Procedure The study will use a sample of convenience. The subject with CLBP will be recruited by the flyers posted at physical therapy clinic, as well as words of mouths. The participants who are interested in the study will undergo screening process for eligibility using the inclusion-exclusion criteria checklist. If the participants meet the eligibility criteria, participants will receive the brief information of the study and the consent process will be performed.

After receiving the informed consents, all participants will be requested to fill out the information sheet for demographic and clinical data. The participants will be asked to change the cloth of the top to be able to expose the lower back region. After collecting demographic and clinical data, 3-condition of LM thickness and cross-sectional area will be measured including 1) resting condition (Rest), maximum voluntary isometric contraction condition (MVIC), and maximum voluntary isometric contraction combined with neuromuscular electrical stimulation (COMB). The landmarks will be identified while the participants lay down in prone position. The landmarks include 1) lumbar spinous process of L2-L5 and 2) L4-5 facet joint (2 cm lateral to lower half of spinous process of L4). Rehabilitative ultrasound imaging (RUSI) will be used to measure LM activation and hand-held dynamometer will be used to measure force in this study across 3 conditions.

The participant will have 10 minutes rest before the transcranial magnetic stimulation (TMS). A pair of surface electrodes (Ag/AgCl, 10 mm) will be place according to SENIAM recommendation. The common ground electrode will be placed at the iliac crest. Two surface EMG electrodes will be placed relative to L4-5 along with the line joining from posterosuperior iliac spine and L1-L2 vertebral interspace (24) on the side of pain. The researcher will determine the hotspot of lumbar multifidus muscle area on the brain. The hotspot of the LM will be determined by least intensity of TMS which at least elicits 50 percent of measurable motor evoked potential (MEP) and this intensity will be the active motor threshold (AMT) of the participants. After the AMT is achieved, the 120% of AMT will be used as the intensity to induce the MEP. The MEP will be used to represent cortical excitability.

After the baseline RUSI and TMS data collection, the participants will be randomly assigned in to 2 groups include 1) NMES and high-frequency repetitive TMS (HF-rTMS) group. For HF-rTMS group, the participants will be in sitting position feet placed firmly on the floor. The stimulation point is the LM hot spot which is pre-determined by single pulse TMS as aforementioned. The HF-rTMS will be applied for participant individually. According to the theory, HF-rTMS results in facilitation of the targeted area; therefore, it will increase the excitability of the LM hot spot and potentially increase LM's neural drive and activation. The parameters will be set at frequency 10 Hz, 50 number of pulses, 40 train of stimulation (5 seconds stimulation and 25 seconds inter-stimulation interval) (34). The stimulation will approximately 20 minutes. The position of coil will be placed tangentially at 45 degrees on the LM hotspot. For the neuromuscular electrical stimulation (NMES) group, participants will be lying in prone position. The landmarks and electrodes placements are the same as mentioned in MVIC combined with NMES and LM thickness measurement. NMES parameters are interferential current 6000 Hz, amplitude 20-50 Hz with scanning mode, duration 20-minute, intensity to elicit the LM contraction (6). After the participants completed receiving the stimulation, the participant will undergo the collecting the LM thickness and force again using the same protocol.

Data will be used to 1) identify changes in cortical excitability, LM activation and force before and after stimulation at cortical level by HF-rTMS, 2) identify changes in cortical excitability, LM activation and force before and after stimulation at muscular level by NMES, and 3) compare the percent changes in cortical excitability, LM activation and force between stimulation at cortical (by HF-rTMS) and muscular (by NMES) levels.