Rectus abdominis muscle thickness change and activation increase during planks performed on different surfaces

The plank is a common exercise used to evaluate core function. Surface electromyography (sEMG) and ultrasound can be used simultaneously to measure muscle activity. We aimed to compare the %-thickness and %-activation during the plank performed on three surfaces and to determine agreement and relationship between rectus abdominis (RA) %-thickness of a rested tabletop position and %-activation normalized to quiet tabletop position during the plank on three surfaces.

In this cross-sectional study, ultrasound and sEMG measured RA muscle function during the first 5-s and last 5-s of a plank performed on a table, yoga mat, and fitness ball. A repeated measures ANOVA compared differences in %-thickness change and Friedman's tests compared differences in %-activation, alpha set a priori p ≤ 0.05. Bland-Altman plots measured agreement between instruments. Spearman's rho determined relationships between instruments.

There was no difference between %-thickness change across surfaces during the first 5-s or last 5-s, or between %-activation during the last 5-s. The %-activation of the RA during the first 5-s performed on the fitness ball was higher than the table and yoga mat (p < 0.001). Ultrasound and sEMG had weak relationships across all surfaces (ρ = - 0.078 to 0.116).

The first 5-s of the plank performed on the fitness ball requires a greater RA activation. Ultrasound could not detect changes in %-thickness of the RA during the plank which may be influenced by the type of contraction. Comparison between these measurement tools during isometric exercise should be used with caution.