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The Journal of Spinal Cord Medicine logoLink to The Journal of Spinal Cord Medicine
. 2019 Mar 18;43(4):476–484. doi: 10.1080/10790268.2019.1582603

Start-up propulsion biomechanics changes with fatiguing activity in persons with spinal cord injury

Fransiska M Bossuyt 1,2,✉, Nathan S Hogaboom 3,4, Lynn A Worobey 3,5, Alicia M Koontz 3,4, Ursina Arnet 1,2, Michael L Boninger 3,4,5
PMCID: PMC7480480  PMID: 30882284

Abstract

Objective: Shoulder pathology is a common condition in wheelchair users that can considerably impact quality of life. Shoulder muscles are prone to fatigue, but it is unclear how fatigue affects start-up propulsion biomechanics. This study determines acute changes in start-up wheelchair propulsion biomechanics at the end of a fatiguing propulsion protocol.

Design: Quasi-experimental one-group pretest-postest design.

Setting: Biomechanics laboratory. Participants: Twenty-six wheelchair users with spinal cord injury (age: 35.5 ± 9.8 years, sex: 73% males and 73% with a paraplegia).

Interventions: Protocol of 15 min including maximum voluntary propulsion, right- and left turns, full stops, start-up propulsion, and rests.

Outcome measures: Maximum resultant force, maximum rate of rise of applied force, mean velocity, mean fraction of effective force, and mean contact time at the beginning and end of the protocol during start-up propulsion.

Results: There was a significant reduction in maximum resultant force (P < 0.001) and mean velocity (P < 0.001) at the end of the protocol. Also, contact time was reduced in the first stroke of start-up propulsion (P < 0.001). Finally, propelling with a shorter contact time was associated with a greater reduction in performance (maximum velocity) at the end of the protocol.

Conclusion: There are clear changes in overground propulsion biomechanics at the end of a fatiguing propulsion protocol. While reduced forces could protect the shoulder, these reduced forces come with shorter contact times and lower velocity. Investigating changes in start-up propulsion biomechanics with fatigue could provide insight into injury risk.

Keywords: Fatigue, Spinal cord injuries, Shoulder, Biomechanical phenomena, Wheelchairs

Introduction

Shoulder pain is prevalent in up to 69% of wheelchair users with spinal cord injury (SCI) and has been suggested to result from musculoskeletal pathology.1–3 Several factors have been related to shoulder pain including: sex,3,4 age,5 weight,6 lesion level,7 the wheelchair and its setup,1 and wheelchair propulsion technique.8 The detrimental impact of shoulder pain on a person’s mobility and quality of life has led to prevention protocols focusing on factors modifiable by training.9 As an example, an intervention program including 7 weeks of wheelchair training (3 times 70 min at 30% of the heart rate reserve per week) based on the Paralyzed Veterans of America’s Clinical Practice Guidelines for Preservation of Upper Limb Function resulted in improved wheelchair propulsion biomechanics.10 Furthermore, a 12-week home-based program including shoulder strengthening and stretching exercises as well as recommendations on wheelchair transfers, raises, and propulsion technique, reduced shoulder pain in persons with SCI.11

Increased superior forces and internal rotation moments at the shoulder during wheelchair propulsion, resulting from increased forces applied to the push rim, have been related to physical and radiological markers for shoulder pathology.8 Wrist pathology has also been related to increased pushrim forces which in turn was related to increased rate of rise of applied force (ROR) and increased propulsion velocity.6 Furthermore, propelling with a high stroke frequency and therefore short contact time was related to wrist pathology.12 This is in line with results of a wheelchair training program demonstrating improved mechanical efficiency accompanied with an increased push time and reduced push frequency.10 A simulation study also found that altering the propulsion biomechanics by minimizing push frequency and therefore prolonging push time had potential in reducing muscle demand and fatigue.13 From a mechanical point of view, propelling with a greater amount of force directed tangentially to the pushrim and therefore with a greater fraction of effective force (FEF) is assumed to be more efficient.14

