Phase I: Preliminary study Subjects aged between 20 and 40 with and without history of low back pain will be recruited. Subjects who are interested in participation will undergo a screening process using inclusion-exclusion criteria checklist and receive brief information regarding the study. If they meet all inclusion criteria, the consent process will be performed.

After obtaining the written informed consent, all subjects will fill out the information sheet for demographic data. The body landmarks will be identified. These landmarks include 1) lumbar spinous process of L2-L5, 2) L4-5 facet joint (2 cm lateral to inferior one half of spinous process of L4), and 3) lumbar multifidus muscle (2 cm lateral to L5 spinous process and 3 cm lateral to L2 spinous process). Bilateral L4-5 facet joints will be used to place ultrasound transducer for capturing lumbar multifidus muscle, while location of lumbar multifidus muscle will be used to place NMES electrodes. Then, subjects will undergo clinical outcome measures including clinical observation of aberrant movement pattern during functional movements, passive SLR, hamstrings muscle length test, modified Thomas's and Ober's tests, abdominal and back muscle strength.

The subjects will be in prone position on treatment table with thorax (T3 level) and pelvis (S2 level) securely fastened to the bed. Hand-held dynamometer will be placed at the thorax to measure back extension force. Then, ultrasound transducer will be placed at right L4-5 facet joint by rater-A. Right lumbar multifidus muscle activation at rest will be recorded. Then, rater-A will move the transducer to left L4-5 facet joint to measure left lumbar multifidus muscle activation. After that, the subject will be asked to perform 2 repetitions of maximum voluntary isometric contraction (MVIC) of back extensor muscle with 1 minute rest between repetitions. Rater-A will measure right and left lumbar multifidus muscle activation during MVIC and record back extension force from hand-held dynamometer for both repetitions. The data from repetition 1 and 2 will be used to establish intra-rater reliability of ultrasound imaging measurement and calculate minimal detectable change, while data from right and left lumbar multifidus muscles will be used to determine the difference between right and left sides. Subject will be asked to rate the rating of perceived exertion (RPE) and take 5 minute rest to prevent muscle fatigue. After rest period, rater-B will perform exact process as rater-A. These data will be used to establish inter-rater reliability of ultrasound imaging measurement. Rater-A and rater-B are blinded to the subject's group.

The NMES electrodes will be placed on bilateral lumbar multifidus muscles (positive electrode at L3 and negative electrode at L5). Ultrasound transducer will be placed at L4-5 facet joint to capture lumbar multifidus muscle activation. The NMES will be first set at faradic mode (20 Hz, 1 ms). The investigator will increase the intensity at 1 mA increment. Ultrasound imaging data will be recorded, while subject will be asked to rate the 0-10 level of discomfort every 5 mA increment. Rater-A and rater-B will record the intensity used to elicit observable lumbar multifidus muscle contraction. The NMES intensity will increase until maximum level of discomfort (10 out of 10). Subject will be asked to rate the RPE and take a rest period for 10 minutes to prevent muscle fatigue. After rest period, the NMES will be set at interferential mode (6000 Hz, beat frequency 20-50 Hz, scanning effect). The investigator will repeat the entire data collection process. Data from two modes of stimulation will be used to determine the appropriate mode and dose to activate lumbar multifidus muscles.

To determine the feasibility of proposed protocol, the NMES will be set at interferential mode. The investigator will increase NMES intensity until lumbar multifidus muscle 10% increase in thickness. Then, subject will be asked to perform MVIC of back extensor muscle. Rater-A will measure right and left lumbar multifidus muscle activation during MVIC and record back extension force from hand-held dynamometer attached to the thorax. The investigator will decrease NMES intensity to zero. Subject will rest for 10 minutes. After that, the subject will be asked to perform another MVIC of lumbar multifidus muscle without NMES. Rater-A will measure lumbar multifidus muscle activation and record back extension force. The NMES electrodes will be removed and subject's skin will be cleaned. The data (MVIC and MVIC+NMES) will be used to preliminarily determine the existence of arthrogenic muscle inhibition of lumbar multifidus muscle. In addition, the MVIC data without NMES between pre- and post-NMES will be used to determine the existence of retention effect of NMES.

To determine the feasibility of proposed intervention, the NMES will be applied using interferential mode on bilateral lumbar multifidus muscles. The investigator will increase the intensity up to the subject's maximum tolerance. Stimulation will be set at 10 seconds on and 60 seconds off to minimize muscle fatigue. The total NMES time is 15 minutes. This intervention dose is based on previous studies by Hicks et al. Quadruped rocking exercise will be used as a motor control training in this study. This exercise is based on movement system impairment concept. Subject will be asked to perform quadruped position with spine in neutral position, shoulder and hip in 90 degrees, hips centered over knees, and feet pointing away from body with the dorsum of the feet on the support surface. Then, the subject will be asked to perform rocking back by pushing back with both hands. This verbal instruction will be given to the subject as "Keep your back straight throughout the movement. Think about moving in your hips and not your back. Move in the hips equally". Subject performs rocking back to the position that cause observable and/or palpable posterior pelvic rotation. The subject will be asked to perform rocking back to this target point supervised by the investigator for 10 repetitions followed by 1 minute resting period to minimize muscle fatigue. Subject will be asked to rate the RPE after completion of 10 repetitions. Then, the investigator will re-evaluate the target point again for the next 10 repetitions. This process will be continuously performed for 15 minutes.

