Preliminary Study of the Sagittal Alignment of the Lumbo-pelvic Complex in a Sitting Position

General Inclusion Criteria: * The patient must have given his/her informed and signed consent * The patient must be insured or beneficiary of a health insurance plan Inclusion Criteria for the Chronic Back Pain Group * Patient has chronic lombalgia (\> 3 months) * Patient has discopathy evidenced by standard imaging of the lumbar spine Inclusion Criteria for the Non Chronic Back Pain Group * Patient does not have chronic lombalgia or neurological symptoms * Patients hospitalized for cardiovascular physical therapy which does not interfere with the sagital alignment of the lombo-pelivien complex (post-infarctus care, cardiac insufficiency) Exclusion Criteria: * The patient is participating in another study * The patient is in an exclusion period determined by a previous study * The patient is under judicial protection, under tutorship or curatorship * The patient refuses to sign the consent * It is impossible to correctly inform the patient * The subject has had more than two x-rays, or similar ray exposure during the previous year * The subject has a professional activity or hobby that exposes him/her to radiation * The subject has a lombo-pelivien malformation