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Muscle tone can be palpated through the skin; more tense muscles feel harder. Hardness is a parameter which may be calculated from the instantaneous slope of a force-displacement curve. It was reasoned then, that this palpable (radial) hardness would be related to the force-deformation curve produced by pressing on the muscle radially with a force measuring instrument. Such an instrument has been developed in our laboratory as part of an upper cervical adjusting study. It uses a stylus, the position of which is determined by a cam driven by a precision stepping motor. This apparatus is mounted on a head, the position of which can be controlled precisely. Monitoring is force and displacement of the stylus tip is accomplished by means of transducer interfaced to a computer. The instrument has been used to generate radial force-deformation data for arm muscles.
The data were plotted for different tensions, and radial hardness values were computed from the slopes of the nerves. In this case the actual muscle tension was calculated from applied load and lever arm mechanics. This made it possible to determine the proportionality factor between muscle tension and radial hardness. For purposes of initial approximation we have assumed that the relationship between muscle tension and radial hardness as seen in the arm muscles is representative of other muscles as well. We have also assumed that any artifacts from overlying skin are negligible. It has therefore been possible to calculate the tension in certain back muscles using force-deformation data from these muscles and the proportionality factor determined from the arm muscle measurements.