Core Muscle Activation and Perceived Exertion Across Side Plank and Copenhagen Plank Variations

García-Jaén, M, Cortell-Tormo, JM, García-Luna, MA, and Gómez-Carmona, CD. Core muscle activation and perceived exertion across side plank and Copenhagen plank variations. J Strength Cond Res XX(X): 000–000, 2026—Plank exercises are fundamental core stability exercises, yet the relationship between muscle activation patterns and perceived exertion remains unclear. This study examined core muscle activation across 10 plank variations and identified which muscles predict rating of perceived exertion (RPE). Fifteen recreationally male active adults performed 10 plank variations in a randomized order: Side Plank (SP), Side Plank Abduction (SP Ab ), Copenhagen Plank (CP), Copenhagen Plank variations with height modifications (CP +20 , CP MAX ), and bench-supported variations with short (CP SLb , CP SLb+20 ) and long lever positions (CP LLb , CP LLb+20 , CP LLbMAX ). Electromyographic activity (%MVIC) was recorded from bilateral rectus abdominis, external oblique, and internal oblique muscles. RPE was assessed postexercise using the 0–10 OMNI Resistance Exercise Scale (OMNI-RES). Right internal oblique demonstrated highest activation across variations (38.3–76.8% MVIC), followed by right external oblique (24.4–46.6% MVIC). RPE ranged from 4.6 ± 1.4 a.u. (SP) to 8.1 ± 0.9 a.u. (CP LLbMAX ). Correlation analyses revealed exercise-specific patterns: right internal oblique showed the strongest association with RPE in side planks ( r = 0.64, p < 0.001), while left external oblique demonstrated the strongest correlation in Copenhagen planks ( r = 0.44, p < 0.001). Multiple regression confirmed right internal oblique exclusively predicted RPE in side planks ( β = 0.64, R 2 = 0.41, p < 0.001), while left external oblique was the sole predictor in Copenhagen planks ( β = 0.44, R 2 = 0.19, p < 0.001). These findings indicate that Copenhagen plank variations, particularly long lever and elevated conditions, elicit greater and more asymmetric oblique activation and higher perceived exertion. Perceived effort showed exercise-specific determinants: right internal oblique predicted RPE in side planks, while left external oblique predicted RPE in Copenhagen planks, indicating that task-specific muscular challenge rather than total activation magnitude determines subjective effort.