RESEARCH REPORT POSTER DISPLAY

EFFECTS OF EXPERIMENTAL SHOULDER MUSCLE PAIN ON SHOULDER-ABDUCTION FORCE STEADINESS. Bandholm T^1,2, Rasmussen L^1,2, Aagaard P^1,3, Diederichsen L¹, Jensen B²; ¹Institute of Sports Medicine – Copenhagen, Bispebjerg University Hospital, Copenhagen, Denmark. ²Institute of Exercise and Sport Sciences, Department of Human Physiology, University of Copenhagen, Denmark. ³Institute of Sports Science and Clinical Biomechanics, University of Southern Denmark, Odense, Denmark

PURPOSE: We have previously demonstrated that shoulder sensory-motor control expressed as abduction force steadiness is mildly impaired in patients with subacromial impingement syndrome (SIS) compared to a group of healthy matched persons, which might be related to shoulder pain associated with SIS. The primary objective of the present study therefore was to examine effects of experimental shoulder muscle pain on shoulder sensory motor control in a group of healthy persons previously matched to patients with chronic shoulder pain (SIS) and to evaluate to which extend the experimental shoulder pain model reflects key features of the SIS. RELEVANCE: In order to investigate interactions between shoulder pain and shoulder sensory-motor control in a clinically relevant model, the experimental pain model needs to resemble key features of the pathology or syndrome that it is supposed to model. PARTICIPANTS: Nine healthy, non-medicated persons with ages ranging from 22 to 37 years (mean, 27.7 years) volunteered to participate in the study. Informed consent was obtained in accordance with the Declaration of Helsinki and the study was approved by our Local Ethics Committee. METHODS: Measurements were performed before, during and after experimental pain induction. Experimental muscle pain was induced by bolus injections of hypertonic saline into the supraspinatus muscle to mimic shoulder pain associated with SIS. The assessed parameters were isometric and isokinetic (concentric and eccentric) shoulder-abduction force steadiness at target forces (TF) corresponding to 20, 27.5, and 35% of the maximal shoulder-abduction torque expressed as the standard deviation and coefficient of variation in exerted force. Muscle activity during contractions was determined using surface and intra-muscular electromyograms (EMG) obtained from eight shoulder muscles. ANALYSIS: Two-way, repeated measures ANOVAs were used to examine main effects of condition (pre, during and post-pain) and load (TF) on the dependent force steadiness and EMG variables. RESULTS: Experimentally induced muscle pain impaired shoulder-abduction force steadiness on average 21% during isometric contractions (P < 0.05) and tended (P = 0.081) to do so also during concentric contractions. Middle deltoid muscle activity during isometric contractions and infraspinatus and lower trapezius muscle activity during concentric contractions increased during experimental pain. CONCLUSIONS: The present data indicate that experimental shoulder muscle pain impairs shoulder sensory-motor control, expressed as shoulder-abduction force steadiness, during isometric contractions in healthy persons. Few minor increases in muscle activity were observed as a result of experimental pain induction and these were not consistent across contraction types. The experimental shoulder pain model seems to reflect some but not all key features of the subacromial impingement syndrome. IMPLICATIONS: Results obtained in experimental pain models should be interpreted with caution when extrapolated to chronic pain conditions. KEYWORDS: Pain, shoulder, steadiness. FUNDING ACKNOWLEDGEMENTS: The study was supported in part by a grant from the Danish Foundation for Research in Physiotherapy. CONTACT: thomas.bandholm@hh.hosp.dk

ETHICS COMMITTEE: De Videnskabsetiske Komitéer for Køben­havns og Frederiksberg Kommuner (ethical approval number KF 01-383/97).