Therefore, it appears that the standing barbell overhead press produces higher levels of rectus abdominis muscle activity than the squat or deadlift exercises. (strengthandconditioningresearch.com)
Assessing the effect of surface stability, Willardson et al (2009) found no difference in transverse abdominis muscle activity between stable (50 % and 75 % of 1RM) and unstable (50 % of 1RM BOSU) conditions. (strengthandconditioningresearch.com)