Outcome measures

Multifidus muscle size and fatty infiltration (e.g., from L1 to L5) will be examined using T2-weighted and IDEAL (lava-flex, 2-echo) sequences obtained using a 3-Tesla GE MRI machine. The cross-sectional area (CSA) of the muscle will be taken on axial T2-weighted slices from the cranial view of L1 and the caudal view of L5; single level and total left and right 3D volume will be calculated. The overall area of lean muscle mass will be assessed using a highly reliable thresholding technique [42, 43], and DIXON axial water and fat images will be used to assess percent-fat signal fraction at each spinal level [44]:

Multifidus muscle thickness at rest and during submaximal contraction will be evaluated by examining the changes in the muscle during contralateral arm lifts. Muscle thickness will be assessed using the Logic E GE ultrasound machine with a 5-MHz transducer. The submaximal and maximal contractions will be performed 3 times on each side in a prone position. Submaximal contraction will be assessed by instructing the participant to lift their arm while holding a handheld weight (e.g. based on subject’s body weight) while the evaluator examines the contralateral multifidus using the ultrasound [45, 46]. The thickness change in the multifidus muscle between sub-maximal (rest) and maximal (contracted) will be calculated using the following equation: %thickness change = (thickness_contracted − thickness_rest)/thickness_rest) × 100. This method of assessing multifidus using ultrasound is both reliable and valid as demonstrated by previous experiments [45, 47, 48].

Lumbar extensor muscle strength will be assessed with the use of the MedX lumbar extension machine. Participants’ hips, knees, and pelvis will be secured to the machine ensuring isolation of the lumbar extensor muscles with the axis of movement being fixed between vertebral levels L5-S1. This dynamometer assesses isometric lumbar extension muscular strength (torque) in a seated position and accommodates the dynamic resistance through a full 72° range of motion (ROM). Therefore, maximum lumbar extension torque will be assessed as maximum voluntary isometric contraction (MVIC) in lumbar extensor muscle strength in seven positions: 72°, 60°, 48°, 36°, 24°, 12° and 0° of flexion [41, 49]. Participants will be seated and positioned in the equipment; initial testing will be performed to verify any limitations in their ROM and adjustment for the counterweight [48]. Participants will first perform a slow controlled warm up for ~ 1 min, and then the maximum strength test will begin [41]. Verbal encouragement will be provided to encourage participants to generate maximum torque. The movement arm of the MedX machine is attached to a load cell that is interfaced with a computer, what will record and calculate torque measurements.

The Oswestry Disability Index (ODI) will be used to measure participants’ level of self-reported disability in relation to LBP. It is a 10-item scale in which each item is rated from 0 to 5, where 0 means that their pain does not influence that situation and a score of 5 indicates severe disability. The categories included in the questionnaire are pain, walking, lifting, sitting, standing, personal care, sleeping, travel, sex life, and social life. Scores are categorized as minimal, moderate, severe, crippled, or bed bound. The ODI has shown good reliability and validity, and therefore is considered to be the gold standard of measuring disability related to low back pain [50].

The 12-item Short Form Health Survey (SF-12) is the condensed form of the previous 36-item survey and will be used to assess participants’ health-related quality of life. The 12-item survey consists of 8 domains that assess both physical and mental components of health: 1) limitations in physical activities because of health problems, 2) limitations in social activities because of physical or emotional problems, 3) limitations in usual role activities because of physical health problems, 4) bodily pain, 5) general mental health (psychological distress and well-being), 6) limitations in usual role activities because of emotional problems, 7) vitality (energy and fatigue) and 8) general health perceptions. Scores from each of the 12 questions are combined to give an overall score between 0 and 100, with a score of 100 indicating the highest level of health. Given that this is a condensed version of a longer and established questionnaire, it has been extensively tested and shown to be both reliable and valid [51, 52].

The Visual Numerical pain rating scale (NPR) will be used to assess participants’ level of pain. The NRP is a self-reported rating system for pain intensity. Ratings range from 0 to 10 with 0 being no pain, 1–3 being mild pain, 4–7 being moderate pain, and 8–10 being extreme pain. This scale has excellent reliability and validity, and can be used detect statistical and clinically significant changes in perceived pain [53, 54].