Instability training has been extremely popular with athletes for more than 10 years. The theory behind the method is that training on unstable surfaces such as Swiss and BOSU balls forces muscles to stabilize the body to perform the exercise, which requires additional muscle activation. However, very few studies support this theory.
A recent Norwegian study measured muscle activation in the upper body and core muscles during a 6-rep maximum bench press performed on stable and unstable surfaces. Muscle activation when training on unstable surfaces, as measured by electromyography, was lower for every muscle group.
In fact, activation of the rectus abdominis was more than 30 percent less when training on the Swiss ball or balance cushion than on a stable surface. Instability training is less effective than traditional training for building strength in prime movers or trunk-stabilizing muscle groups.
Source: Journal Strength Conditioning Research, 27: 1101-1107, 2013)