Altered neuromuscular control mechanisms of the trapezius muscle in fibromyalgia

Abstract

Background: fibromyalgia is a relatively common condition with widespread pain and pressure allodynia, but unknown aetiology. For decades, the association between motor control strategies and chronic pain has been a topic for debate. One long held functional neuromuscular control mechanism is differential activation between regions within a single muscle. The aim of this study was to investigate differences in neuromuscular control, i.e. differential activation, between myalgic trapezius in fibromyalgia patients and healthy controls.

Methods: 27 fibromyalgia patients and 30 healthy controls performed 3 minutes bilateral shoulder elevations with different loads (0-4 Kg) with a high-density surface electromyographical (EMG) grid placed above the upper trapezius. Differential activation was quantified by the power spectral median frequency of the difference in EMG amplitude between the cranial and caudal parts of the upper trapezius. The average duration of the differential activation was described by the inverse of the median frequency of the differential activations.

Results: the median frequency of the differential activations was significantly lower, and the average duration of the differential activations significantly longer in fibromyalgia compared with controls at the two lowest load levels (0-1 Kg) (p < 0.04), but not at the two highest load levels (2 and 4 Kg).

Conclusion: these findings illustrate a different neuromuscular control between fibromyalgia patients and healthy controls during a low load functional task, either sustaining or resulting from the chronic painful condition. The findings may have clinical relevance for rehabilitation strategies for fibromyalgia.

Figure 1

Experimental setup. Posterior view of a subject, electromyographical electrode device (rectangle), and weight harness. The centre of the electrode-device was placed in the middle of the line between processus spinosus of the C7 vertebra and the lateral edge of acromion. The harness was used to attach weights symmetrically on the shoulders. Subjects performed isometric shoulder elevation such that their shoulders were held in the horizontal plane.

Figure 2

Examples of a normalized muscle activity from a high-density EMG recording (caudal - cranial, channel 1 to 10, 5 mm's apart) from one healthy control (A) and one patient with fibromyalgia (C) performing a 1 kg weight of isometric shoulder elevation. The activity difference (B and D) in normalized activity between caudal and cranial regions segments (see A right) were used to calculate the power spectral median frequency.

Figure 3

Examples of the power spectral density (Psd) of the differential activity signal between the cranial and caudal parts of the trapezius and its corresponding median frequency (MDF), for a healthy control (A) and a patient with fibromyalgia (B).

Figure 4

Mean values and 95% confidence intervals for median frequency of the differential activations for healthy controls and patients with fibromyalgia at different weights. Furthest to the right is shown the difference between the two groups.