Sagittal plane theory is a model of foot function initially articulated by Dr Howard Dananberg, DPM and is concerned with facilitating motion of the foot in the sagittal plane.
Any number of different entities can restrict motion in the sagittal place and increase the risk for musculoskeletal conditions.
For more, see:
Sagittal Plane Theory and Plantar Fasciitis
Sagittal plane blocks
Dananberg Sagittal Plane Theory