The research questions are if persons with low back pain (LBP) have lower trunk muscle strength, lower trunk muscle strength endurance and lower trunk muscle activity than healthy controls and if functional limitations because of LBP are related to the level of muscle strength, strength endurance and activity. To investigate this investigators will carry out a clinical assessment including questionnaires and a biomechanical assessment comprising muscle strength tests, muscle strength endurance tests and electromyographic (EMG) analysis of selected trunk muscles.

Objectives

The current state of research in the field and of the investigators own clinical experience clearly shows that the role of trunk muscle function in LBP is poorly understood. Investigators will address the following research questions:

• Do persons with LBP have lower trunk muscle strength, lower trunk muscle strength endurance and lower trunk muscle activity than healthy controls?
• Are functional limitations because of LBP related to the level of muscle strength, strength endurance and activity?

To answer these questions, investigators will establish an experimental framework to study limitations in muscle function to address the following aims:

• To compare trunk muscle strength, strength endurance and activity between persons with LBP and healthy control persons.
• To determine the relationship between trunk muscle strength, strength endurance and activity, and the Oswestry Disability Index (ODI) score.

The experimental setting consists of a clinical assessment using a questionnaire and a biomechanical assessment comprising muscle strength tests, muscle strength endurance tests and electromyographic analysis. Electromyographic activity of selected trunk muscles will be recorded during isokinetic and isometric trunk flexion and extension strength tests and during isometric trunk flexion and extension strength endurance tests.

Hypotheses Investigators hypotheses are that i. persons with LBP have lower trunk muscle strength, strength endurance and activity than healthy controls subjects. ii. the intensity of the EMG signal in lower back muscles decreases faster during the strength endurance test in persons with LBP than in healthy control subjects. iii. clinical scores correlate with trunk muscle strength, strength endurance and activity.

Procedures First, written informed consent will be obtained before participants will undergo a clinical exam (inspection and palpation of the lower back, measurement of active range of motion) performed by a trained physical therapist (PS). All participants will complete questionnaires to obtain pain and functional scores and information on their typical physical activity level (approximate duration: 30 minutes). Bipolar skin electrodes will be placed on selected trunk muscles (approximate duration: 15 minutes) and participants will walk on a treadmill for 10 min (warm-up). Participants will be seated on a strength testing system and practice trunk flexion and extension movements for 1 min. Subsequently, subjects will be asked to perform isokinetic trunk flexion and extension tests at maximum effort. EMG signals of the trunk muscles will be recorded during the maximum isokinetic contractions (approximate duration: 20 minutes). Subsequently, subjects will lie supine on a bench with their legs and pelvis supported by the bench. Their upper legs will be secured to the bench using a 15 cm wide Velcro belt. Subjects will be asked to hold their trunk in a horizontal position as long as possible while EMG signals of the trunk muscles will be recorded (approximate duration: 5 minutes).

Inclusion Criteria:

• age ≥ 20 years
• age ≤ 55 years
• primarily seated occupational activities for at least 4 hours per working day
• patients:
• low back pain for at least 12 weeks

Exclusion Criteria:

• neurological symptoms
• diagnosed spinal damage
• cancer,or infectious diseases that could lead to chronic LBP
• other diseases that could affect physical performance
• surgery for chronic LBP
• pregnancy

20 persons with LBP 20 age and sex matched healthy control persons