The repetitive demands from wheelchair propulsion impact the shoulder musculature, which is susceptible to fatigue that could negatively affect movement patterns and increase the risk of shoulder injuries and pain.15,16 The higher loads on the shoulder17,18 and different propulsion styles19 found during start-up wheelchair propulsion, as compared to steady state propulsion, support that start-up propulsion is likely the most demanding subtask from a biomechanical perspective (i.e. inertial effect); when propelling on a level surface, this task may be the first one to be impacted by fatigue. Furthermore, the amount of starts and stops found during daily life of wheelchair users ranged, depending on the lesion level, from 89.2 to 158.5 per 1000 m.20 This demonstrates the value of investigating start-up propulsion with fatigue and the relation to injury development. However, previous experimental studies have mainly investigated propulsion biomechanics during steady state level-ground propulsion on a wheelchair ergometer15,16,21–23 or treadmill.24,25 While it seems likely that at times wheelchair users propel while fatigued, it should be noted that there is no evidence on the frequency with which this occurs.

There is no understanding of how fatiguing overground wheelchair propulsion affects start-up propulsion biomechanics. Therefore, this study investigates how wheelchair users’ start-up propulsion biomechanics changes at the end of a fatiguing overground propulsion protocol. First, we hypothesized that at the end of the propulsion protocol, there will be adverse acute changes towards more straining propulsion biomechanics during the start-up phase of propulsion (i.e. increased applied forces, increased ROR, reduced FEF, and reduced contact time). Secondly, we hypothesized that subject’s body weight and subject’s propulsion biomechanics (i.e. contact time and percentage difference in applied force at the end of the protocol) will be related to the development of fatigue (i.e. percentage difference in propulsion velocity at the end of the protocol).

Methods

Study design and study population

This study with a quasi-experimental, one-group, pretest-posttest design recruited persons with SCI by distributing flyers and word-of-mouth advertisement. All participants met the following inclusion criteria: (1) age between 18 and 65 years old, (2) more than one year since initial SCI, (3) SCI occurring after the age of 15 to reduce the effect of different adaptations to the injury in the young adolescent musculoskeletal system and exclude congenital injuries, (4) neurological impairment secondary to SCI, (5) use a manual wheelchair as primary means of mobility (≥ 40 h per week), and (6) English speaking. Participants were excluded if they: (1) had a history of fractures or dislocations in the shoulder, elbow, and/or wrist that had not fully recovered, (2) had upper limb dysthetic pain as a result of a syrinx or complex regional pain syndrome, (3) were pregnant, (4) did not have a quick-release axle, or (5) had a history of cardiovascular or cardiopulmonary disease. Prior to data collection, ethical approval was obtained from the Institutional Review Board and all participants read and signed the informed consent. Sample-size estimation was based on a study investigating changes in maximum resultant force (FRmax) at a moderate vs high intensity of wheelchair propulsion because this may resemble magnitudes of changes with fatigue and resulted in the requirement of 28 participants to observe a 16.8 N difference in FRmax at 80% statistical power and alpha = 0.05.26

Data collection

Participants were invited for one testing session at the Human Engineering Research Laboratories (VA Pittsburgh HealthCare System, Pittsburgh, PA, USA). Demographic variables (age, sex, time since injury (TSI), and lesion level) were collected via a questionnaire. Weight was collected with a wheelchair scale by subtracting the weight of the wheelchair from the total weight. Subsequently, all participants propelled on a cement floor at self-selected speed and completed a figure 8 protocol (15 min) that has been described previously (Figure 1).27 This protocol was selected based on the included tasks as the effect of fatigue has been found to be task dependent and for comparability reasons.28 To complete the protocol, participants were asked to propel on a cement floor for as many laps as possible for three, four-minute intervals with 90 s of rest between each interval. One lap consisted of a right and left turn with a complete stop after each half-lap. The provided rest-periods increased the likelihood that all participants would be able to complete the entire protocol.

