Low back pain (LBP) is currently the most common cause of functional impairment with regard to the musculoskeletal system1. LBP occurs in men and women in all age groups and has a peak incidence between 30 and 65 years. Although 10% of the persons with LRP can be diagnosed with a specific underlying pathology (e.g., disc herniation, canal stenosis, spondylolysis, ...), 90% have symptoms with an unclear origin. 23% of the latter group will eventually develop chronic nonspecific low back pain (CNSLBP)2.

Exercise therapy is seen as an important component in the rehabilitation of persons with CNSPLBP3. Because previous research showed reduced muscle strength of the back muscles4, exercise therapy that improves / recovers muscle strength is being investigated. However, there is still a lack of clarity concerning the effect that reduced muscle strength has on the development and further development of back pain5-6. The objective testing of back muscle strength remains a recurring issue. Up to now, back muscle strength has been mainly tested by a battery of clinical tests (eg Sorensen test) or the use of "iso machines"7. These Iso machines contain a dynamometer that can very specifically reflect the isometric or isokinetic force on an axis of rotation. Different types of these iso machines (eg Cybex, Biodex) have already been developed, each with specific designs (eg differences in the hip angle, differences in the stabilization of the participant). However, to date no standardized protocols are available for evaluating both abdominal and back muscle strength. Furthermore, it is also essential that before such systems can be used in clinical intervention studies, the reliability of such systems is investigated and that reference data from healthy subjects are collected with which data from patients can be compared.

The aim of the current research is therefore to investigate the intra- and inter-operator reliability of a standardized protocol drawn up according to the latest scientific evidence. A second goal is to set standard values for healthy persons for the developed protocol using the Biodex 3 dynamometer system.

Inclusion Criteria:

1. medically diagnosed with non-specific chronic low back pain,
2. between 25-60 years old,
3. able to understand Dutch (spoken and written).

Exclusion Criteria:

1. invasive surgery at the lumbar spine in the last 18 months (arthrodesis was excluded, microsurgery was allowed),
2. radiculopathy (uni- or bilateral),
3. co-morbidities: paresis and/or sensory disturbances by neurological causes, diabetes mellitus, rheumatoid arthritis, pregnancy,
4. ongoing compensation claims and/or (work)disability > 6 months,
5. rehabilitation/exercise therapy program for LBP in the past 6 months.