Comparison of Shoulder Muscle Electromyographic Activity During Standard Manual Wheelchair and Push-Rim Activated Power Assisted Wheelchair Propulsion in Persons With Complete Tetraplegia

Article Outline

• Abstract
• Purpose and Hypotheses
• Methods
  • Participants
  • Instrumentation
    • Electromyography
    • Kinematics
    • Ergometer set-up
    • Shoulder strength testing
  • Procedures
    • Electromyographic activity
    • Kinematics
    • Ergometer calibration and set-up
    • Wheelchair propulsion tasks
    • Shoulder strength testing
  • Data Management
• Results
  • Pushrim-Activated Power-Assisted Wheelchair Propulsion Matched to the Standard Wheelchair
    • Propulsion characteristics
    • Shoulder muscle electromyographic activity
  • Self-Selected Free Pushrim-Activated Power-Assisted Wheelchair Propulsion
    • Propulsion characteristics
    • Shoulder muscle electromyographic activity
  • Shoulder Isometric Torque
• Discussion
  • Study Limitations
• Conclusions
• References
• Copyright

Abstract 

Lighthall-Haubert L, Requejo PS, Mulroy SJ, Newsam CJ, Bontrager E, Gronley JK, Perry J. Comparison of shoulder muscle electromyographic activity during standard manual wheelchair and push-rim activated power assisted wheelchair propulsion in persons with complete tetraplegia.

Objectives

To compare spatio-temporal propulsion characteristics and shoulder muscle electromyographic activity in persons with cervical spinal cord injury propelling a standard pushrim wheelchair (WC) and a commercially available pushrim-activated power assisted wheelchair (PAPAW) design on a stationary ergometer.

Design

Repeated measures.

Setting

Motion analysis laboratory within a rehabilitation hospital.

Participants

Men (N=14) with complete (American Spinal Injury Association grade A or B) tetraplegia (C6=5; C7=9).

Intervention

Participants propelled a standard pushrim WC and PAPAW during 3 propulsion conditions: self-selected free and fast and simulated 4% or 8% graded resistance propulsion.

Main Outcome Measures

Median speed, cycle length, cadence, median and peak electromyographic activity intensity, and duration of electromyographic activity in pectoralis major, anterior deltoid, supraspinatus, and infraspinatus muscles were compared between standard pushrim WC and PAPAW propulsion.

Results

A significant (P<.05) decrease in electromyographic activity intensity and duration of pectoralis major, anterior deltoid, and infraspinatus muscles and significantly reduced intensity and push phase duration of supraspinatus electromyographic activity at faster speeds and with increased resistance were seen during PAPAW propulsion.

Conclusions

For participants with complete tetraplegia, push phase shoulder muscle activity was decreased in the PAPAW compared with standard pushrim WC, indicating a reduction in demands when propelling a PAPAW.

Key Words: Rehabilitation, Spinal cord injuries, Wheelchairs

List of Abbreviations: ADLs, activities of daily living, MMT, maximal muscle test, PAPAW, pushrim-activated power assisted wheelchair, SCI, spinal cord injury, WC, wheelchair

LOSS OF LOWER-LIMB function after SCI, which shifts the weight-bearing demands of locomotion and functional mobility to the upper extremities, often results in upper-limb pain and injury after as little as 2 years, causing a reduction in independence among many persons.¹, ², ³ Upper-limb pain in those who use a manual WC is most often related to musculoskeletal pathology, specifically diagnoses related to subacromial impingement syndrome, rotator cuff tendinopathy,⁴, ⁵, ⁶ chronic orthopedic inflammatory syndromes,⁷ and carpal tunnel syndrome.⁸

Manual WC propulsion is particularly difficult for those with muscular weakness as well as upper-extremity pain and/or injury. In particular, those with cervical-level SCI—that is, tetraplegia—have reduced muscle strength reserve⁹, ¹⁰ and are more prone to develop upper-extremity pain and injury with prolonged manual WC use.¹¹ Newsam et al¹² documented reduced efficiency for persons with tetraplegia propelling a manual WC, with significantly slower propulsion speeds and shorter cycle distances recorded, compared with subjects with paraplegia. When Kulig et al¹³ covaried for this reduction in velocity, similar shoulder joint moments but significantly higher superior shoulder joint forces were documented in those with tetraplegia. Despite slower propulsion speeds, Mulroy et al¹⁴ found similar intensities of push phase shoulder muscle activity in addition to prolonged duration of pectoralis major and infraspinatus activity compared with persons with paraplegia. These findings suggest that persons with tetraplegia may have a greater likelihood of compressing their subacromial structures because of the greater shoulder reaction forces during WC propulsion, combined with weakness of the thoraco-humeral depressors and rotator cuff musculature¹³ and increased severity of sensory-motor trunk impairments contributing to malalignment and instability of the shoulder joint.

The upper-limb demands of locomotion can be virtually eliminated in those with tetraplegia and/or upper-limb impairments by using a power WC. Transition from a manual to a power WC, however, can lead to some loss of independence because of the challenges associated with transportation and maneuvering environmental barriers. Furthermore, use of a power WC may result in cardiovascular and skeletal muscle deconditioning, in addition to increased cost to purchase and maintain the chair and the perception of a stigma associated with its use. Thus, an alternative mode of WC propulsion is needed¹⁵ that is less demanding for the upper extremities, yet allows similar access to the environment provided by a manual WC.

Recent development of the PAPAW offers an alternative mode of manual propulsion for persons with SCI¹⁶ and limited reserve capacity for propulsion because of muscular weakness, upper-extremity pain, and excessive body mass (marginal users). This system is designed to provide additional power to the propulsion effort, resulting in a considerable reduction in the push force during WC propulsion. A commercially available PAPAW (Quickie Xtender^a) (fig 1A) uses a set of linear compression springs to regulate the pushrim force while a potentiometer senses the relative motion between the pushrim and hub. Potentiometer signals from both wheels are interfaced to a microcontroller that coordinates the direct current motors in each wheel hub, resulting in increased power output. The motors are powered by a rechargeable nickel cadmium battery pack located behind the backrest. The system is also designed to be disassembled, similar to a manual WC, for ease of transport.

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• Fig 1. 

