The main objective of this study is to investigate the influence of seat type (straight chair versus "ergonomic" chair) on the sagittal alignment of the lumbo-pelvic complex in two different postures compared to standing for each individual in both a chronic-back-pain and a non-chronic-back-pain population.

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

Two populations are compared: one with chronic back pain, and the other a group of hospitalized patients with no lombo-peliven complications.