Figure 1.

Figure 1

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Course of figure 8 fatigue protocol. Arrows indicate direction of travel. Triangles are located at turning points. At the stop sign participants came to a complete stop and immediately continued to the next cone.

Prior to the start of the protocol, a SmartWheel (Three Rivers Holdings, Inc., Mesa, AZ, USA) was attached to the non-dominant side of the participant’s personal wheelchair, opposite an identical wheel without force transducers. The non-dominant arm was chosen to control for handedness. The SmartWheel measures forces and moments in the three global reference planes (Fx, Fy, Fz, Mx, My, Mz). Biomechanical data were collected during the first and last minute of each interval of the protocol. No familiarization period was included as the task involves normal activity for experienced wheelchair users. Furthermore, the Rating of Perceived Exertion (RPE) scale ranging from no perceived exertion (6), light exertion (11), hard exertion (15), up to maximal exertion (20), was collected after propulsion at a self-selected speed on a cement floor and at the end of the protocol.29

Data analysis

Pushrim forces and moments were calculated using a custom Matlab program (MathWorks, Inc. Natick, MA, USA). Raw data were collected at 240 Hz, filtered through a 4th order Butterworth filter with cutoff frequency at 10 Hz, and corrected for the camber of the wheelchair.30 The push phase was started when a positive torque around the wheel axle was found provided the torque became greater than 2 Nm. Push phases were selected automatically and subsequently confirmed with visual inspection. The present analysis only includes data from the first minute of the first interval and the last minute of the last interval. To investigate fatigue, the first four strokes of each of the first three complete course laps (beginning of 15-minute interval) and the first four strokes of each of the last three complete course laps (end of 15-minute interval) were analyzed. We collected the first four strokes because this represents the start-up phase of propulsion and includes all strokes before propulsion at steady state.17 The distance traveled within the analyzed four strokes of all participants was a maximum of 6.3 m, confirming that all participants were still propelling straight and had not started turning. Fatigue was defined based on a previous conceptualization as “a disabling symptom in which physical and cognitive function is limited by the rate of change in performance fatigability and perceived fatigability”.31 Performance fatigability (i.e. decline in an objective measure of performance) was quantified as a reduction in maximum velocity (i.e. maximum mean velocity (distance / contact time) of the first four pushes of start-up propulsion) at the end of the protocol. Perceived fatigability (i.e. change in sensation that regulates the integrity of the performer) was quantified as an increase in RPE at the end of the figure 8 protocol as compared to overground propulsion at a self-selected speed. The resultant force (FR) was calculated as the vector sum of all forces applied to the pushrim (i.e. Fx, Fy, and Fz). The radial force (Frad – directed to the hub) and propulsive or tangential force (Ftan – force perpendicular to Frad and coplanar with the pushrim) were calculated as described previously.32 From these variables, FRmax, maximum Frad (Fradmax) and maximum Ftan (Ftanmax) were calculated. Furthermore, maximum ROR was determined from instantaneous slopes (RORmax; i.e. maximum (FR / contact time)), mean FEF (FEFmean; i.e. mean (Ftan2 / FR2)), mean velocity, and mean contact time were defined accordingly. Finally, for all variables, the average of the first, second, third and fourth strokes across the three beginning and end laps were calculated for each participant.