  (A) PAPAW. (B) Standard WC placed on ergometer. (C) Custom adjustable wheel axle plate for standard pushrim WC.

Performance evaluation of the PAPAW on an ergometer has shown that it can reduce metabolic energy consumption between 17% and 43%,¹⁷ improve the mechanical efficiency by an average of 80%, and reduce user power requirements from 56% to 79%.¹⁸ Over an outdoor track, it has been shown to reduce oxygen consumption and heart rate while increasing speed during 20 minutes of propulsion at a self-selected pace.¹⁹ In an evaluation under controlled environmental conditions, Cooper et al¹⁶ tested 10 subjects (including 9 with paraplegia from SCI) propelling a PAPAW and their personal WC over a 5-task ADLs obstacle course and reported an overall decrease in heart rate and oxygen consumption with higher ergonomic rating during ADLs, except for car loading of the rear wheel. Moreover, Levy et al²⁰ examined elderly WC users (with varying diagnoses or reasons for WC use) during propulsion over a 110-m level surface, over 21m of carpet, and during 3 trials of ascending and descending a 6-m ramp with a 5° incline. They determined that the PAPAW reduced heart rate and lowered perceived exertion.

In addition to energy cost and efficiency, joint range of motion¹⁷, ²¹, ²², ²³, ²⁴ and muscle activity analysis¹⁴, ²⁵, ²⁶ have been used to evaluate upper-limb effort and propulsion demand during WC propulsion. Evaluation of upper-limb joint range of motion during PAPAW propulsion at 3 resistance levels and 2 speeds on an ergometer showed reduced excursion and similar stroke frequency compared with standard manual propulsion.²¹ To evaluate the demand at the individual muscle level, Levy²⁰ recorded surface electromyographic activity from upper-limb and torso muscles of elderly subjects while they wheeled their own WC and a prototype PAPAW on a level surface, carpet, and an incline. They found a decrease in extensor carpi radialis, triceps brachii, pectoralis major, latissimus dorsi, and anterior-medial deltoid muscles, particularly when pushing over carpet and up inclines. To determine more fully the demands on the specific shoulder muscles, particularly the anterior deltoid and deep rotator cuff muscles, we evaluated a PAPAW in a group of full-time manual WC users with complete tetraplegia.

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Purpose and Hypotheses 

The primary purpose of this study was to compare spatio-temporal propulsion characteristics and the activity of 4 key shoulder muscles responsible for providing the propulsive forces during standard pushrim WC and PAPAW propulsion in subjects with complete tetraplegia propelling at similar speeds (initially self-selected in a standard pushrim WC) during 3 conditions at varying speeds and resistances (free, fast, and graded propulsion). Related to our primary purpose, we hypothesized that PAPAW propulsion at a velocity similar to standard pushrim WC propulsion would result in (1) reduced cadence and increased cycle length and (2) a reduction in push phase shoulder muscle activity, with an increased effect during high-intensity propulsion conditions (fast and graded). The secondary purpose of this study was to compare spatio-temporal propulsion characteristics and the activity of 4 key shoulder muscles during self-selected (unmatched) free PAPAW propulsion to self-selected free and fast standard pushrim WC propulsion in the same group of subjects. We hypothesized that self-selected free (unmatched) PAPAW propulsion velocity and cycle length would be greater than that recorded during self-selected free standard pushrim WC propulsion and similar to self-selected fast standard pushrim WC propulsion. We additionally hypothesized that electromyographic activity during self-selected free PAPAW propulsion would be more similar to the activity recorded during standard pushrim fast propulsion than standard pushrim free propulsion.

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Methods 

Participants 

A sample of convenience was recruited from the outpatient services at Rancho Los Amigos National Rehabilitation Center in Downey, CA. Subjects were included if they had complete (American Spinal Injury Association grade A or B) tetraplegia (C6=5; C7=9) and pushed a manual WC for at least 50% of their locomotion. Patients with a duration of tetraplegia of less than 1 year, full-thickness rotator cuff tears, adhesive capsulitis, cervical radiculopathy, or suprascapular nerve palsy were excluded. Fourteen men met the inclusion criteria. The mean age of subjects was 43 years (range, 23.8–56.5y), and the average duration of injury was 19.9 years (range, 5–34y) (table 1). Prior to data collection, volunteers were provided with a copy of the Bill of Rights of Human Subjects and were asked to read and sign an informed consent form that had been approved by the Rancho Los Amigos Institutional Review Board. All testing was performed at the Pathokinesiology Laboratory located at Rancho Los Amigos National Rehabilitation Center.

Table 1. Demographic Data and Mean Shoulder Isometric Torque

Abbreviation: NA, not available.

Instrumentation 

Indwelling bipolar, 50-μm Ni-Cr alloy wire electrodes (California Fine Wire^b) were used to record shoulder muscle activity. A fiber optic cable system transmitted the electromyographic signal to the data acquisition computer^c (PDP-11/73). The electromyographic signal was filtered using a bandwidth of 150 to 1000Hz, with an overall system gain of 1000. The data were sampled at a rate of 2500Hz.

A 6-camera VICON motion analysis system^d was used to record 3-dimensional motion of the right upper limb and trunk during WC propulsion. This instrumentation and procedure were described at length in a previous report.²⁷

Two test WCs, a standard pushrim (see fig 1B), and a PAPAW (see fig 1A) were used. The standard pushrim WC was a rigid-frame, lightweight WC (Quickie GPV^a). To accommodate subjects of varying sizes, 2 standard pushrim test WCs were available with either 40.6 or 45.7 cm (16-in or 18-in) seat width. A custom-fabricated wheel axle mounting plate (see fig 1C) allowed vertical and horizontal adjustability to achieve a standardized seat position for each subject in the standard pushrim WC, as previously described.²⁷ The standardized seat position aligned the rear-most axle opening of the custom axle plate horizontally with the center of the right glenohumeral joint (seat anterior, or wheel back position) (see fig 1C), allowing similar placement of the standard pushrim wheel to the PAPAW wheel. The custom axle plate was then adjusted vertically, with the wheel in the center axle position, such that the subject's elbow was flexed to 80° (relative to 0° full extension) with the hand grasping the wheel at top center (12 o'clock position) of the right pushrim. A 45.7 cm (18-in) wide rigid-frame, lightweight WC (Quickie GPV) with a fixed axle plate was used for PAPAW propulsion. The PAPAW axle plate was not adjustable because of the motor mechanism. The standard pushrim WC and PAPAW were placed on the ergometer as previously described.²⁵

Right shoulder maximum isometric torques were recorded with a Lido isokinetic dynamometer.^e

Procedures 

Data collection, including electromyographic activity, kinematics, WC propulsion, and isometric strength testing were performed in a single test session lasting approximately 5 hours. Frequent rest breaks were provided to avoid muscular fatigue and to allow for equipment set-up.

