Effects of exercises for the low back muscles on disability in people with chronic low back pain

The intervention group will attend two, 30-minute exercise sessions per week over a 12-week period. All exercise sessions will be supervised by physiotherapists and exercise physiologists who are experienced in delivering strengthening exercises to people with chronic low back pain. These sessions will be conducted as a one-on-one. The intervention group will complete both strengthening and neuromuscular retraining exercises. A description of both follows: i) Neuromuscular exercise: For neuromuscular retraining exercises, participants will be seated on a MedX dynamometer with their trunk in 12 degrees of flexion and lower limbs fixed in order to restrict posterior pelvic rotation and other lower limb movements. Using their lumbar extensor muscles, participants will be required to push back into the pad placed at the level of their thoracic spine and match a fluctuating target force varying between 20-50% of their lumbar extension maximal voluntary isometric contraction (MVIC; Pranata et al., 2017). This target force, as well as a real-time measure of the participant's force, will be displayed on a tablet placed one meter in front of the participant. Participants will perform this force matching task at 0.08 Hz (medium speed) for 60 seconds as a warm-up. Participants will then perform three, 60-second force matching sets at three different frequencies: i) 0.08 Hz (5 waves per minute; as per the warm-up), ii) 0.14 Hz (8.4 waves per minute; fast speed) and, iii) 0.05 Hz (3 waves per minute; slow speed). The order of the three frequencies will be random. Participants will receive 30 seconds rest between the warm-up and each of the three sets performed at different frequencies. The parameters for progression of this exercise over the 12-week intervention include: i) The amplitude of the target wave, whilst remaining at 20-50% MVIC, will change according the MVIC value assessed at the beginning of each training session. ii) The duration of the sets will increase from 60 to 90 seconds at the beginning of week 5 (the warm-up set will remain the same). Set duration will increase again from 90 to 120 seconds at the beginning of week 7 and the duration will not increase for the remainder of the trial. iii) At the beginning of week 9, each set will incorporate the three frequencies, rather than single frequencies in isolation for each set. Frequencies will vary randomly and will change throughout the 120 seconds. ii) Strength exercise: In addition to the neuromuscular exercise described above, the intervention group will also complete global (i.e., whole body) and isolated (i.e., lumbar extensor muscles) strengthening exercises. Four of the strengthening exercises will be standardised across both the intervention and control groups. Standardised exercises include lumbar extension (using the MedX), hip extension, leg press and trunk flexion. At their discretion, treating clinicians can incorporate other strength exercises into the interventional program which will be documented in training logs. Of the standardised strengthening exercises, participants will complete the lumbar extension (using the MedX dynamometer) and one of the other three listed exercises each session, at the discretion of the treating clinician. Adherence to the intervention will be monitored via an exercise record taken by the treating clinician. Treating clinicians will contact participants who miss an exercise session, within 2 days. Missed appointments will be documented.