Table 3 Eligibility criteria for the asymptomatic reference sample

Inclusion criteria: Age 20–59; No current low back pain or back pain-related disability; No significant back pain (resulting in absence from work or interruption of normal activities for more than one day), in the last three years. Primary means of communication is the English language.

Exclusion criteria: Pregnancy; current intention to become pregnant; Any history of low back pain, which resulted in absence from work or interruption of normal activities for more than one day, in the last three years; upper motor neuron lesion; frank psychiatric illness; traumatic fracture of any vertebra leading to permanent neurological damage or persistent disability; non-ambulatory or using devices to assist gait; known spondylolysis or spondylolisthesis; any spinal surgery.