Four push-phase muscles of the right shoulder were sampled using indwelling fine-wire electrodes.²⁶ These muscles included the sternal or clavicular portion of pectoralis major, anterior deltoid, supraspinatus, and infraspinatus. A 25-gauge needle was used to insert bipolar electrodes into each muscle.²⁸ For persons with C6 tetraplegia lacking innervation of sternal pectoralis major, an electrode was placed in the clavicular portion of pectoralis major, between the first and second proximal one third of the muscle belly. For those with adequate innervation of sternal pectoralis major, the electrode was placed 1 finger-width cephalo-medial to the axilla, in the center of the bulk of the muscle belly. The electrodes for anterior deltoid, supraspinatus, and infraspinatus were placed in the center of the muscle bellies. Confirmation of electrode placement was achieved by palpation of muscle belly contraction and/or tendon movement during mild electrical stimulation through the wires.

A 5-second period of electromyographic data collection initially occurred with participants seated in their own WCs, at rest, to measure electronic noise inherent to the acquisition system. This served as a threshold for the detection of myoelectric activity. Electromyographic activity was then recorded for each muscle during a 5-second maximum voluntary isometric contraction in an MMT.²⁵

Twelve reflective markers were taped to bony landmarks on the trunk and right upper limb to identify upper-limb kinematics: manubrium, xiphoid process, T3 and T10 spinous processes, greater tubercle,²³ posterior lateral mid-humerus, lateral and medial epicondyles, radial and ulnar styloid processes, and the dorsal third metacarpal and medial fifth metacarpal phalangeal joints. Three markers were taped to the right wheel of the standard pushrim and PAPAW WCs.²⁷ For the purpose of this analysis, only the third metacarpal, lateral epicondyle, and wheel marker trajectories were used.

Two forms of propulsion, standard pushrim and PAPAW, were tested. Subjects sat on their own cushion in the test WC. The backrest and footrest of the test WCs were adjusted as close as possible to each individual's own WC configurations. The test WC was then mounted on a stationary ergometer for testing, with the rear wheels resting on 2 independent rollers of the ergometer. Further support of the WC frame was provided by hydraulic jacks. An additional support bar clamped to the footrest prevented movement of the WC during testing. Removable flywheels proportional to the weight of the participant were added to the roller axle to simulate translational inertia experienced during level propulsion over a firm surface (eg, cement). Prior to acquisition of WC propulsion data, the friction force between the tire and ergometer was determined during a coast-down test with the subject sitting in the test WC mounted on the ergometer. The 2 independent rollers were first locked and spun by an electric drill motor at 182m/min as determined by the ergometer-controlling computer. The drill motor was then removed, and the wheels were allowed to coast down to 35m/min. The friction force recorded during the coast-down test was used by the ergometer-controlling computer to determine the resistance applied to the rollers in subsequent data acquisition trials. Additional resistance through the ergometer rollers for simulation of graded propulsion was delivered by a controlling computer from a bicycle ergometer (production discontinued), as previously described.²³

Wheelchair propulsion trials began with the standard WC. Prior to data collection, subjects propelled the WC on the ergometer for 3 to 5 minutes to accommodate to the test environment, followed by a 5-minute rest period. Subjects propelled the WC approximately 5 to 10 seconds prior to initiation of 10 seconds of collection of electromyography and kinematics data for each propulsion trial. A single trial each of standard pushrim WC propulsion was recorded at a self-selected free velocity and at a self-selected fast velocity without additional load applied to the ergometer rollers, simulating propulsion over level ground. Next, 1 trial of standard pushrim propulsion was recorded with the front end of the ergometer elevated with wooden blocks and resistance added to the ergometer rollers to simulate an 8% grade (or a 4% grade if it was demonstrated during the accommodation period that the subject was unable to sustain propulsion with resistance simulating the 8% grade). Propulsion speeds were noted from the display of the ergometer-controlled computer.

After completion of the standard pushrim WC condition, the PAPAW was placed on the ergometer, and participants propelled the PAPAW for 5 to 8 minutes to accommodate to the test environment. After a 5-minute rest period, PAPAW propulsion was performed with verbal feedback from the tester and visual feedback from a speedometer in attempt to match propulsion speed, plus or minus 5%, to that recorded during the similar matched standard pushrim WC propulsion condition (free, fast, and graded). Because we observed an artificially prolonged recovery phase of the push cycle during free PAPAW propulsion resulting from efforts to match the speed initially self-selected in the standard pushrim WC during data collection for the first subject, an additional unmatched (self-selected) free PAPAW propulsion trial was also collected.

The Lido dynamometer was calibrated according to the manufacturer's specifications prior to each test session. All shoulder maximum isometric torque testing was performed after WC propulsion tasks and an approximately 30-minute rest break. Subjects were seated in their personal wheelchairs for strength testing. Right shoulder scaption was tested with the elbow extended and the shoulder at 30° of elevation in the scapular plane (30° anterior to the frontal plane). Shoulder external rotation was assessed with the shoulder in 90° of abduction and neutral rotation and the elbow flexed 90°. The humerus was supported in a specially designed stand with a trough for the upper arm that could be raised or lowered to align the subject's shoulder in 90° of abduction. Subjects performed two 5-second trials for each condition.

Data Management 

Electromyographic activity during the MMT was full-wave–rectified and integrated over .02-second intervals. The greatest 1 second of electromyographic muscle activity during MMT was located via a moving window, and the time average served as the normalization value (100% MMT). The electromyographic muscle activity recorded during WC propulsion was full-wave–rectified and integrated over .01-second intervals, and intensity was expressed as a percentage of the MMT.

