THE IMPACT OF FATIGUE ON SHOULDER MOTOR CONTROL IN A HEALTHY POPULATION DURING A SUSTAINED ELEVATION TASK

F. Dupuis^1,2, G. Sole³, C. Wassinger⁴, M. Beilmann^1,2, L. Bouyer^1,2, J.-S. Roy^1,2

Background: Shoulder control is dependant on the central nervous system to integrate the sensory information and to create an appropriate motor command. Given the complexity of the shoulder joint, various daily encountered factors can potentially compromise its control, such as muscular fatigue. 

Purpose: The aim of this study was to explore how muscle fatigue influences shoulder control, characterized by upper limb kinematic and spatiotemporal variables.

Methods: Forty healthy participants were randomly assigned to one of two groups: 1) Control group and 2) Fatigue Group. Participants completed the two assessments of a reaching task with their dominant arm; 1) Baseline phase and 2) Post-experimental phase. Following the baseline phase, the Fatigue Group completed an upper limb fatigue protocol. Electromyographic activity of shoulder muscles were collected to assess the presence of fatigue, and spatiotemporal data and joint kinematics were collected to characterize motor control. The spatiotemporal variables were the time taken to reach the targets, initial angle of endpoint deviation (iANG: based on the initial trajectory of the hand), final error (fERR: shortest arc distance between the actual and expected arrival points into the targets) and area under the curve (total movement error). Upper limb kinematic variables were initial and final joint angles and angular excursion at the elbow, shoulder, sternoclavicular joint and trunk.. Repeated-measures ANOVA were used to determine the effects of Time, Group and of the interaction between these factors.

Results: In the Fatigue Group, all shoulder muscles showed signs of fatigue during the post-experimental phase, as shown by the drop of the mean median power frequency and the increased EMG amplitude (p < 0.05; except for the middle deltoid, p = 0.09). Fatigue affected learning and inter-joint coordination (significant Group x Time interaction, p < 0.05), as shown by the lack of performance improvement; iANG and fERR did not improve in the Fatigue group and speed decreased, while controls improved their performance (time, area under the curve, iANG : p > 0.05). Changes in joint kinematics reflecting less glenohumeral elevation and more trunk and sternoclavicular movements were also observed in the fatigued state.

Conclusion(s): During the fatigued state, learning process was altered evident by the lack of performance improvement. Furthermore, to counteract muscular fatigue, a new and simplified inter-joint coordination involving less glenohumeral movement and more trunk and sternoclavicular movements was used.

Implications: The consequences of the adaptations  in a fatigued state on shoulder structures are  unknown, but   they are similar to those evidenced in glenohumeral and scapular sensorimotor control in individuals suffering from shoulder pain, which have been proposed to cause a reduction of the acromiohumeral distance and the compression of subacromial structures. As we demonstrated that muscle fatigue affects kinematics, we can hypothesize that movement repetitions in a fatigued state might lead to a consolidation of maladapted motor patterns, leading to higher physical stress on tendons and muscles. It should also be noted here that muscle fatigue seems to have affected the between-test learning process, suggesting that fatigue should be avoided when targeting motor learning during rehabilitation.

Funding, acknowledgements: F. Dupuis is financially supported by the Fonds de Recherche du Québec en Santé (FRQS)

Keywords: Shoulder motor control, Muscular fatigue, Upper limb kinematics

Topic: Musculoskeletal: upper limb

Did this work require ethics approval? Yes

Institution: CIUSSS-Capitale Nationale

Committee: Sectorial Rehabilitation and Social Integration Research Ethics Committee of the CIUSSS-CN

Ethics number: 2017-480