Palpable rigidity of the epaxial (paraspinal) muscles, lordotic flattening, and spinal flexion accompanying back pain generally are ascribed to epaxial muscle spasm. However, palpable rigidity without muscle spasm occurs in compartment syndromes and epaxial muscle contractions extend the spine, increasing lordosis. Epaxial compartment syndromes are proposed as a possible cause of palpable rigidity, lordotic flattening, and spinal flexion accompanying idiopathic back pain. This article demonstrates the following: existence of an epaxial compartment by latex and dye injections; simulation of epaxial compartment syndromes in unembalmed cadavers by saline injections; and a "Bourdon tube effect" producing spinal flexion with lordotic flattening during epaxial compartment syndrome simulation in embalmed cadavers. In addition, resting and exercising epaxial compartment pressures were measured in 18 normal volunteers with a slit catheter.