The 3-dimensional kinematic data were sampled at a rate of 50 frames a second and acquired on the PDP-11 storage computer. Data were smoothed with a 4-Hz low-pass second-order recursive digital filter with forward and backward passes to eliminate phase shift. Data were then copied to a personal computer for identification of marker trajectories using visual 3D.^f Because push-rim force data were unavailable for calculation of PAPAW propulsion cycle timing, the lateral epicondyle marker trajectory was used to determine propulsion cycle (time normalized to 100%) to provide consistent comparable cycle timing between each WC. Duration of the push phase was comprised of anterior lateral epicondyle movement from beginning to end, and recovery phase was defined by posterior lateral epicondyle excursion. Pilot data from the first 10 subjects comparing anterior lateral epicondyle movement to positive wheel torque in the standard WC indicated that these events were highly associated (r=.99). Electromyographic activity timing for every 1% of the propulsion cycle was calculated for both WCs during each propulsion condition. For each muscle and test condition, a median electromyographic activity intensity pattern for each participant was derived by computing the median electromyographic activity intensity for each 1% of the push cycle from 5 consecutive propulsion cycles collected. Electromyographic activity onsets, cessations, and durations were determined from this composite cycle such that electromyographic activity greater than or equal to 5% of MMT and lasting at least 5% of the propulsion cycle was selected. Peak electromyographic activity was identified as the greatest intensity over 1% of the push cycle.

Peak isometric torques for shoulder scaption and external rotation were determined for each subject from the absolute highest value recorded.

Data were analyzed using SPSS 12.0 software.^g Group propulsion characteristics and electromyographic activity data were compared between the standard WC and PAPAW in 3 analyses: (1) the matched velocity trials of standard pushrim versus PAPAW in the free, fast, and graded conditions; (2) free standard pushrim versus self-selected (unmatched) free PAPAW; and (3) fast standard pushrim versus self-selected free PAPAW propulsion. The Shapiro-Wilk statistic determined that propulsion velocity, cycle length, and electromyographic activity data were not normally distributed, while cadence data were normally distributed. Conservatively, therefore, medians were determined and compared using the nonparametric Wilcoxon signed-ranks test for all data. A Bonferroni correction was used for standard pushrim free and fast comparisons, and thus a significance level of .025 was selected. A significance level of .05 was selected for the matched standard pushrim and PAPAW graded comparisons.

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Results 

Pushrim-Activated Power-Assisted Wheelchair Propulsion Matched to the Standard Wheelchair 

Subjects were able to match the self-selected free standard pushrim WC propulsion velocities (table 2) during free PAPAW propulsion. During fast propulsion (fig 2), PAPAW velocity was statistically but not clinically significantly reduced (86.6m/min; interquartile range, 70.5–101.2m/min) compared with fast propulsion in the standard pushrim WC (86.6m/min; interquartile range, 74.7–113.0m/min; P=.02). PAPAW velocity during graded propulsion (41.2m/min) was significantly faster, however, than the standard pushrim (30.8m/min; P=.01).

Table 2. Median Velocity, Cycle Length, and Cadence (Interquartile Range, 25%–75%)

	Free		Fast		Graded
	Self-Selected Standard	Matched PAPAW	Self-Selected Standard	Matched PAPAW	Self-Selected Standard	Matched PAPAW
Velocity (m/min)	55.6(42.4–76.2)	59.4(45.8–76.3)	86.6(74.7–113.0)	86.6⁎(70.5–101.2)	30.8(21.5–36.3)	41.2⁎(32.8–46.6)
				P=.024		P=.008
Cycle length (m)	0.88(0.75–1.08)	1.01(0.76–1.25)	1.04(0.85–1.26)	1.31⁎(1.02–1.62)	0.51(0.40–0.58)	0.73⁎(0.54–0.93)
				P=.001		P=.001
Cadence (cycles/m)	68.7(53.8–75.5)	63.2(49.4–67.5)	89.7(85.0–92.9)	70.3⁎(59.0–76.0)	60.4(56.3–66.6)	58.5(47.6–65.9)
				P=.001

Abbreviations: Fast, fast propulsion; Free, free propulsion; Graded, graded propulsion.

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• Fig 2. 

  Velocity-matched fast propulsion. *Significantly different from PAPAW.

Cycle length (see table 2) during the matched propulsion trials was significantly greater in the PAPAW for fast and graded propulsion compared with the standard pushrim WC (P=.001). Cadence (see table 2) was significantly reduced during fast PAPAW propulsion (70.3 cycles/min) compared with the standard pushrim WC (89.7 cycles/min; P=.001) but similar to the standard pushrim WC during free (63.2 cycles/min PAPAW vs 68.7 cycles/min standard pushrim WC) and graded propulsion (58.5 cycles/min PAPAW vs 60.4 cycles/min).

Median electromyographic activity intensities during matched free, fast, and graded (table 3) propulsion trials were reduced during PAPAW propulsion compared with the matched standard pushrim condition, particularly for the 2 muscles primarily responsible for providing the forward propulsive force during the push phase, pectoralis major and anterior deltoid (fig 3). Median intensity in both muscles was significantly reduced, to less than 20% MMT, in all 3 conditions in the PAPAW, compared with intensities in the standard pushrim WC propulsion condition that ranged from 43% to 66% MMT for pectoralis major and from 19% to 49% MMT for anterior deltoid. Median intensity was also significantly reduced during PAPAW propulsion in the 2 rotator cuff muscles, infraspinatus and supraspinatus. Infraspinatus was reduced to less than 15% MMT in fast (fig 4A) and graded conditions (see fig 3) compared with 23% and 37% MMT in the standard pushrim chair. Median intensity of supraspinatus was unchanged in (matched) free propulsion, mildly reduced in fast, and decreased to 17% MMT during graded PAPAW propulsion, compared with 41% MMT in the standard pushrim WC.

