Ultrasound-Measured Abdominal Muscle Thickness During Prone and Side Plank Exercises Performed with Bent Knees, with and Without the Abdominal Drawing-In Maneuver

Health training complements rehabilitation of chronic musculoskeletal pain by consolidating therapeutic effects and limiting recurrence. Thirty-eight adults (18 women, 20 men; 19–47 years; 24.6 ± 6.5) underwent ultrasound assessment. Bilateral thickness of the external oblique (EO), internal oblique (IO), transversus abdominis (TrA), and rectus abdominis (RA) was measured at the umbilical level in relaxed supine, prone plank, and side plank. For both plank exercises, measurements were repeated during the abdominal drawing-in maneuver (ADIM). Prone plank increased thickness of all abdominal muscles bilaterally (p < 0.001) and elicited a largely symmetric pattern; ADIM further increased IO and TrA thickness (p < 0.001). Side plank increased muscle thickness predominantly on the support side (p < 0.001), with greater support-side responses for the EO (p < 0.001), IO (p < 0.01), and RA (p < 0.001), while the TrA showed a more bilateral pattern. On the non-support side, IO (p < 0.05), TrA (p < 0.001), and RA (p < 0.01) also increased. ADIM augmented TrA bilaterally (p < 0.001), increased IO on the non-support side (p < 0.001), and promoted symmetry for the EO, IO, and RA. Overall, prone plank was mostly symmetric, while side plank was asymmetric except for the TrA; ADIM amplified TrA responses in both exercises and in prone plank, enhanced IO on the side opposite the support.