A spectral analysis of rotator cuff musculature electromyographic activity: surface and indwelling

Abstract

Electromyography (EMG) of the shoulder girdle is commonly performed; however, EMG spectral properties of shoulder muscles have not been clearly defined. The purpose of this study was to determine the maximum power frequency, Nyquist rate, and minimum sampling rate for indwelling and surface EMG of the normal shoulder girdle musculature. EMG signals were recorded using indwelling electrodes for the rotator cuff muscles and surface electrodes for ten additional shoulder muscles in ten healthy volunteers. A fast Fourier transform was performed on the raw EMG signal collected during maximal isometric contractions to derive the power spectral density. The 95% power frequency was calculated during the ramp and plateau subphase of each contraction. Data were analyzed with analysis of variance (ANOVA) and paired t tests. Indwelling EMG signals had more than twice the frequency content of surface EMG signals (p < .001). Mean 95% power frequencies ranged from 495 to 560 Hz for indwelling electrodes and from 152 to 260 Hz for surface electrodes. Significant differences in the mean 95% power frequencies existed among muscles monitored with surface electrodes (p = .002), but not among muscles monitored with indwelling electrodes (p = .961). No significant differences in the 95% power frequencies existed among contraction subphases for any of the muscle-electrode combinations. Maximum Nyquist rate was 893 Hz for surface electrodes and 1,764 Hz for indwelling electrodes. Our results suggest that when recording EMG of shoulder muscles, the minimum sampling frequency is 1,340 Hz for surface electrodes and 2,650 Hz for indwelling electrodes. The minimum sampling recommendations are higher than the 1,000 Hz reported in many studies involving EMG of the shoulder.

Keywords: electromyography; isometric contraction; rotator cuff; shoulder.

Fig. 1

A representative surface EMG vs. time curve along with the ramp and plateau phases

Fig. 2

The time and frequency domains of a typical shoulder indwelling and surface EMG signal. For the analysis at the narrow bandwidth (10–350 Hz), the percentage of total power spectrum contained in the frequency domain from 10 to 350 Hz was calculated and reported

Fig. 3

Two SDs about the mean for the ramp and plateau phases of the MVIC. The muscle names are italicized when an indwelling electrode was used for recording

Fig. 4

The percentage of the power frequency (mean ± SD) for the ramp and plateau phases of the MVIC for narrower 10 to 350 Hz bandwidth condition data. The muscle names are italicized when an indwelling electrode was used for recording