Is the figure-8 fatigue protocol a suitable tool to investigate shoulder muscle fatigue in wheelchair users with spinal cord injury?
Authors: Minder Ursina , Arnet Ursina , Müller Erich , Boninger Michael , Bossuyt Franziska 
Published: February 2023

Introduction
Shoulder pain is common in persons with spinal cord injury (SCI) and has been associated with wheelchair use. Previous studies identified fatigue related compensation strategies that could impact the development of shoulder injury and pain (Bossuyt et al., 2020). The aim of this study was to determine the presence of performance and perceived fatigability during a figure-8 fatigue protocol (F8F; Collinger et al., 2010) and thereby validate the protocols suitability to assess fatigue in wheelchair users with SCI.
Methods
Wheelchair users with paraplegia (N = 43, age = 50.2 ±10.1 years, body weight = 72.1 ±13.3 kg, time since injury = 27.1 ±11.6 years) completed a F8F introduced by Collinger et al. (2010). The F8F consists of 3 bouts of 4 minutes maximum voluntary propulsion including turns and full stops with 90 seconds of rest between the bouts thereby simulating wheelchair propulsion during daily life. The surface EMG signal of the biceps brachii, pectoralis major (pars sternalis), deltoideus pars acromialis, lower and upper trapezius of the non-dominant shoulder was recorded. To analyse performance fatigability, root mean square (RMS) (expected to increase with fatigue) of the five muscles was calculated with a short-time Fourier transformation. RMS means were normalised to the starting RMS values. In addition, heart rate (HR) was measured using a heart rate monitor. To analyse perceived fatigability, rate of perceived exertion (RPE) was captured with a 20-point Borg scale. A one-way repeated measure ANOVA was used to analyse changes in the dependent variables RMS, HR and RPE over time (α = 0.05). Correlation coefficients (Spearman- or Pearson-Test) between changes in HR, RPE and RMS of muscles with significant effects for time, were calculated.
Results and Discussion
RPE as well as HR and RMS of pectoralis major and upper trapezius showed the expected increases at the end of the demanding parts of the protocol. The changes in RMS demonstrate a shift in the power of the EMG signal indicating fatigue. The presence of fatigue in only a subset of the muscles may lead to muscular imbalances and over time induce injury and/or pain. Weak to moderate correlations were found between changes in HR and RPE as well as between changes in RMS of pectoralis major and RMS of upper trapezius. The correlation of changes in HR and RPE further demonstrates the association of more global measures of performance and perceived fatigability. Nevertheless, these global measures of fatiguability did not correlate with the changes in RMS, a local measure of performance fatiguability. The lack of association between global and local parameters demonstrates the importance of recording local parameters like RMS and suggests the integration of local RPE-scores for future research. Exercise programs targeting the trapezius and pectoralis major may have a role in preventing fatigue during wheelchair propulsion.
Conclusions
The detected changes in markers of performance and perceived fatigability support F8F as an adequate choice to evaluate the effect of wheelchair propulsion induced fatigue in wheelchair users with paraplegia. Furthermore, these findings support the value of investigating biomechanical changes during fatiguing wheelchair propulsion that resembles daily life activities, when the neuromuscular system is prone to adaptations, as it may give us further insights into the development of shoulder injury and pain.
References
Bossuyt, F. M., Arnet, U., Cools, A., Rigot, S., de Vries, W., Eriks-Hoogland, I., Boninger, M. L., & Group, S. S. (2020). Compensation strategies in response to fatiguing propulsion in wheelchair users: Implications for shoulder injury risk. American Journal of Physical Medicine & Rehabilitation, 99(2), 91-98. https://doi.org/10.1097/PHM.0000000000001267
Collinger, J. L., Impink, B. G., Ozawa, H., & Boninger, M. L. (2010). Effect of an intense wheelchair propulsion task on quantitative ultrasound of shoulder tendons. PM&R, 2(10), 920-925. https://doi.org/10.1016/j.pmrj.2010.06.007