Table 3. Electromyographic Median Intensity (Interquartile Range, 25%–75%)

	Free		Fast		Graded
	Self-Selected Standard	Matched PAPAW	Self-Selected Standard	Matched PAPAW	Self-Selected Standard	Matched PAPAW
PECMAJ (%MMT)	42.5(15.8–61.5)	9.3⁎(0.0–26.0)	60.8(30.0–75.8)	15.5⁎(9.0–33.5)	65.5(31.8–111.5)	16.5⁎(10.4–22.3)
		P=.002		P=.002		P=.001
ADELT (%MMT)	18.5(11.3–36.4)	0.0⁎(0.0–6.8)	34.0(23.4–57.3)	8.0⁎(0.0–13.8)	48.5(34.9–60.5)	10.8⁎(3.8–15.3)
		P=.002		P=.001		P=.001
SUPRA (%MMT)	13.3(8.5–40.9)	12.8(8.5–18.3)	25.0(18.8–41.1)	22.8⁎(9.5–28.3)	40.8(20.0–50.3)	16.5⁎(8.1–23.0)
				P=.020		P=.003
INFRA (%MMT)	11.3(0.0–24.8)	5.8(0.0–12.4)	22.5(14.9–33.4)	13.5⁎(0.0–26.4)	37.3(24.0–50.3)	10.5⁎(0.0–14.4)
				P=.012		P=.001

Abbreviations: ADELT, anterior deltoid; Fast, fast propulsion; Free, free propulsion; Graded, graded propulsion; INFRA, infraspinatus; PECMAJ, pectoralis major; SUPRA, supraspinatus.

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• Fig 3. 

  Electromyographic activity of anterior deltoid (ADELT), pectoralis major (PECMAJ), supraspinatus (SUPRA), and infraspinatus (INFRA) during simulated 8% graded propulsion with a standard WC and a PAPAW from a subject with C7 tetraplegia. Solid bars indicate push phase.

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• Fig 4. 

  (A) Median electromyographic intensity—matched fast propulsion. (B) Peak electromyographic intensity—matched fast propulsion. (C) Electromyographic duration—matched fast propulsion. Abbreviations: ADELT, anterior deltoid; INFRA, infraspinatus; PECMAJ, pectoralis major; SUPRA, supraspinatus. *Significantly different from standard WC .

Peak electromyographic activity intensity (table 4) during matched propulsion was significantly reduced during PAPAW propulsion (0% to 43% MMT) compared with standard pushrim WC propulsion (30% to 107% MMT) for all muscles (pectoralis major, anterior deltoid, supraspinatus, infraspinatus) during all conditions (free, fast [fig 4B], and graded), with the exception of supraspinatus during free propulsion (35.0% vs 38.5% MMT).

Table 4. Electromyographic Peak Intensity (Interquartile Range, 25%–75%)

	Free		Fast		Graded
	Self-Selected Standard	Matched PAPAW	Self-Selected Standard	Matched PAPAW	Self-Selected Standard	Matched PAPAW
PECMAJ (%MMT)	78.0(34.0–108.5)	20.0⁎(0.0–43.5)	106.5(78.3–186.0)	36.0⁎(17.5–100.3)	93.5(80.5–157.8)	35.5⁎(21.5–44.0)
		P=.001		P=.001		P=.001
ADELT (%MMT)	32.5(20.0–65.5)	0.0⁎(0.0–10.0)	76.0(55.3–154.3)	14.5⁎(0.0–29.8)	72.5(59.0–101.3)	17.5⁎(10.5–34.5)
		P=.001		P=.001		P=.001
SUPRA (%MMT)	38.5(18.8–74.5)	35.0(14.8–42.3)	78.0(46.5–114.8)	43.0⁎(25.5–62.0)	67.5(48.3–97.3)	34.5⁎(13.0–51.3)
				P=.002		P=.002
INFRA (%MMT)	30.0(0.0–52.5)	11.0⁎(0.0–31.8)	62.5(40.5–84.5)	24.5⁎(0.0–59.8)	81.0(53.5–95.8)	12.0⁎(0.0–34.5)
		P=.011		P=.001		P=.002

Abbreviations: ADELT, anterior deltoid; Fast, fast propulsion; Free, free propulsion; Graded, graded propulsion; INFRA, infraspinatus; PECMAJ, pectoralis major; SUPRA, supraspinatus.

Duration of electromyographic activity as a percentage of the push cycle (table 5) during matched PAPAW propulsion was significantly reduced for pectoralis major (by 36% to 66% of the push cycle) during all 3 conditions (free, fast [fig 4C], and graded). Duration of anterior deltoid and infraspinatus electromyographic activity was significantly less during fast (see fig 4C) and graded PAPAW propulsion. Additionally, the decrease in anterior deltoid duration recorded during free PAPAW propulsion nearly reached statistical significance (.025<P<.05). There was no significant difference in duration of supraspinatus electromyographic activity between the matched PAPAW and standard pushrim WC propulsion trials (free, fast, and graded).

Table 5. Electromyographic Activity Duration (Interquartile Range, 25%–75%)

	Free		Fast		Graded
	Self-Selected Standard	Matched PAPAW	Self-Selected Standard	Matched PAPAW	Self-Selected Standard	Matched PAPAW
PECMAJ (% push cycle)	44(37–57)	15⁎(0–23)	54(46–67)	25⁎(18–38)	72(62–83)	46⁎(31–49)
		P=.001		P=.016		P=.002
ADELT (% push cycle)	28(17–37)	0(0–12)	41(35–50)	19⁎(0–24)	62(56–69)	27⁎(9–45)
				P=.002		P=.004
SUPRA (% push cycle)	49(11–61)	42(8–79)	50(36–57)	36(24–70)	39(20–71)	34(19–59)
INFRA (% push cycle)	30(0–47)	12(0–35)	46(30–63)	20⁎(0–49)	58(44–66)	28⁎(0–53)
				P=.001		P=.001

Abbreviations: ADELT, anterior deltoid; Fast, fast propulsion; Free, free propulsion; Graded, graded propulsion; INFRA, infraspinatus; PECMAJ, pectoralis major; SUPRA, supraspinatus.

Self-Selected Free Pushrim-Activated Power-Assisted Wheelchair Propulsion 

Self-selected free PAPAW propulsion velocity was more than twice as fast as self-selected free standard pushrim WC propulsion (table 6). In fact, self-selected free PAPAW propulsion velocity was similar to standard pushrim WC fast propulsion velocity.

