Table 1 Inclusion and exclusion criteria

• Age between 18 and 80 years

• Clinical symptoms of LSS

• Not responding to at least 3 months of non-surgical treatment

• Radiological findings (foraminal, central canal, or lateral recess stenosis) corresponding to the clinical symptoms such as back pain, leg pain, or neurologic claudication

• Understanding the Norwegian language (spoken and written)

• Previous surgery at the level of stenosis

• Previous fracture or fusion of the thoraco-lumbar spine

• Cauda equina syndrome (bowel or bladder dysfunction) or fixed complete motor deficit

• ASA grade 4 or 5

• More than 20° lumbosacral scoliosis

• Distinct symptoms in lower limbs due to other diseases

• Stenosis in more than three lumbar levels

• Being unable to comply fully with the protocol

• Isthmic defect in pars interarticularis at the level of stenosis

• Participation in another clinical study that could interfere with the present trial

• Alcohol or substance abuse

• ≥ 3 mm spondylolisthesis verified on upright lateral view x-ray

• Axial T2-weighted MR images not covering the paraspinal and the psoas muscles at both sides of the spine or angulated more than 5° to the upper endplate of the vertebra at the level of measurement