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Abstract
We investigated the postural effects of trains of electrical stimulation (TES) applied unilaterally or bilaterally on the trapezius muscle in 20 healthy subjects (mean age: 23.1 ± 1.33 years; F/M: 8/12). The anterior–posterior (AP) displacements (AP axis), medio-lateral displacements (ML axis) and total travelled distances (TTW) of the centre of pressure (COP) remained unchanged with TES. However, detailed spectral analysis of COP oscillations revealed a marked decrease of the magnitudes of peak power spectral density (peak PSD) following application of TES. Peak PSD was highly correlated with the intensity of stimulation (P < 0.001 both the AP and ML axes). For the AP axis, the integrals of the sub-bands 0–0.4, 0.4–1.5, 1.5–3 Hz were significantly decreased (P < 0.001), the integrals of the sub-bands 3–5 and 5–8 Hz were not significantly affected (P>0.30) and the integrals of the sub-band 8–10 Hz were significantly increased (P < 0.001). The ratios of the integrals of sub-bands 8–10 Hz/0–3 Hz were markedly enhanced with bilateral TES (P < 0.001). For the ML axis, the effects were striking (P < 0.001) for the sub-bands 0–0.4, 0.4–1.5 and 8–10 Hz. For both the AP and ML axes, a significant inverse linear relationship was found between the intensity of TES and the average speed of COP. We show that TES applied over the trapezius muscles exerts significant and so far unrecognised effects upon oscillations of the COP, decreasing low-frequency oscillations and enhancing high-frequency oscillations. Our data unravel a novel property of the trapezius muscles upon postural control. We suggest that this muscle plays a role of a distributor of low-frequency versus high-frequency sub-bands of frequency during stance. Previous studies have shown that patients with supra-tentorial stroke show an increased peak PSD in low frequencies of body oscillations. Therefore, our findings provide a rationale to assess neurostimulation of the trapezius muscle in the rehabilitation of postural deficits in supra-tentorial stroke.