Take the simple action of a firm dragging action of a digit or the thumb across an area of skin, in which hyperaemia is produced - a ‘red reaction’.

It is fascinating to go back through the history of bodywork and to find how many ways this simple method has been utilised.

Many researchers and clinicians have described an assortment of responses in the form of such ‘lines', variously coloured from red to white and even blue-black, after application of local skin dragging friction, with a finger or probe.

In the early days of osteopathy in the 19th Century the phenomenon was already in use.

Carl McConnell D.O.stated the following in 1899:
'I begin at the first dorsal and examine the spinal column down to the sacrum by placing my middle fingers over the spinous processes and standing directly back of the patient draw the flat surfaces of these two fingers over the spinous processes from the upper dorsal to the sacrum in such a manner that the spines of the vertebrae pass tightly between the two fingers; thus leaving a red streak where the cutaneous vessels press upon the spines of the vertebrae. In this manner slight deviations of the vertebrae laterally can be told with the greatest accuracy by observing the red line. When a vertebra or section of vertebrae are too posterior a heavy red streak is noticed and when a vertebra or section of vertebrae are too anterior the streak is not so noticeable'.

Much more recently, Marshall Hoag D.O. writes as follows regarding examination of the spinal area using skin friction:

'With firm but moderate pressure the pads of the fingers are repeatedly rubbed over the surface of the skin, preferably with extensive longitudinal strokes along the paraspinal area. The blunt end of an instrument or of a pen may be used to apply friction, since the purpose is simply to detect colour change, but care must be taken to avoid abrading the skin. The appearance of less intense and rapidly fading colour in certain areas as compared with the general reaction is ascribed to increased vasoconstriction in that area, indicating a disturbance in autonomic reflex activity. The significance of this red reaction and other evidence of altered reflex activity in relation to (osteopathic) lesions has been examined in research. Others give significance to an increased degree of erythema or a prolonged lingering of the red line response'.

John Upledger D.O. writes of this phenomenon:
'Skin texture changes produced by a facilitated segment [localised areas of hyper-irritability in the soft tissues involving neural sensitisation to long term stress] are palpable as you lightly drag your fingers over the nearby paravertebral area of the back. I usually do skin drag evaluation moving from the top of the neck to the sacral area in one motion. Where your fingertips drag on the skin you will probably find a facilitated segment. After several repetitions, with increased force, the affected area will appear redder than nearby areas. This is the 'red reflex'. Muscles and connective tissues at this level will:

1. have a 'shotty' feel (like BBs under the skin);
2. be more tender to palpation;
3. be tight, and tend to restrict vertebral motion; and
4. exhibit tenderness of the spinous processes when tapped by fingers or a rubber hammer'.

Roger Newman Turner N.D., D.O. describes the research of another osteopath/naturopath, Keith Lamont N.D., D.O., who first described the 'black line' phenomenon:
'It is a common observation of osteopaths who use a spinal meter, to detect the most active lesions, that pressure on either side of the spine with a hemispherical probe of approximately 0.5 cm diameter, will, in some patients, illicit a dark blue or black line. The pressure of the probe is usually very light since it is intended to register variations in skin resistance, but it has a pinching-off effect on the arterioles and venules of the capillary network beneath the skin. Local engorgement of the capillary bed with deoxygenated venous blood causes the appearance of the line which slowly fades as the circulation returns.'