Background: Core stability exercises are widely applied in rehabilitation and training programs, but evidence regarding the effects of activation strategies and surface conditions on abdominal muscle recruitment during plank exercise remains inconsistent. Objects: This study aimed to investigate how different activation strategies (hollowing vs. bracing) and surface conditions (stable vs. unstable) influence selective activation of abdominal muscles during the plank exercise. Methods: Thirty-six healthy male adults in their 20s participated. Ultrasound imaging was used to measure the relative thickness of the transversus abdominis (TrA), internal oblique (IO), and external oblique (EO) under four randomized conditions: stable-hollowing, stable-bracing, unstable-hollowing, and unstable-bracing. Relative thickness was calculated as [(contraction – rest) ÷ rest] × 100. Data were analyzed using three-way repeated-measures ANOVA with Bonferroni comparison. Results: No significant three-way interaction was found (p = 0.840). However, a significant muscle × surface interaction indicated that TrA thickness increased more on a stable surface (p < 0.05), while IO and EO showed no difference. A significant muscle × strategy interaction revealed that hollowing induced greater relative thickness in all muscles, particularly in TrA, compared with bracing (p < 0.001). No significant strategy × surface interaction was observed (p = 0.512). Conclusion: Abdominal hollowing and a stable surface independently enhanced TrA thickness during plank exercise. These findings suggest that stable-surface plank exercise with hollowing is an effective strategy for selectively recruiting deep abdominal muscles, providing practical implications for clinical and sports settings. These findings may be specific to anklesling– induced instability and may not extend to other types of unstable surfaces.

Pollutants removal efficiency in pretreatment(GAC filter, multi-media filter, disk filter) and RO facilities was investigated for the Jeju Samyang spring water source where raw water intake has been stopped due to sea water intrusion. In addition, preliminary feasibility analysis was conducted between RO and groundwater intake systems. Turbidity removal in 4 different pretreatment processes was less than 25% due to low concentration of turbidity(i.e., less than 0.21 NTU), while multi-media filter is recommended for the pretreatment facility based on the low organic content in raw water as well as cheaper operation and maintenance cost. The average concentration of Cl- in raw water was 691.4 mg/L, while that of RO permeate was 9.1 mg/L(i.e., removal efficiency was 98.4%). In addition, TDS removal efficiency was 98.1%, which was quite high. The production cost for RO system(Q=4,000 m3/d) was 362.1 won/m3 considering installation, operation and maintenance cost for 30 years. While that of groundwater was 262.6 won/m3 which was low compared to the RO system. However, it is recommended to introduce RO system for Samyang water source rather than new groundwater development because Samyang water source has been discharged to the sea without any usage, while groundwater can be used for other purpose as a sustainable water source.