Table 6. Median Velocity, Cycle Length, and Cadence (Interquartile Range, 25%–75%)

	Self-Selected Free Standard	Self-Selected Free PAPAW	Self-Selected Fast Standard
Velocity (m/min)	55.6(42.4–76.2)	111.7⁎(89.0–118.0)	86.8(74.7–113.0)
		P=.002
Cycle length (m)	0.88(0.75–1.08)	1.51⁎†(1.41–1.81)	1.04(0.85–1.26)
		P=.001
Cadence (cycles/min)	68.7(53.8–75.5)	61.2†(52.6–82.9)	89.7(85.0–92.9)
		P=.003

Cycle length during the self-selected free PAPAW trial (see table 6) was significantly greater than both free and fast standard pushrim WC propulsion (1.51m vs .88m and 1.04m, respectively). Self-selected free PAPAW cadence (see table 6) was similar to the free standard pushrim WC trial, but significantly less than the standard pushrim WC fast trial.

Median electromyographic activity intensities during self-selected free PAPAW propulsion (table 7; fig 5) were similar to those recorded during standard pushrim free propulsion, despite the velocity being more than twice as fast. Median electromyographic activity intensities during the self-selected free PAPAW trial, however, were significantly reduced, by 38% to 59% MMT, compared with fast propulsion in the standard pushrim WC (see fig 5) for 3 of the 4 muscles tested (pectoralis major, anterior deltoid, infraspinatus), while velocities were statistically similar.

Table 7. Electromyographic Median Intensity (Interquartile Range, 25%–75%)

	Self-Selected Free Standard	Self-Selected Free PAPAW	Self-Selected Fast Standard
PECMAJ (%MMT)	42.5(15.8–61.5)	30.0⁎(18.0–48.0)	60.8(30.0–75.8)
		P=.009
ADELT (%MMT)	18.5(11.3–36.4)	14⁎(10.3–22.5)	34.0(23.4–57.3)
		P=.003
SUPRA (%MMT)	13.3(8.5–40.9)	19.0(6.0–28.3)	25.0(18.8–41.1)
INFRA (%MMT)	11.3(0.0–24.8)	14.0⁎(6.0–25.0)	22.5(14.9–33.4)
		P=.008

Abbreviations: ADELT, anterior deltoid; Fast, fast propulsion; Free, free propulsion; INFRA, infraspinatus; PECMAJ, pectoralis major; SUPRA, supraspinatus.

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• Fig 5. 

  Electromyographic activity of the anterior deltoid, pectoralis major, supraspinatus, and infraspinatus during self-selected free propulsion with a standard pushrim, self-selected free propulsion with a PAPAW, and self-selected fast propulsion with a standard pushrim and a subject with C7 tetraplegia. Solid lines indicated push phase of propulsion. Abbreviations: ADELT, anterior deltoid; FREE, free propulsion; INFRA, infraspinatus; PECMAJ, pectoralis major; SS FREE, self-selected propulsion; STANDARD, standard pushrim; SUPRA, supraspinatus.

Peak electromyographic activity intensities during self-selected free PAPAW propulsion (table 8) for all muscles tested were similar to those recorded during free standard pushrim WC propulsion and significantly less (reduced by 26% to 46% MMT) than those recorded during fast standard pushrim WC propulsion. These substantial reductions in peak demand for each of the 4 muscles were seen despite similar velocities between the self-selected free PAPAW and fast standard pushrim WC trials. Durations of electromyographic activity during self-selected free PAPAW propulsion (table 9) were similar to standard pushrim WC free propulsion for all muscles tested, despite propelling at greater than twice the speed of standard pushrim WC propulsion. Electromyographic activity durations during self-selected free PAPAW propulsion, however, were reduced by 30% to 42% of the push cycle for pectoralis major, anterior deltoid, and infraspinatus compared with standard pushrim WC fast propulsion.

Table 8. Electromyographic Peak Intensity (Interquartile Range, 25%–75%)

	Self-Selected Free Standard	Self-Selected Free PAPAW	Self-Selected Fast Standard
PECMAJ (%MMT)	78.0(34.0–108.5)	59.0⁎(44.5–85.0)	106.5(78.3–186.0)
		P=.013
ADELT (%MMT)	32.5(20.0–65.5)	38.0⁎(16.0–44.0)	76.0(55.3–154.3)
		P=.003
SUPRA (%MMT)	38.5(18.8–74.5)	58.0⁎(19.0–71.5)	78.0(46.5–114.8)
		P=.005
INFRA (%MMT)	30.0(0.0–52.5)	34.0⁎(9.8–69.0)	62.5(40.5–84.5)
		P=.005

Abbreviations: ADELT, anterior deltoid; Fast, fast propulsion; Free, free propulsion; INFRA, infraspinatus; PECMAJ, pectoralis major; SUPRA, supraspinatus.

Table 9. Electromyographic Activity Duration (Interquartile Range, 25%–75%)

	Self-Selected Free Standard	Self-Selected Free PAPAW	Self-Selected Fast Standard
PECMAJ (% push cycle)	44(37–57)	34⁎(21–56)	54(46–67)
		P=.006
ADELT (% push cycle)	28(17–37)	24⁎(11–31)	41(35–50)
		P=.001
SUPRA (% push cycle)	49(11–61)	35⁎(11–60)	50(36–57)
INFRA (% push cycle)	30(0–47)	29⁎(6–49)	46(30–63)
		P=.004

Abbreviations: ADELT, anterior deltoid; Fast, fast propulsion; Free, free propulsion; INFRA, infraspinatus; PECMAJ, pectoralis major; SUPRA, supraspinatus.

Shoulder Isometric Torque 

Shoulder scaption strength, mean ± SD (n=13), was 3.9±1.4kg·m. Shoulder external rotation torque, mean ± SD (n=13), was 2.9±0.9kg·m (see table 1).

