Passive flexion of the trunk (relative to the legs) may be accompanied by contraction of the psoas muscles, even when the subject has been told not to contract any muscles. The psoas contraction is involuntary and cannot be controlled by the subject. This lack of passivity might be concomitant with lower back pain: the impairment may be present when lower back pain is present and/or absent when lower back pain is absent.

The study's primary objective is thus to determine the sensitivity and/or specificity of a clinical test for impaired hip flexor passivity in cases of lower back pain during passive flexion of the trunk (from the supine position,).

The secondary objective is to show that a negative test (after administration of correcting measures) is correlated with a decrease in pain (i.e. pain intensity and the functional repercussions of pain).

Inclusion Criteria:

• Age between 18 and 30
• Social security coverage
• Ability to understand the study's objectives, procedures and risks and to provide informed, written consent
• Having been informed about the results of the prior clinical examination.

Exclusion Criteria:

• Past or concomitant infectious or cancerous diseases of the spine
• Vertebral osteosynthesis
• Acute neck or arm pain (recurrent dislocations of the glenohumeral joint, trauma in the previous 12 months, etc.)
• Lower back pain with irradiation of pain below the buttocks
• Known specific lower back pain or pain for which a differential diagnosis suggests an inflammatory, traumatic, cancerous or infectious cause
• Subjects under legal guardianship
• Known pregnancy
• Simultaneous participation in another biomedical research study of a drug or medical device.