Now we mentioned it again and again and we will do so! Stable training reigns supreme against training on unstable surfaces for core strength.
Most health clubs and University fitness programs offer core strength and stability classes.
Exercising on unstable surfaces, such as BOSU balls, Swiss balls and wobble boards is a central component of most of these core-strengthening programs. Most studies show that instability training is not effective for building the core. A study from Li Yongming from the University of Leipzig in Germany showed that muscle activation in the lower body and spine was similar when squats were performed on stable and unstable surfaces. Researchers measured muscle activation during the squat performed on stable and unstable surfaces (Reebok core board), using electromyography (measures the electrical activity of muscles). Subjects performed the squats using low loads. Studies using higher load showed that exercising on firm ground is best for building muscles during the squat.
(Journal Strength Conditioning Research, 27: 1349-1353, 2013)