact that either one or two wheelchairs were used for all subjects兲. Differences in methods and subsequent results may illustrate the importance of using a wheelchair fitted to each subject during data collection. The biceps brachii burst duration throughout propulsion for participants with paraplegia appears to be longer than what has previously been reported; however, both our findings and reported results demonstrate activity in the initial stages of propulsion 关8,16,25,35兴. Able-bodied biceps burst duration is slightly shorter, although still longer than what has been previously reported. Our triceps findings for paraplegic participants reveal activation early on in the push stroke. This coincides with McLaurin and Brubaker’s report from one participant 关15兴, although it confounds the majority of previous investigators reporting on multiple participants. However, triceps activity for the able-bodied group coincides well with previous results 关8,16,25,35兴. The fact that participants with paraplegia have early onset of the triceps simultaneous to biceps activity may be explained by the fact that without full trunk control, this initial contact may be used as both a propulsion technique 共to propel them forward兲 and to drive the trunk upright for stability. This move may utilize momentum throughout propulsion but may also be taxing on muscles. It is also possible that the longer duration times of both the biceps and triceps in the 021015-6 / Vol. 131, FEBRUARY 2009

group with paraplegia is related to the fact that they spend more time on the wheel, as determined by their earlier contact and later release angles, requiring longer contraction time. Further inspection into the burst durations for the investigated muscles reveals that the order of peak activation for all subjects during self-selected propulsion speeds is as follows: biceps brachii, anterior deltoid, long head of the triceps, and posterior deltoid. These results are in line with the activation patterns as reported by Veeger et al. 关36兴 under similar testing scenarios. However, the difference in the magnitude of peak firing between the groups for nearly all muscles was noticeable. Previous findings report similar results comparing those with paraplegia and able-bodied individuals: For the posterior deltoids, biceps, and triceps, the paraplegic participants were less efficient, in terms of using a greater percentage of MVC, to achieve the same goal of propelling and maintaining a self-selected speed 关16,25,35,38兴. The total muscle energy of these muscles was greater for those with paraplegia than able-bodied participants 共Table 3兲. While the self-selected speed was not significantly different between the two groups, the higher average speed of the individuals with paraplegia might account for the difference in muscle energy of this group compared to the able-bodied group. The difference in results may also be compensatory, perhaps due to the fact that trunk muscles are compromised in participants with paraplegia. As mentioned previously, an impaired trunk may burden the muscles of the upper limb. The less trunk control a participant has 共which generally correlates to injury level兲, the greater a burden 共as a percentage of MVC兲 the upper limbs face. This, in general, holds true for the majority of participants with paraplegia; P-5, whose injury level is L1/L2, used the lowest percent of MVC in the group for all muscles while P-1, whose injury level is T3, used the greatest percent of MVC for the majority of muscles. The remaining participants fell somewhere between the two extremes. Peak biceps firing in all participants occurred prior to 90 deg 共vertical兲 on the push rim 共participants with paraplegia, 77.4 deg⫾ 2.1 deg; able bodied, 80.5 deg⫾ 3.1 deg兲 and coincided with previous findings 关16,25兴. Similarly, peak triceps firing between groups were nearly identical; those with paraplegia had peak firing at 108.1 deg⫾ 3.9 deg, and able-bodied peaks were at 112.0 deg⫾ 2.7 deg. What did vary was the peak firing of both the anterior and posterior deltoids between groups. Participants with paraplegia had peak anterior deltoid firing prior to able-bodied subjects 共91.8 deg⫾ 5.5 deg versus 99.1 deg⫾ 3.6 deg兲 and peak posterior deltoid firing subsequent to able-bodied subjects 共126.5 deg⫾ 5.4 deg versus 117.6 deg⫾ 9.4 deg兲. The fact that those with paraplegia had peak muscle activation for all muscles prior to the able-bodied group may be explained by their contact and release angles; to elucidate on why the established order of peak activation 关36兴 is slightly earlier in the group with paraplegia, this group grabs and releases the wheel nearly 7 deg and 3 deg earlier, respectively, than the able-bodied group. Further investigation matches peak muscle activities to peak hand rim forces; our results support the fact that the previously defined relationship of peak muscle activity to peak hand rim force was strongest for the shoulder and elbow extensors, suggesting the triceps and posterior deltoid may be noteworthy in wheelchair propulsion 关8兴. There is also a strong relationship of peak shoulder and elbow flexors with the previously defined impact spike 关26兴 as well, suggesting the biceps and anterior deltoid may play a role in the initial force generation for propulsion. Future research should focus on how the synergies of the anterior deltoid and biceps brachii, and the posterior deltoid and long head of the triceps, sequence their contractions in individuals with paraplegia. Ultimately, we hypothesize that the magnitude of the loads in the glenohumeral 共GH兲 joint are the ultimate cause of shoulder pain. The simultaneous contribution of participant kinematics, kinetics, and EMG data drives these GH loads, and the role of each will be investigated in a future study with the use of a patient-specific model. A rigid-body dynamics model, constructed via objectTransactions of the ASME