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Discussion 

Contrary to our initial hypothesis, we found that during free PAPAW propulsion at a velocity similar to that initially self-selected in the standard pushrim WC, cycle length and cadence were not significantly different in the PAPAW in subjects with tetraplegia. During high-intensity graded PAPAW propulsion, however, despite attempts to match the speed initially self-selected during standard pushrim graded propulsion, subjects pushed at a velocity 25% greater than the standard pushrim WC. Their increased velocity in the PAPAW was accomplished with a significantly increased cycle length but a similar cadence. In terms of muscular activity and consistent with our initial hypothesis, PAPAW propulsion during low-intensity (free) propulsion at a similar speed resulted in decreased push phase shoulder muscle activity compared with the standard pushrim WC. Specifically, PAPAW substantially reduced electromyographic activity of the power muscles, anterior deltoid and pectoralis major, by 66% to 100%, and decreased peak intensity of 1 of the 2 rotator cuff muscles (infraspinatus). While we did not find any significant difference in supraspinatus intensity or duration during matched free PAPAW propulsion, we did observe an artificially prolonged recovery phase of the push cycle resulting from efforts to decrease cadence to match the speed initially self-selected in the standard pushrim WC. Thus, the additional trial of self-selected free PAPAW propulsion was added for subsequent subjects. During the higher intensity matched conditions (fast, graded), PAPAW substantially reduced activity of all muscles tested by 40% to 100%, in spite of the increased velocity achieved during graded PAPAW propulsion.

Consistent with our secondary hypothesis, subjects self-selected a substantially faster free PAPAW velocity than that selected in the standard pushrim WC and accomplished this increased velocity with a similar cadence but a greater cycle length. Further, there was no difference between push phase muscle activity during self-selected free PAPAW and standard pushrim free WC propulsion, in spite of achieving a free PAPAW velocity that was twice as great. In fact, velocity during this self-selected free PAPAW trial was similar to that selected during fast standard pushrim WC propulsion, and yet shoulder muscle electromyographic activity in the PAPAW was reduced from 26% to 59% for all muscles except supraspinatus.

Given these findings, PAPAW appears to maximize the effectiveness of the push effort during manual WC propulsion at similar velocities by reducing the activity of the power muscles during propulsion. The reduced power muscle activity (pectoralis major, anterior deltoid) during matched free PAPAW propulsion was consistent with that seen in the self-selected free PAPAW and fast standard pushrim comparison, with an additional reduction in infraspinatus activity as well as increased push distance and decreased cadence demonstrated during self-selected free PAPAW propulsion.

This study additionally demonstrated that PAPAW propulsion under higher intensity conditions (self-selected free and graded) allowed those with tetraplegia to achieve greater propulsion velocities by increasing the distance traveled a cycle. PAPAW self-selected free velocity was twice that selected during standard pushrim free propulsion without any increase in muscular effort required. This finding is also consistent with our previous report documenting increased self-selected PAPAW velocity with reduced energy cost during prolonged free propulsion compared with a personal manual WC.¹⁹ Further, supraspinatus peak activity was significantly less during self-selected free PAPAW propulsion than standard pushrim fast propulsion, and median intensity approached a significant reduction. Given these findings, we suggest that subjects in the laboratory had increased supraspinatus activity associated with a prolonged recovery phase in attempt to match their slower free velocity self-selected initially in their standard pushrim WC. Outside of the laboratory, however, subjects would likely select a faster PAPAW free velocity, reducing the amount of time spent pushing in the PAPAW to arrive at a particular destination, thus decreasing the demand on supraspinatus compared with a standard pushrim WC.

The increased velocity provided by PAPAW is clinically substantial, given that the median standard pushrim self-selected free propulsion velocity in this study was 55.6m/min and yet normative walking velocity is about 80m/min. PAPAW could allow persons with tetraplegia to propel themselves at normative free speeds without an increase in effort from the primary push phase muscles studied here. During higher-intensity propulsion, such as up inclines or on uneven surfaces, PAPAW could allow persons similar to our subjects to push at a similar or slightly increased velocity while reducing the muscular effort required from the 4 push phase muscles studied here.

Despite the high demands of standard pushrim manual WC propulsion, well documented in recent literature, and the increased prevalence of shoulder pain and pathology in the spinal cord injury population,²⁹ the reluctance of clients, unable to tolerate these demands, to transition to power wheelchair propulsion is understandable, given the associated disadvantages. Recent findings delineating the reduced demands of PAPAW propulsion versus the standard pushrim manual WC, in terms of heart rate,³⁰ energy cost,¹⁹ upper-limb joint range of motion,²¹ and surface electromyographic activity²⁰ certainly provide a compelling case for preserving manual propulsion with transition to PAPAW mobility. Our current findings are consistent with the benefits of PAPAW propulsion previously reported, but supplement the electromyographic activity evidence in 2 ways. First, we provide an analysis of different components of muscular activity. In order to assess the maximum power output necessary to sustain propulsion, we included comparison of peak electromyographic activity intensity, whereas muscle endurance measures are delineated more from evaluation of median electromyographic activity intensity as well as duration of electromyographic activity over the push cycle. The combination of intensity and duration of muscle activity determines the overall muscular demand during prolonged, cyclic activities. We additionally documented a reduction in activity specific to the anterior portion of the deltoid, as well as the deep rotator cuff muscles, supraspinatus and infraspinatus, during high-demand conditions.

The rotator cuff muscles are crucial to maintaining normative shoulder mechanics by counterbalancing the shear force of the power muscles, anterior deltoid and pectoralis major, during WC propulsion.²⁶ Our electromyographic activity findings documenting reduced electromyographic activity of pectoralis major during low-intensity (matched level free) and high-intensity conditions (matched fast and graded) and decreased deltoid activity during graded PAPAW propulsion are consistent with those reported by Levy et al.²⁰ In contrast with Levy et al,²⁰ however, our group additionally documented significantly decreased anterior deltoid activity during matched level free and fast PAPAW propulsion. These findings may be attributed to the fact that we used indwelling wire electrodes, recording the muscle activity of the anterior portion of the deltoid, whereas Levy²⁰ recorded from surface electrodes placed on the anteriomedial portion of the deltoid, potentially resulting in cross-talk from PECMAJ and the medial head of the deltoid.

Study Limitations 

While important information was gained from this investigation, several limitations could be addressed in future work. First, our study examined a sample of convenience. Thus, while women were not intentionally excluded, only men with complete tetraplegia volunteered. Further research is needed to explore the applicability of these findings to female subjects with similar injuries.