Statistical analysis

Two-way within-subjects repeated measures analysis of variance was used to assess the effect of time (2 levels: beginning and end), stroke (4 levels: stroke 1 – first stroke after complete stop, stroke 2, stroke 3, and stroke 4) and the interaction effect on the dependent variables FRmax, Fradmax, Ftanmax, RORmax, FEFmean, mean contact time, and mean velocity during the figure 8 protocol. Planned contrasts with Bonferroni corrections were used to do post hoc testing across strokes. Due to a significant reduction in velocity at the end of the protocol and to take into account participants capacity quantified with propulsion velocity, an ANCOVA analysis controlling for the velocity at each time point and stroke was run; only findings when controlling for velocity were reported. Huynh-Feldt corrected P values were used when Greenhouse-Geisser epsilon ≥ 0.75, and Greenhouse-Geisser corrected P values were used if Greenhouse-Geisser epsilon ≤ 0.75. Effect sizes (Eta-squared (η2), η2 > 0.2: small effect, η2 > 0.5: medium effect, η2 > 0.8: large effect33) and confidence intervals (95% CI) were reported. Finally, a multiple linear regression analysis was performed to determine how much of the percentage difference in maximum velocity between the beginning and end (dependent variable) could be explained by body weight, beginning contact time at stroke four and the percentage difference in FRmax at stroke four between the beginning and end. All statistical analyses were performed with Stata version 13 (StataCorp LP, College Station, TX). Relationships with an α = 0.05 were considered statistically significant.

Results

Demographics

Thirty-one participants were recruited, and data from 26 participants were investigated (age: 35.5 (9.8) years, weight: 76.6 (8.2) kg, TSI: 12.0 (8.2) years, lesion level: 7 tetraplegia (lesion level between C5 and C8) and 19 paraplegia (lesion level between T1 and T6 (n = 8), lesion level between T7 and T12 (n = 7), lesion level below L1 (n = 3), exact lesion level of one participant with a paraplegia was missing), sex: 19 males and 7 females, ethnicity: 21 Caucasians, 4 African Americans and 1 other). Two participants did not meet inclusion criteria: One person did not have a neurological impairment secondary to SCI and the other person had a fracture that had not fully recovered. Three individuals were excluded due to technical problems with the SmartWheel.

Figure 8 protocol

During the figure 8 protocol, participants performed on average 8 (2) laps per interval (the amount of laps did not change between intervals). As expected, RPE was higher when measured at the end of the figure 8 protocol as compared to the RPE reported after propulsion at a self-selected speed on a cement floor (resp. 15 (3) and 7 (2), t = 12.05, P < 0.001).

Propulsion velocity and contact time

Mean velocity was significantly reduced in the second, third and fourth strokes of start-up propulsion of the final figure 8 interval compared with the first figure 8 interval (P < 0.001, η2 = 0.56: medium effect, 95% CI = 0.39–0.65; Figure 2, Table 1). A significantly reduced mean contact time during the first stroke of start-up propulsion was found in the final figure 8 interval compared to the first (P < 0.01, η2 = 0.29: small effect, 95% CI = 0.11–0.41). From the first up to the fourth stroke of start-up propulsion, mean velocity increased significantly (P < 0.001, η2 = 0.88: large effect, 95% CI = 0.82–0.90) and mean contact time reduced significantly (P < 0.001, η2 = 0.47: small effect, 95% CI = 0.29–0.58). No other differences were observed (Figure 2, Table 1).

Figure 2.

Figure 2

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Mean (circle or diamond) and standard deviation (vertical line) of the mean velocity, mean contact time, maximum resultant force, and maximum rate of rise of applied force (ROR) for the first four strokes of start-up propulsion at the beginning (blue diamond) and at the end (red square) of the figure 8 protocol. § denotes a significant effect of time at the given stroke after post hoc testing with Bonferroni corrections (α = 0.05). # denotes a significant difference between assigned strokes and all other strokes after post hoc testing with Bonferroni corrections. If # is accompanied with a number, the assigned stroke is only significant to the stroke of the accompanied number.

Table 1. Mean velocity, mean contact time, maximum resultant force (FRmax), maximum tangential force (Ftanmax), maximum radial force (Fradmax), maximum rate of rise of applied force (RORmax), and mean fraction of effective force (FEFmean) at the beginning and at the end of the figure 8 protocol for each of the four strokes.

