Background: Plank exercises are widely used for core stabilization, but the effects of applying instability to different support surfaces on trunk muscle activation remain unclear. Objectives: This study aimed to investigate the effect of support surface instability on the electromyographic activity of trunk muscles including the rectus abdominis, internal oblique and transversus abdominis, multifidus, and longissimus during the plank exercise. Design: This study is quasi-experimental design. Methods: Thirty healthy university students performed plank exercises under four conditions: stable surface, unstable elbows, unstable feet, and unstable both. Muscle activity was measured using wireless EMG. Data were analyzed using one-way repeated measures ANOVA and Scheffé post hoc tests. Results: The rectus abdominis showed a statistically significant increase in muscle activation when both elbows and feet were placed on an unstable surface compared with the stable condition (P<.05). Conclusions: Unstable support surfaces during planks significantly enhance rectus abdominis activation, increasing global muscle recruitment for postural control. Conversely, stable surfaces may be preferable for training deep stabilizers without excessive superficial muscle dominance.

INTRODUCTION
SUBJECTS AND METHODS
    Subjects
    Outcome Measures
    Experimental Procedures
    Data and Statistical Analysis
RESULTS
DISCUSSION
CONCLUSION
FUNDING
CONFLICTS OF INTEREST
AUTHOR CONTRIBUTIONS
REFERENCES

• Chihwan Kim(Department of Physical Therapy, Kaya University, Gimhae, Republic of Korea)
• Sunghak Cho(Department of Physical Therapy, Kaya University, Gimhae, Republic of Korea) Corresponding author
• Juhun Lee(Department of Physical Therapy, Kaya University, Gimhae, Republic of Korea)