oriented programming to the anthropometrics specifications of each participant, will be driven by the recorded subject kinematics and kinetics data from the current study. Inverse dynamics analysis will be run, after which point joint forces will be investigated and compared 共EMG will be used to validate the model兲. Differences in joint forces will be compared to differences found in the current study.

Conclusion Muscles act as joint stabilizers, and their recruitments will determine the magnitude and direction of the resultant joint force. There are two main conclusions that can be drawn from this integrative investigation: 共1兲 A greater “muscle energy,” as measured by the percentage of EMG throughout propulsion, results in a greater resultant joint force in the shoulder and elbow, thus potentially resulting in shoulder pathology; 共2兲 Similarly, a greater muscle energy may result in fatigue and play a factor in the development of shoulder pain and pathology over time; fatigue may compromise an effective propulsive stroke placing undue stresses on the joint capsule. In summary, differences in these muscle forces are responsible for increasing shoulder joint forces, which may in turn cause shoulder pain or pathology. Limitations. Variations between the previously reported results and our current findings may have been affected by any number of external factors. Wheelchair criteria were slightly different for all reported papers. Mulroy used one of two wheelchairs for all participants, depending on the participant’s size 关16,25兴, Rodgers used one wheelchair for all participants 关8兴, Schantz’s participants each used their own wheelchair 关35兴, and for the our study, paraplegic participants used their own wheelchair and able-bodied participants used one wheelchair, regardless of their size. Other sources of error may include human error in finding true maximal voluntary contraction, population differences, and small sample size. EMG data were collected and analyzed differently in each paper as well. Fine-wire electrodes were used in Mulroy’s experiments 关16,25兴; however, all other studies including ours used surface electrodes 关8,35,36,38,39兴. Signal analysis varied as well, from visual inspection and manual measurement 关35兴 to the method we used where onset and cessation was determined via threshold-based calculations 关31兴. It is also important to note that none of the able-bodied participants uses a manual wheelchair as a means of transportation, and therefore their novice experience may affect the way in which they propel and may result in any inconsistencies within that population.

Acknowledgment The first author 共S.R.D.兲 was supported through funding from the National Science Foundation GK-12 Fellowship. The second author 共S.A.S.兲 was supported by the National Institute on Disability and Rehabilitation Research CULP-SCI Grant 共H133A011107兲. This study was performed toward the partial fulfillment of the requirements for the degree of Doctor of Philosophy at Rutgers, The State University of New Jersey, for the first author. They would like to acknowledge the Kessler Medical Rehabilitation and Research Education Center 共KMRREC兲 and the Henry H. Kessler 共HHK兲 Foundation for providing the facility and the software with which to work. They would also like to thank Mathew Yarossi for his help in analyzing participant EMG data, and Andrew Kwarciak for his help in analyzing participant grab angle.

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