  Beginning figure 8 End figure 8 Time Stroke Time × Stroke
  1 2 3 4 1 2 3 4 F η [CI] F η [CI] F η [CI]
Velocity (m/s) 1.1 (0.3) 1.5 (0.3) 1.7 (0.4) 1.8 (0.4) 1.1 (0.3) 1.4 (0.3) 1.5 (0.3) 1.6 (0.3) 2.9 0.10 [0–0.34] 180.6§ 0.88 [0.82–0.90] 31.4§ 0.56 [0.39–0.65]
Contact time (s) 0.77 (0.15) 0.43 (0.10) 0.35 (0.09) 0.32 (0.07) 0.68 (0.22) 0.45 (0.13) 0.38 (0.08) 0.35 (0.09) 1.1 0.04 [0–0.25] 22.3§ 0.47 [0.29–0.58] 9.9† 0.29 [0.11–0.41]
FRmax (N) 126.6 (34.3) 126.3 (31.1) 119.7 (33.0) 110.9 (32.4) 117.4 (44.4) 112.2 (32.2) 97.5 (31.9) 94.0 (29.1) 8.1† 0.24 [0.02–0.47] 2.3 0.08 [0–0.19] 1.0 0.04 [0–0.12]
Ftanmax (N) 92.4 (33.9) 89.9 (29.3) 83.3 (30.0) 77.4 (29.7) 81.6 (32.5) 72.2 (28.0) 62.3 (24.6) 58.2 (23.6) 12.8† 0.34 [0.06–0.55] 2.3 0.08 [0–0.19] 1.1 0.04 [0–0.13]
Fradmax (N) 83.3 (30.7) 89.7 (24.3) 88.6 (23.1) 81.1 (22.7) 85.9 (37.7) 85.1 (24.7) 75.8 (23.7) 72.6 (22.7) 0.1 0.00 [0–0.14] 1.9 0.07 [0–0.17] 0.9 0.03 [0–0.11]
RORmax 1.16 (0.67) 1.92 (0.80) 2.03 (1.01) 1.77 (0.93) 1.29 (0.84) 1.76 (0.76) 1.53 (0.62) 1.55 (0.58) 1.1 0.04 [0–0.25] 6.6† 0.21 [0.05–0.34] 3.1* 0.11 [0–0.23]
FEFmean 0.60 (0.30) 0.46 (0.19) 0.43 (0.18) 0.41 (0.18) 0.62 (0.33) 0.43 (0.19) 0.38 (0.16) 0.35 (0.15) 2.8 0.10 [0–0.33] 1.3 0.05 [0–0.14] 2.22 0.08 [0–0.19]
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Note: All data are means (standard deviations). F of the main effect of time, main effect of stroke and the interaction effect of time and stroke were presented when controlling for velocity.* represents P < 0.05, † represents P < 0.01 and § represents P < 0.001. η represents Eta-squared with the 95% confidence interval (CI). Significant differences between strokes after post hoc testing are not presented in this table but can be found in Figure 2 (α = 0.05).

Forces applied on the pushrim

FRmax was significantly reduced at the first three strokes of start-up propulsion in the final interval compared to the first interval (P < 0.05, η2 = 0.24: small effect, 95% CI = 0.02–0.47). Ftan was significantly reduced at each stroke of start-up propulsion in the final interval compared to the first interval (P < 0.05, η2 = 0.34: small effect, 95% CI = 0.06-0.55) (Figure 2, Table 1). No significant differences were found in FEFmean. There was a significant reduction in RORmax in the third stroke of the final interval compared with the first interval (P < 0.05, η2 = 0.11, 95% CI = 0 - 0.23) (Figure 2, Table 1). No other significant differences were observed (Figure 2, Table 1).

