The terms ‘relative stiffness’ and ‘relative flexibility’ were coined by American physiotherapist Prof. Shirley Sahrmann to describe how the body achieves a particular movement using the relative flexibility available at a series of joints. Her theory posits that the body will use the path of least resistance or region of greatest relative flexibility to achieve a particular range of motion.
Consider a rower at the catch position. In this position the rower must have her hands past her feet in order to generate good drive. If, for some reason the rower has tight hips and can’t flex them effectively, her body will find somewhere else to move to compensate for the lack of hip flexibility. Usually this range in global movement is found in lumbar and thoracic flexion. This may present as lumbar or thoracic dysfunction and pain.
Consider the tennis player with reduced thoracic extension mobility. To serve the ball, they need good overhead movement at the shoulder. With a reduction in T-spine mobility, they may find that movement in gleno-humeral flexion and lumbar extension. This may present as low back pain and shoulder pain.
An important thing to remember is that with relative stiffness/flexibility, the “stiff” area may have adequate mobility when tested in isolation or tested passively. The stiff region is relatively stiff. Thus one must consider motor patterning of the movement as much as passive mobility modalities.