Patients' back flexibility will be assessed during the medical examination or physiotherapy session through one of the following non-invasive tests:

• Thoracic Stiffness Test (TST)
• Scoliosis stiffness test (SST)
• Kyphosis Stiffness Test (KST) Each test will be carried out at the beginning of the visit and repeated at the end 2 times by different operators, or once by the same operator to verify that the measurements taken at different times are reliable and that the measurements taken by two different operators are reliable.

The clinical stiffness tests will be applied twice to the same patient by the same operator to verify intra-operator variability and twice by two operators at the same time to verify inter-operator variability.

The verification of diagnostic accuracy with respect to the radiographic reference standard will be carried out retrospectively by using the data already available and systematically collected in electronic format during medical visits and physiotherapy sessions.

Test-retest reliability of the three stiffness assessment tests. Inter class correlation will be calculated

Inter-Rater and Intra-Rater Repeatability of the three stiffness assessment tests

diagnostic sensitivity and specificity of each test