After completion of intervention, subject will undergo post-intervention data collection using the same protocol, except only rater-A will perform the post-intervention data collection. The NMES electrodes will be re-applied to bilateral lumbar multifidus muscles. Ultrasound transducer will be re-placed at L4-5 facet joint, and hand-held dynamometer will be re-placed at the thorax. Subject will be asked to perform 2 repetitions of MVIC of back extensor muscle with 1 minute rest between repetitions. Rater-A will measure right and left lumbar multifidus muscle activation during MVIC and record back extension force from hand-held dynamometer for both repetitions. The investigator will increase NMES intensity until lumbar multifidus muscle 10% increase in thickness. Then, subject will be asked to perform MVIC of back extensor muscle. Rater-A will measure right and left lumbar multifidus muscle activation during MVIC and record back extension force from hand-held dynamometer attached to the thorax. The investigator will decrease NMES intensity to zero. Subject will rest for 10 minutes. After that, the subject will be asked to perform another MVIC of lumbar multifidus muscle without NMES. Rater-A will measure lumbar multifidus muscle activation and record back extension force. The NMES electrodes will be removed and subject's skin will be cleaned. Pre- and post-intervention data will be used to determine the feasibility of the proposed intervention.

Phase II: Randomized controlled trial study Phase II study will use a sample of convenience between the ages of 20 and 40. Subjects with and without history of low back pain will be recruited by flyers posted in 2 university physical therapy clinics, as well as word of mouth from subjects and friends. Subjects who are interested in participation will undergo a screening process using inclusion-exclusion criteria checklist and receive brief information regarding the study. If they meet all inclusion criteria, the consent process will be performed.

After obtaining the written informed consent, all subjects will fill out the information sheet for demographic data. Then, subjects will be asked to change their cloth to lab tank top to expose their lower back area. The same body landmarks as preliminary study will be identified. The subjects will be in prone position on treatment table with thorax (T3 level) and pelvis (S2 level) securely fastened to the bed. Hand-held dynamometer will be placed at the thorax to measure back extension force. The NMES electrodes will be placed to bilateral lumbar multifidus muscles, and the system will be set at interferential mode. Then, ultrasound transducer will be placed at right L4-5 facet joint by rater-A.

Subject will be asked to perform 2 repetitions of MVIC of back extensor muscle with 1 minute rest between repetitions. Rater-A will measure right and left lumbar multifidus muscle activation at rest and during MVIC, as well as record back extension force from hand-held dynamometer for both repetitions. The investigator will increase NMES intensity until lumbar multifidus muscle 10% increase in thickness. Then, subject will be asked to perform MVIC of back extensor muscle. Rater-A will measure right and left lumbar multifidus muscle activation during MVIC and record back extension force from hand-held dynamometer attached to the thorax. The NMES electrodes will be removed and subject's skin will be cleaned. The data (MVIC and MVIC+NMES) will be used to determine the existence of arthrogenic muscle inhibition of lumbar multifidus muscle. After pre-intervention data collection, subjects with impaired LPC will be randomly assigned to either combined NMES with motor control training group (COMB), or motor control training alone group (MCT) using randomly generated number in concealed envelopes.

The COMB group will receive the NMES using interferential mode on bilateral lumbar multifidus muscles. The intensity will be set at the subject's maximum tolerance. Stimulation will be set at 10 seconds on and 60 seconds off to minimize muscle fatigue. The total NMES time is 15 minutes. After the NMES, the COMB group will receive 30 minutes motor control training based on movement system impairment concept. This motor control training will focus on the control of the lumbar multifidus muscle in various functional tasks. It is believed to restore the motor units in individuals with arthrogenic muscle inhibition. This motor control training with take approximately 30 minutes. The COMB group will receive intervention twice a week for 8 weeks. We use the 8-week motor training program because several studies have showed significant improvement after 8-week exercise program.1,8 For subjects in MCT group, they will receive placebo NMES (no electrical current) and the same motor control training program as the COMB group. Therapist who provides motor control training will be blinded to the group assignment. After completion of the intervention program, each subject will undergo post-intervention data collection immediately after the first session (Post-1) and after 8-week (Post-2) of intervention to determine the effect of combined neuromuscular electrical stimulation and motor control training in individual with impaired lumbopelvic control.