Paths of the cervical instantaneous axis of rotation during active movements—patterns and reliability

The instantaneous helical axis (IHA) is a characteristic of neck movement that is very sensitive to changes in coordination and that has potential in the assessment of functional alterations. For its application in the clinical setting, normative patterns must be available, and its reliability must be established. The purpose of this work is to describe the continuous paths of the IHA during cyclic movements of flexion-extension (FE), lateral bending (LB), and axial rotation (AR) and to quantify their reliability. Fifteen healthy volunteers participated in the study; two repetitions were made on the same day (by different operators) and over an 8-day interval (by the same operator) to evaluate the inter-operator and inter-session reliability, respectively. The paths described by the IHA suggest a sequential movement of the vertebrae in the FE movement, with a large vertical displacement (mean, 10 cm). The IHA displacement in LB and AR movements are smaller. The paths described by the IHAs have a very high reliability for FE movement, although it is somewhat lower for LB and RA movements. The standard error of measurement (SEM) is less than 0.5 cm. These results show that the paths of the IHA are reliable enough to evaluate changes in the coordination of intervertebral movement.