The erectores spinae muscles are described by topographical anatomists as extensors of the trunk. Borelli (1710), Duchenne (1867) and Beevor (1904) asserted that they contract when the trunk is flexed from the upright position and so act as antagonists to gravity. Weddell, Feinstein & Pattle (1944) stated that the lumbar and thoracic sacro-spinalis muscles are easily relaxed with ' satisfactory positioning', but gave no details of postures in which this occurs. Akerblom (1948) reported briefly that the electromyogram of the lumbar sacro-spinalis muscle, in eight out of twelve subjects sitting in the sunken position (i.e. with full flexion of the trunk), showed practically no difference from the resting electromyogram. Allen (1948) stated that the erector spinae is quiescent when full trunk flexion has been reached from the orthograde position. Kelton & Wright (1949) found in two subjects that the erectores spinae muscles were electrically silent for long periods of time in the 'easy standing position '. Floyd & Silver (1950 described the action of the erectores spinae during 'straining' and in flexion and extension of the trunk.The present paper describes the functions of the erectores spinae muscles in certain postures and movements and during weight-lifting, as studied in 150 human subjects by electromyographic, photographic and radiographic methods. A preliminary account of this work has been given in brief communications to the Anatomical and Physiological Societies during 1949-52. METHODSIn most experiments surface electrodes were placed directly over the muscles in the lumbar region. Concentric needle electrodes were also used in order to record the activity of the deeper parts of the muscles. The action potentials were amplified and recorded with either a 4-or 6-channel Ediswan electroencephalograph.

The conception that skeletal muscle consists of slow-acting and quick-acting elements, commonly described as red and white fibres, is long-standing (Ranvier, 1874). Recent pharmacological and histochemical studies have renewed interest in the subject (Paton & Zaimis, 1952) as has the technique of electromyography, used for example in investigations of postural activity. Following an investigation of the fibre content of the human gastrocnemius and soleus muscles (Walls, 1953), it occurred to us, if current descriptions of its action are correct, that of all the striped musculature of the body the external anal sphincter might be said to be the one most constantly in a state of tonic contraction. This muscle might therefore be expected to exhibit most convincingly the structure of so-called red muscle.The first step therefore was to study the activity of the sphincter in normal subjects. Although workers in the field of electromyography have already seized upon most of the readily accessible surface muscles, only Beck (1930) appears to have studied the anal sphincter in this way. Using a string galvanometer and steel needle electrodes inserted at the muco-cutaneous junction to a depth of 4 cm, Beck studied the effects of distension of the anal canal in a number of dogs and in one man. Briefly his findings were these: during both contraction (shortening) and relaxation (lengthening) of the sphincter action currents were recorded; but once the muscle had become contracted or relaxed the action currents gradually lessened and sometimes disappeared. This observation seemed to Beck to provide evidence for a locking mechanism similar to that noted by Fr6hlich & Meyer (1912) in the mollusc.Beck published only records from dogs but stated that his one human subject gave similar results. His findings are clearly of great interest and well worth while re-investigating with the refined electromyographic apparatus

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