An exercise fitness ball forces the abs to contract because they have to in order to keep the body in a stable position. The instability provided by the ball itself is forcing this contraction.
However, the contractions that can be achieved are far from maximum. A study entitled “. The Effectiveness of Resistance Exercises Performed on Unstable Equipment” by Willardson, Jeffrey M. MS, said the following:
“The performance of resistance exercises on unstable equipment has increased in popularity, despite the lack of research supporting its effectiveness. Endurance exercise performed on unstable equipment that cannot be effective in developing the type of balance, proprioception and core stability needed for successful athletic performance, free weight exercises performed while standing on a stable surface has been proven more effective in improving skills related to sports. “
If “free weight exercises performed while standing on a stable surface has proven most effective for the improvement of sports-related skills”, then a stable base is definitely best when training for growth.