Regression model to predict the effect of fatigue

The percentage difference in FRmax at stroke four between the beginning and end, and contact time at stroke four in the beginning accounted for 32% of the variation in percentage difference in maximum velocity between the beginning and end (F = 3.5, P = 0.033, Table 2). No significant association was found with body weight.

Table 2. Multivariable linear regression analyses to predict percentage difference in maximum velocity at the fourth stroke between the beginning and end of the figure 8 protocol.

  Unstandardized coefficients Standardized coefficients 95% CI t P
B SE β Lower Upper
Constant 34.63 13.59   6.44 62.81 2.55 0.02
Contact time of 4th stroke at the beginning −72.70 32.18 −0.40 −139.43 −5.97 −2.26 0.03
Difference FRmax stroke 4 beginning and end (%) 0.33 0.15 −0.17 0.03 0.64 2.25 0.04
Body weight −0.09 0.10 0.40 −0.29 0.11 −0.92 0.37
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Note: B (Beta) and SE (standard error) represent the unstandardized coefficients, β (Beta) the standardized coefficient, CI the confidence interval, t the associated t-score and P the significance level (α = 0.05).

Discussion

This study provides insight into changes in start-up, overground propulsion biomechanics with fatiguing wheelchair propulsion. Overall, the figure 8 protocol induced fatigue by the rate of change in both performance fatigability (the significant reduction in velocity at the end of the protocol) and perceived fatigability (significant increased RPE of 15 (3), corresponding to a sensation of ‘hard’, at the end of the protocol). Based on a study that collected the RPE scale in wheelchair users during a graded exercise test until volitional exhaustion (VO2peak), an RPE of 13 corresponded to 80% VO2peak.34 The amount of laps did not change between the beginning and end interval. Because the lap count did not measure partial laps completed and velocity was measured precisely and only during start up, velocity, can change in the absence of a change in lap count. The changes with fatiguing wheelchair propulsion partly confirmed our first hypothesis as we only found a significant reduction in contact time during the first stroke of start-up propulsion at the end of the protocol. In contrast with our hypothesis, however, persons propelled with a significantly reduced FRmax, reduced Ftanmax, and no change in FEFmean at the end of the protocol when controlling for velocity at each time point and for each stroke. The second hypothesis was partly confirmed as a greater percentage difference in FRmax with fatiguing propulsion and a shorter contact time in the beginning were significant predictors for a greater reduction in maximum velocity at the end of the protocol. This suggests a greater susceptibility to fatigue in participants who propel with a shorter contact time at the beginning of the protocol.

The significant reduction in FRmax at the end of the protocol differs from previous findings describing either an increase or no difference in hand rim forces with fatigue.16,23 However, the latter studies induced fatigue on a stationary, instrumented wheelchair positioned on a roller system with a graded tolerance test until maximum exhaustion. As these studies constrained the propulsion velocity (3 km/h) and investigated steady state propulsion, there was no room for the observation of natural changes in propulsion biomechanics. The changes in FRmax point towards less strain on the shoulder at the end of the protocol. It has been described that fatigue is not a result of one mechanism but rather a combination of a variety of mechanisms.31 It was also highlighted that underlying mechanisms and effects of fatigue are task dependent and vary with the induced muscle contractions.28 This supports the investigation of different types of wheelchair propulsion (including steady state and start-up) to provide insights into the development of injury risk.

An unexpected finding was that FEFmean did not change with straining wheelchair propulsion due to a reduction in both FRmax and Ftanmax. However, a simulation study described a relationship between reduced upper extremity demands and an optimal FEFmean rather than a maximum FEFmean.35 More specifically, an optimal FEFmean maximized mechanical efficiency and subsequently minimized overall upper extremity demands on the shoulder. In this study, it could be assumed that persons maintained the optimal FEFmean by reducing FRmax and Ftanmax at the end of the protocol rather than redistributed the available force more or less towards Ftanmax. It is important to bear in mind that this study did not investigate steady state propulsion and that there can be different changes in FEFmean with fatigue during start-up or steady state propulsion.

