An electromyographic study of the abdominal muscles during postural and respiratory manoeuvres.

Abstract

SUMMARY A method was developed for making EMG recordings from the four individual muscles of the anterior abdominal wall. It was then demonstrated that these muscles have different and distinguishable actions on trunk movement, but act together in breathing. The level of ventilation at which the abdominal muscles become active in expiration was shown to be posture dependent.Previous studies of the EMG activity of the abdominal muscles in man have used recordings made from surface electrodes' or needle electrodes2 placed in rectus abdominis and external oblique muscles. The EMG activity of the external oblique muscles during breathing has been assumed to be representative of the antero-lateral muscle group as a whole, though there is no evidence for this assumption. The anterolateral musculature consists of three layers comprising external oblique outermost, internal oblique next, and transversus abdominis innermost. These layers are in such close proximity that it has been difficult to separate their electrical activities.34 The fibres of the four muscles of the anterior abdominal wall run in different directions, thus these muscles may have different actions. The aim of this study was to develop a method of sampling EMG activity from individual abdominal muscles, and to record their activity during breathing. Methods and materials SubjectsSix normal volunteers took part in the study; none was suffering from any acute or chronic, neurological or respiratory illness. Three subjects were male, and the group had a mean age of 28 years. Apparatus EMG recordings from the abdominal muscles were made with bipolar fine wire electrodes manufactured in the manner of Basmajian,' from 0-06mm stainless steel wire insulated with Diamel. Ventilation was measured as flow with a conical pneumotachograph, accurate to flow rates of 300 1/min. This was connected to a differential pressure transducer (Elema-Schonander EMT 32c), and the signal generated processed by a purpose built integrating amplifier (with a time lag in response to a square wave of OOls). The ventilation signal in the form of volume was displayed with the EMG signals on a "Medelec MS6" EMG module fitted with four AA6 amplifiers, and recorded on light sensitive paper run at 5 cm/s. Recordings from the wire electrodes were made at a gain of 200mA. The high pass filter was set to 16HZ (-3dB cut oft) and the low pass filter to 1-6KHZ (-3 dB cut oft). Both filters rolled off at 12 dB/octave.Methodfor location ofindividual muscle layers Computed tomographic (CT) scans of the abdomens of 20 patients were performed with an Elscint 2002 whole body scanner at Brompton Hospital. These subjects were selected at random, they represented 20 consecutive referrals for abdominal CT scan. The scan taken at the mid-point between xiphisternum and pubis was selected from a "scout" view, and on this the individual muscle layers of the antero-lateral abdominal wall were clearly visible (fig 1). The cursors available on the viewing module were used as electronic calipers to measure the d...

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