Power spectral analysis of the surface electromyogram before and after a warning signal in a reaction movement

The present study was performed to substantiate the premise that frequency components of the surface EMG before a reaction movement change after a warning signal (S1). Twelve subjects extended rapidly the trunk from a bent position in response to a moving signal (S2) 2 sec after S1. The movement was repeated 20 times. Bipolar surface EMG recordings were made from the erector spinal muscle. A minicomputer calculated power spectra of the surface EMGs for 2 sec before S1 and for 2 sec from S1 until S2 in each trial. From each spectrum, the mean frequency (MF) was calculated. EMG itself was examined. Statistical comparisons of differences between the mean values of the MF before and after S1 showed that a significant difference was found in 5 subjects. For the subjects, the mean value of the MF after S1 was a lower value than that of the MF before S1. Increase of EMG power in frequency range from 30 to 50 Hz was observed in the averaged EMG spectrum after S1 in the subjects. Slowing of the surface EMG during the preparatory period from S1 until S2 was often observed in the subjects. These results show that the power spectrum of the surface EMG after S1 shifted to lower frequencies and suggest that the EMG spectral shift is due to the slowing of the surface EMG in association with motor preparation to perform the movement.