The contact time of the first stroke commencing wheelchair propulsion detrimentally changed at the end of the protocol, whereas there was no change in the following strokes. More specifically, the first stroke showed a reduced contact time when controlling for velocity. It is important to note that the effect size was small and results should be interpreted with caution. An intervention study investigating steady state propulsion found that the increased contact time and decreased push frequency as a result of training, improved mechanical efficiency and metabolic cost of propulsion.10 In the second and third stroke, with fatiguing propulsion, persons produced less force and propelled slower but spent the same amount of time on the pushrim and propelled with the same RORmax at the second stroke as RORmax was only reduced at the third stroke and the effect size was very small (resp. 0.11). This suggests less efficient and non-optimal propulsion mechanics.

The reported findings may have implications for wheelchair skills training programs and the target population. Besides strengthening the capacity of the wheelchair user (muscle strength36 and aerobic capacity37) and adjusting the wheelchair to the user,38 the ability to maintain optimal propulsion mechanics with fatigue may play a role in preserving mobility. With practice, time to failure of submaximal contractions of the elbow flexor muscles (e.g. the biceps brachii muscle, active during the push phase of wheelchair propulsion) was increased by ∼ 60%.39 This was due to reflex inhibition in motor units of the m. biceps brachii explained by plasticity of the nervous system in response to physical activity. Also in persons with SCI, exercise affected reflex responses in the m. biceps brachii. Caution is needed as practice-related adaptations in the nervous system are task specific. Future research should further investigate subject and injury characteristics associated with increased susceptibility to fatigue, specific training methods that prevent fatigue-induced changes in start-up propulsion, and their potential impact on injury development.

Several limitations need to be acknowledged. First, this study did not include objective measures to investigate the mechanisms of fatigue. Including electromyography could give an objective indication of neuromuscular fatigue as previous studies reported a decrease in the median power frequency and an increase in the root mean square of the electromyography signal with fatigue.28 Secondly, even though the figure 8 protocol includes several starts, stops, and turns, it does not mirror typical propulsion that occurs during daily mobility related activities which vary in length, velocity, direction and incline. Also, the protocol has not been validated to induce fatigue. Thirdly, due to a lack of measures of pathology, we do not know if the changes are adaptive and prevent injury, nor if they promote injury development. Fourthly, the small sample limited the number of predictors that could be included in the regression. Finally, the sample consisted of highly independent wheelchair users fulfilling specific inclusion criteria and thus may not be representative of the entire SCI population.

Conclusions

This study demonstrated changes in start-up propulsion biomechanics at the end of a physically challenging overground propulsion protocol. Fatiguing wheelchair propulsion induced a reduction in the applied forces on the pushrim during start-up, when controlling for propulsion velocity. Interestingly, there was a potentially detrimental change in contact time at the first stroke, with persons who propelled with a shorter contact time at the beginning of the fatiguing protocol displaying more changes in performance fatigability. A next step is to investigate underlying mechanisms in the changes found in this study by investigating muscular activation and upper extremity mechanics and how these changes affect markers for pathology in the long-term. These findings support intervention programs aiming at maintaining efficient start-up propulsion biomechanics in persons with SCI.

Disclaimer statements

Contributors MLB, NSH, LAW and AMK initiated the present study. NSH performed the data collection. All authors substantially contributed to conception and design. FMB performed statistical analyses. All authors interpreted the data. FMB drafted the first manuscript version. All authors critically revised the manuscript and gave final approval of the version to be published.

Conflicts of interest Authors have no conflict of interests to declare.

Funding Statement

This article is the result of work supported with resources and the use of facilities at the Human Engineering Research Laboratories, VA Pittsburgh Healthcare System. The contents of this paper do not represent the views of the Department of Veterans Affairs or the United States Government. Finally, the authors would like to thank the International Society of Biomechanics for their support with the International Travel grant (July 1st, 2016).

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