This study used a sample of convenience between the ages of 18 - 40. Sixty healthy participants were recruited by flyers posted in 2 Mahidol university physical therapy clinics in Pinklao and Salaya, as well as word of mouth from participants and friend. Sample size calculation was performed using G\*Power software (G\*Power version 3.1.9.2, University Kiel, Germany). To our knowledge, no study has investigated the immediate effect of core stabilization exercise on trunk proprioception neither in healthy individuals, nor patients with low back pain. However, based on our clinical experiences, we expected to find an improvement (1-tailed) with a medium effect size (Cohen's d = 0.5); therefore, a total of 60 participants were required at confidence level (α) of .05 and power (1-β) of .80.

Participants who were interested in participation underwent a screening process using inclusion-exclusion criteria checklist and receive brief information regarding the study. If they met all inclusion criteria, the consent process was performed. To ensure they understood the study, they were simply asked regarding the study (i.e. objectives, benefits, etc.) before signing the consent form.

After the participants provided a written informed consent, all participants filled out the information sheet for demographic data. After that, they changed their cloth to lab tank top and shorts to expose only their lower back (L1 to S2). The body landmarks, including lumbar spine (L1) and sacrum (S2) were identified and marked with a skin pen for iPhone placement. Each iPhone was placed in the iPhone pocket with transparent film in order to see the iPhone monitor. The pockets were attached to the participant using a Velcro strap around his/her trunk and pelvis. These body landmark identification and iPhone placement process was performed by male or female investigator based upon participant's gender. Once the participant had completed setting up, the data collection process began.

Participants stood on a drawing paper with feet shoulder width apart. The foot print was drawn using a marker. This foot print was used for post-test positioning. The participants stood in a natural stance with arms down by sides. Participants were instructed to stand up straight and maintain that position for recording lumbar and sacral angles in neutral position. After recording, participants were instructed to perform 2 set of 3 repetitions of standing forward bend as far as they can and then return to upright position. Lumbar and sacral angles at full trunk flexion, as well as lumbar and sacral angles at completion of returning to upright were recorded for each repetitions. Data for each set were used to determine test-retest (intra-trial) reliability of an iPhone application, as well as establish minimal detectable change (intra-trial).

Participants underwent trunk proprioception test (pre-test) process by first performing practice trials. Participants performed trunk flexion at 30°, 45°, and 60° (4 repetitions for each degree) with feedback from investigators using those iPhones attached to lumbar spine and sacrum. This practice trials aimed for participants to get familiar with the testing protocol, as well as minimize the learning effect. After completion of practice trials, participants underwent pre-test data collection by perform 3 repetitions of trunk flexion at randomly selected angles each repetition (30°, 45°, and 60°) without feedback. Investigators recorded all lumbar and sacral angle data. After completion of pre-test data collection, all iPhones were removed and body landmarks were erased.

Each participant was randomized into either control or exercise group. Participants in control group waited for 15 minutes (newspaper and magazines were provided during waiting period), while participants in exercise group underwent the assessment of core stability in order to select an appropriate level of core stabilization exercise. After that, each participants in this group received a 30-minute of core stabilization exercise. Clinically, physical therapists would prescribe a 30-minute core stabilization exercise program for patients with non-specific low back pain. This exercise focuses on motor control rather than strength, endurance, or cardiovascular system.37 It is a low intensity exercise to develop an ability to automatically control co-contraction of deep abdominal and back muscles during functional movement. Therefore, the possibility of experiencing muscle fatigue is minimal. However, the participants were instructed to perform stretching exercise after the data collection was completed, and perform in following days as well.

Once participants in control group had finished 15-minute rest, and participants in exercise group had completed core stabilization exercise, they underwent practice trials and post-test data collection using the same protocol starting with marking all body landmarks. Data from test (first 2 sets of 3 repetitions) and retest (last 2 sets of 3 repetitions) from control group were used to determine test-retest (inter-trial) reliability, as well as derive minimal detectable change (inter-trial) of the protocol. Approximate time of the test protocol was 45 minutes including 15-minute rest period for the control group, whereas 60 minutes including 30-minute core stabilization exercise for the exercise group.

Rating of perceived exertion (RPE) and heart rate were used to monitor each participant throughout the protocol to avoid fatigue. Pre- and post-test data from the control and exercise groups were used to determine the immediate effect of core stabilization exercise.

All Statistical analyses were performed using statistical package for the social science (SPSS) software (IBM SPSS Statistics for Windows, Version 21.0. Armonk, New York, USA). Study objective 1 was to determine test-retest reliability of an iPhone application measurement and protocol. Mean angular displacement data from the iPhone application were used to calculate test-retest reliability by using intraclass correlation coefficient (ICC2,k) with confidence intervals. Significance level was held at .05 for all analysis. Study objective 2 was to establish minimal detectable change (MDC) of measurement. Minimal detectable change is defined as the smallest amount of change that likely reflects true change rather than measurement error from the iPhone application and protocol. Study objective 3 was to determine the immediate effect of a 30-minute core stabilization exercise on trunk proprioception in healthy individuals. Prior to utilization of statistical tests, descriptive statistics were performed and statistical assumptions were tested. Transformation will be performed if the data were not normally distributed. Appropriate statistical tests (parametric vs. non-parametric) were performed to determine the effects of 30-minute core stabilization exercise on trunk proprioception. Significance level was held at .05 for all analyses. Post-hoc power analysis was also performed.