The effect of movement-evoked shoulder pain on scapular orientation and muscle activation in healthy individuals.

Introduction: Tonic experimentally induced shoulder pain alters the activation of periscapular muscles1. However, tonic pain may not entirely reproduce the symptoms experienced by people with painful shoulder conditions2 since they commonly report an increase of symptoms during movement. In this study we adopted a novel experimental pain model3 to investigate motor control adaptations of the scapular muscles during movement-evoked pain. Methods: Fifteen healthy participants performed a repeated box-lifting task for twelve sets. Pain was induced over the posterior acromion using electrical stimulation, modulated to induce a pain intensity of 4/10 when the activity of the anterior deltoid was equal or higher than the peak activity measured at baseline, and of 1/10 when lower than 80%. Muscle activity from the Upper (UT), Middle (MT) and Lower (LT) divisions of the Trapezius, and elevation of the scapula, were compared between baseline (sets 1-2), early-and late adaptation (respectively sets 3-4 and 11-12) using a Friedman test and post-hoc comparisons. Results: Early adaptation was characterised by lower scapular elevation (p = 0.002) and decreased activation of the UT (p = 0.006). In the late adaptation phase, participants exhibited lower scapular elevation (p = 0.032), decreased MT activation (p = 0.032) and increased LT activation (p = 0.006). Discussion: Movement-evoked pain results in lower scapular elevation, and different muscle strategies may be responsible for the changes in scapular kinematics in the early-and late-adaptation phase. Those results may provide new insights about motor adaptations and motor control in clinical condition. Process evaluation: Stimulation artifacts in the EMG were removed by filtering. [ABSTRACT FROM AUTHOR]