Stay off the exercise balls

Canadian scientists reported that doing dumbbell chest presses on an exercise ball resulted in a 40% decrease in strength compared to doing the exercise on a bench. And a study from Appalachian State University Boone, North Carolina found that when athletes performed squats on instability discs, they had less force production and muscle activity of the quadriceps compared to when they did squats on firm ground. Since any marked increase in strength or muscle mass is going to come through moving progressively heavier weights on key lifts, you might want to avoid exercise balls and instability boards. Plant yourself for better gains.