Experimental muscle pain changes the spatial distribution of trapezius muscle activity during dynamic tasks

Cescon, Corrado and Falla, Deborah and Lindstrøm, Renè and Farina, Dario (2008) Experimental muscle pain changes the spatial distribution of trapezius muscle activity during dynamic tasks. In: Proc. of the XVII Congress of the International Society of Electrophysiology and Kinesiology, 18-21 June 2008, Niagara Falls, Ontario, Canada.

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INTRODUCTION The economic cost of work-related muscle pain in the neck and shoulder region necessitates research in the area. In particular, trapezius myalgia is a common complaint with increasing incidence (Juul-Kristensen et al., 2006). Most studies investigating the interaction between pain and trapezius muscle activity have been limited to isometric contractions. However, dynamic contractions are more frequent in everyday life and repetitive tasks are considered one of the major risk factors for neck and shoulder pain (Miranda et al., 2008 add Malchaire et al. 2001). Recent studies using high-density surface electromyography (EMG) (Madeleine et al., 2006; Falla et al, 2008) have shown a change in the spatial distribution of trapezius muscle activity during sustained isometric contractions following noxious stimulation of the upper trapezius muscle. This finding suggests that during painful contractions of the trapezius, the distribution of activity within the muscle is altered with respect to normal activation. This altered motor strategy may have implications for the development of chronic work-related neck / shoulder pain. The aim of this study was to evaluate whether experimentally induced upper trapezius muscle pain resulted in a reorganization of the distribution of trapezius muscle activity during a repetitive dynamic task consistent with previous observations in isometric contractions. METHODS Ten healthy male volunteers (age 26.2 ± 3.1 yrs) participated in the experiment. Subjects were required to lift a 1 kg box between shelves positioned at hip (P1) and shoulder height (P2) with a cycle time of 3 s for 50 cycles. Experimental muscle pain was induced by injection of 0.4 ml of sterile hypertonic saline (5.8%) into the right upper trapezius along the line between the spinous process of C7 and the acromion, 1 cm lateral with respect to the main innervation zone. Isotonic saline (0.5 ml, 0.9%) was injected at a similar location as a control injection. The order of the two injections was randomized. The box lifting task was performed in 4 conditions: baseline, immediately after injection of isotonic and hypertonic saline and 20 min after the last injection (recovery). EMG signals were detected from the right trapezius with a 13  5 grid of electrodes (Figure 1). Two uniaxial accelerometers were mounted on the box to obtain the start and end points of the cyclic movement. Surface EMG average rectified value (ARV) averaged across the grid of electrodes was computed in a 500 ms interval starting from the instant when the box was lifted from position P1. The centroid of the ARV map was calculated to quantify the spatial distribution of muscle activity. Regression lines of ARV barycentre positions and average values were computed across the 50 values extracted from the cycles. RESULTS AND DISCUSSION One way repeated measures analysis of variance (ANOVA) showed a significant decrease of the initial value of average ARV during the painful condition (P<0.01). A significant shift of the centroid of the ARV map towards the caudal region of the trapezius (P<0.05) was also observed . The shift of the centroid implies a relatively larger decrease in activity in the cranial with respect to the caudal region of the muscle which is in agreement with previous observations during isometric shoulder abduction (Madeleine et al. 2006). SUMMARY/CONCLUSIONS This study shows that it is possible to extract maps of EMG parameters and to track the position of the barycentre of these maps during a prolonged dynamic task. We confirmed that local excitation of nociceptive afferents selectively inhibits motor neurons innervating muscle fibers with specific spatial localization in the muscle and we showed that the activation strategy to perform a repetitive task in a controlled environment is reorganized in a similar way as during static contractions.

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