Therapists' conceptualization and characterization of the clinical concept of spinal stiffness

Background and Purpose. The clinical concept of spinal stiffness provides one basis for applying spinal manipulation. Because the terms used to describe the perceptual results of manual spinal stiffness testing are poorly defined, the nature and number of attributes contained in the concept remain unclear. This study attempted to clarify the concept of spinal stiffness by examining the relationships among 31 published spinal stiffness descriptors using cluster analysis. Subjects and Methods. Each stiffness descriptor was printed on a magnetized rubber strip. Physical therapists in Houston (Tex) and Sydney (New South Wales, Australia) judged the similarity of the stiffness descriptors by arranging them on a board. The squared Euclidean distance between words was calculated, and cluster analysis was performed using Ward's method. Results. Cluster analysis reduced the 31 stiffness descriptors to three superclusters (limited mobility, increased mobility, and viscoelasticity) in both the Houston and Sydney data. Conclusion and Discussion. In a step toward improving the reliability of spinal stiffness judgments, this study has identified the fundamental characteristics of the clinical concept of spinal stiffness. Research is now needed to clearly define these characteristics and then develop protocols that will allow physical therapists to reliably rate these attributes.