Another limitation of this study was the fact that, because of the small sample size, we opted to combine findings from those with C6 and C7 tetraplegia into a single analysis. However, we did not expect a particularly different response to PAPAW between the 2 groups, with the exception that persons with C6 tetraplegia may have demonstrated a more substantial reduction in electromyographic activity than those with C7 tetraplegia. Future research to determine whether those with C6 tetraplegia would benefit more from PAPAW use would be beneficial.

Next, the participants in the current investigation may have been weaker than the average individual with C6 or C7 tetraplegia. The shoulder muscle strength of our subjects, as assessed by isometric torque testing of scaption and external rotation, was mildly reduced compared with the subjects with tetraplegia tested by of Powers et al³¹ (scaption, 79%; external rotation, 88%). This reduction in strength could be associated with the fact that the mean age of subjects in the current study was approximately 10 years greater and duration of injury was more than twice that of the subjects tested by Powers et al.³¹ Future investigations including younger participants with shorter durations of injury would be informative.

The current investigation attempted to standardize the position of the shoulder joint relative to the rear wheel between subjects propelling the standard pushrim WC, such that personal preference in WC set-up would not affect response to standard pushrim WC propulsion. The disadvantage of this is that the standard pushrim WC rear wheel position may have varied from the subjects' usual set-up. Next, the rear wheel axle of the PAPAW was not adjustable, and the PAPAW available for the current investigation had a seat that was 18in wide, whereas for propulsion in the standard pushrim WC, a seat width of 16 or 18in was selected based on the size of the subject. For subjects with more narrow builds, the wider seat in the PAPAW, compared with the 16-in-wide standard pushrim WC seat, may have required increased shoulder abduction during PAPAW propulsion. This may have contributed to the similar supraspinatus activity documented during PAPAW propulsion for those subjects. Further, exploration of shoulder kinematics to determine whether increased shoulder abduction occurs during matched free PAPAW propulsion compared with the standard pushrim free propulsion would be beneficial. Verbal feedback to avoid abducting the shoulder during recovery at slower PAPAW propulsion speeds might lead to reduced supraspinatus activity. Future studies exploring propulsion of a more adjustable PAPAW would assist in clarifying some of these issues.

Next, the low end of our electromyographic band pass filter at 150Hz is uncommon and differs from the recommended standards. We selected this filter to eliminate cross-talk or signal from other nearby muscles. Signal from other muscles is filtered as it passes through layers of tissues. Consequently, it has a lower frequency than that of the inserted muscle. With intramuscular electrodes, the signal from the inserted muscle does not pass through tissues prior to recording. Consequently, it has a higher frequency than either surface electromyographic activity or activity from other muscles.³² By using a higher cut-off frequency, we ensured that the signal recorded came only from the muscle of interest. Despite the fact that electromyographic activity recorded with an intramuscular electrode has a higher frequency than surface electromyograpic activity, some of the lower-frequency signal that was eliminated below 150Hz in our data was likely from the muscle of interest. To counter this, the raw signal was normalized by a maximal effort contraction trial that was also recorded with the 150-Hz filter, resulting in a relative effort (% of maximum) that included only activity from the intended muscle above 150Hz.³³ The 150-Hz filter was applied uniformly to all electromyographic activity in our study. Consequently, the impact of our filtering on the results, except to eliminate activity from other muscles, would be minimal. It is possible, however, that our filter may have slightly reduced the duration of muscle activity recorded, but that would be consistent across both wheelchair propulsion conditions.

Finally, while the muscles explored in this study are certainly key muscles providing the propulsive force, future work should explore changes in activity of additional push phase muscles, such as subscapularis and latissimus dorsi, during PAPAW propulsion. Additionally, given that propulsion was performed in the laboratory on an ergometer, the results may not be completely applicable to community PAPAW propulsion.

Practically, the difficulty of subjects in matching their fast and graded PAPAW propulsion velocity to that initially self-selected in the standard pushrim WC suggests that in a real-world situation, subjects would have selected a propulsion speed that was comfortable for the PAPAW. In fact, when subjects were allowed an additional PAPAW propulsion trial at a self-selected free speed, they propelled at a velocity that was twice their self-selected standard pushrim free WC velocity. Additionally, the fact that the PAPAW was a novel WC, as opposed to participants being long-time standard pushrim manual WC users, would further explain limitations in matching velocity in the PAPAW to the standard pushrim chair. A longer accommodation period to use the PAPAW would aid in teasing these factors out. Clinicians and researchers also need to assess whether persons who are able to disassemble their standard pushrim manual WC and load it into a car independently could physically do the same with a PAPAW (in spite of the increased weight of the wheels added by the motor) without compromising their shoulders. For instance, the heaviest portion of the PAPAW model studied here is the frame, weighing 10kg, while each wheel weighs 6.8kg. Finally, the implications of extra wheel width added by the motors on functional mobility and the consequence of loss of battery power during community mobility need to be explored.

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Conclusions 

Our study suggests that during low-intensity propulsion (level, free), substitution of a PAPAW can decrease key shoulder muscular demands by at least 50% while propelling at a velocity similar to standard pushrim manual WC propulsion for persons with C6 and C7 tetraplegia. Conversely, use of a PAPAW can allow them to increase their velocity to up to twice that of a standard pushrim manual WC, allowing them to arrive at their destination more quickly, without increasing key shoulder muscular demands. These benefits of PAPAW propulsion appear to be enhanced under higher intensity propulsion conditions. Substituting a PAPAW for a standard pushrim manual WC has the potential to preserve shoulder joint function for persons with SCI or other disabilities while maintaining many of the benefits of manual propulsion.

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• a Sunrise Medical, 7477 E Dry Creek Pkwy, Longmont, CO 80503.
• b Indwelling wire electrodes; California Fine Wire Co, 338 S Fourth St, Grover Beach, CA 93433-0199.
• c PDP-11/73; Digital Equipment Corp, 111 Powdermill Rd, Maynard, MA 01754-1499.
• d Oxford Metrics LTD, 14 Minns Estate, West Way, Oxford, OX2 OJB, United Kingdom.
• e Loredan Biomedical Inc, 1632 Da Vinci Court, PO Box 1154, Davis, CA 95617.
• f C-Motion Inc, 20030 Century Blvd, Ste 104A, Germantown, MD 20874.
• g SPSS Inc, 233 S Wacker Dr, 11th Fl, Chicago, IL 60606.