Comparison of Abdominal Muscle Thickness During Plank Exercise by Activation Strategies and Surface Conditions

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.