This 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 university physical therapy clinics in 2 locations, 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 the participants meet all inclusion criteria, the consent process will be performed. To ensure the participants understand the study, the participants will be simply asked regarding the study (i.e. objectives, benefits, etc.) before signing the consent form. Total data collection including intervention between pre- and post-intervention will take approximately 2 hours.

After obtaining the written informed consent, all participants will fill out the information sheet for demographic data. Participant will have the right to refuse answering the demographic questionnaire, but the participants can participate in biomechanical data collection. Then, participants will asked to remove the metal accessories that could influence the electromagnetic field, change the cloth to lab tank top and shorts to expose the lower back and right knee, and wear the shoes throughout the test protocol. The body landmarks will be identified. These landmarks include 1) lumbar spine level 1 (L1), 2) sacrum level 2 (S2), 3) lateral epicondyle of right (RE) and left (LE) knees, and 4) head of fibula of right (RF) and left (LF) leg. These locations will be used to place motion sensors for outcome measures. The preparation process will take approximately 10 minutes.

The subjects will stand in a natural stance with arms down by sides. The participants will be instructed to perform functional movements including active forward bend, right and left straight leg raise tests, right and left hip abduction with knee flexion tests, quadruped backward rock test, prone with right and left knee flexion tests, prone with right and left hip internal/external rotation tests, prone with right and left hip extension tests. Each subject performs 2 trials of 3 consecutive repetitions of each functional movement tests. Heart rate and rating of perceive exertion (RPE) will be monitored after each functional movement tests. Two physical therapists (PT-A and PT-B), who are blinded to the group membership, will simultaneously observe and independently rate aberrant movement patterns. After physical therapists completing the clinical observation, the participants will hand the rating form to the principal investigator to input into a spreadsheet for further analysis. These data will be used to determine inter-rater reliability and diagnostic accuracy of clinical observation of aberrant movement patterns during functional movements to identify individuals with a history of CLI. The clinical observation will take about 20 minutes.

Prior to performing SLR, motion sensors will be placed on the subject's body landmarks. Subjects will be asked to perform one repetition of active forward bend, while motion data are simultaneously collected. Then, subjects will be asked to lay on the treatment table in supine position. PT-A will perform passive SLR, and maximal angle will be recorded. This SLR test is intended to determine whether the subject has a sign of CLI, rather than to assess neural tension (to rule out HNP and dural tension) or hamstrings muscle length. In participants with clinical lumbar instability, pelvis/hip will not be stabilized when performing passive SLR. In this case, when pelvis/hip has reached its restriction by hamstrings muscle tightness, lumbar spine would easily allow compensatory motion. Thus, leg raising greater than 91° would indicate clinical lumbar instability. This test is differed from hamstrings muscle tightness (passive knee extension in supine with 90° hip flexion position). This test the clinician will fix the pelvic/hip at 90°; therefore, compensatory motion from pelvis/hip will be minimized. After that, PT-A will perform hamstrings muscle length test. Subjects will be first asked to lay on the treatment table with tested hip and knee in 90 degrees flexion. PT-A will passively extend subject's knee to maximal tolerable point. Angle in sagittal plane between lateral epicondyle and head of fibula sensors will be measured to represent hamstrings muscle length. PT-A will perform additional clinical examination including modified Thomas test (muscle length test for iliopsoas, quadriceps, hip abductor, and hip adductor muscles), Ober's test (muscle length test for tensor fascia latae muscle), as well as abdominal and back muscle strength test assessed by a handheld dynamometer. These outcome measures will be assessed for baseline. To reduce the risk of muscle fatigue, the investigators will monitor heart rate and verbally ask the RPE during the test, and provide adequate resting period between repetitions. The clinical examination will take approximately 10 minutes.

After clinical data collection, the subjects will undergo corticospinal excitability measurement using transcranial magnetic stimulation (TMS). The investigator will identify transversus abdominis muscle (2 cm medial and 2 cm inferior to anterior superior iliac spine on the inguinal line), and lumbar multifidus muscle (2 cm lateral to lumbar spinous process of level 4). These locations will be used for EMG electrode placement. Before placing surface EMG electrodes, the skin will be lightly abraded using abrasive paper and cleaned using cotton with alcohol to lower the skin impedance. Electrodes will be aligned parallel to the muscle fibers. Then, Subjects will be seated comfortably in a reclined chair with both arms supported. The participants will be asked to wear a swim cap used to identify stimulation areas for transverse abdominis and lumbar multifidus muscles. The investigator will use a double cone coil to find the hotspot for both ipsilateral and contralateral to the pain in each muscle. Figure 1 illustrates TMS set up. After corticospinal excitability measurement, subjects will be randomly assigned into 2 groups (lumbopelvic stabilization, LPS and lumbopelvic relaxation, LPR). The TMS data collection will take approximately 60 minutes.

The subjects in both groups will perform hamstrings muscle stretching in standing position. The participants will be asked to place the heel on the tested leg on the compression load cell mounted on the adjustable treatment table. The adjustable treatment table will be initially set at the subject's knee level. Treatment table will gradually move up until force at the heel reaches 15% of the body weight. This standardized force will be controlled by the compression load cell. Our preliminary study indicates that 15% of the body weight is the maximal stretching force in which subjects have reported very hard level of exertion (16-18 out of 20) on RPE during stretching exercise. During applying stretching force, subjects in LPS will be asked to keep the lumbar and pelvic positions as rigid as possible without holding the breath, while subjects in LPR will be asked to relax the lumbopelvic region and do not hold against stretching force. Each subject will perform 4 repetitions of 30-second stretching with 15 seconds rest between repetitions.

After the intervention, the subjects will be reassessed for lumbopelvic motion during forward bend and hamstrings muscle length test. The intervention and reassessment process will take approximately 20 minutes. The data at baseline and post-intervention will be used to determine the effect of lumbopelvic stabilization during hamstring muscle stretching in individuals with and without a history of CLI. In addition, the investigator will call each subject for 1-year follow up to check whether subjects have any episode of low back pain and how often the participants have low back pain.