Participants with non-specific chronic low back pain (CLBP) aged 20-65 years old were recruited from the community by posters. Non-specific chronic low back was defined as discomfort in the low back area, which persisted over a day during the past three months and was diagnosed as non-specific low back pain. Participants who 1) had gone through surgery at the low back or lower extremity; 2) presented neurological symptoms, such as numbness or tingling; 3) could not walk or stand independently; 4) had cancer; 5) had systemic inflammation, such as ankylosing spondylitis, rheumatoid arthritis, and systemic lupus erythematosus; 6) were in pregnancy were excluded from the study.

Asymptomatic participants aged 20-65 years old were recruited from the community by posters. Participants who 1) had gone through surgery at the low back or lower extremity; 2) presented neurological symptoms, such as numbness or tingling; 3) could not walk or stand independently; 4) had cancer; 5) had systemic inflammation, such as ankylosing spondylitis, rheumatoid arthritis, and systemic lupus erythematosus; 6) were in pregnancy were excluded from the study.

The deep core muscles, including transversus abdominis, psoas major, and multifidus, are observed by USI. During USI measurements, participants are instructed to perform specific movement within normal range of motion to activate muscles.

The deep core muscles, including transversus abdominis, psoas major, and multifidus, are observed by MRI. Data collected by MRI is analyzed through ImageJ.

The deep core muscles, including transversus abdominis, psoas major, and multifidus, are observed by MRI. Data collected by MRI is analyzed through ImageJ with threshold method, to acquire funcitonal cross-sectional area of core muscles.

Proprioception was evaluated by lumbar repositioning error with the Zebris motion analysis system (Zebris Medizintechnik GmbH, Isny, Germany). The smaller the error suggests the better proprioception. Repositioning test was performed in a standing position. Participants were first instructed and practiced anterior and posterior pelvic tilting with minimal paraspinal muscle activation, and the neutral position of the lumbar spine was identified. Then, anterior and posterior pelvis tilting to the half and maximal range were practiced with eyes closed. For the actual tests, participants performed 5 repetitions of half range of anterior tilt, back to neutral spine, half range of posterior tilt, and back to neutral spine position. The performance of the first repetition served as the reference for the repositioning test. The error between the reference and the following 4 repetitions indicated the proprioception of the lumbar spine.

Pain intensity is measured by Visual Analog Scale in which the 10 indicates the most severe pain participants can imagine.