Gym balls are in fashion but are they effective? 

Over the last number of years, the gym ball has become a central component of most gym equipment collections.
Evidence for use of a gym ball to activate the ‘core’ muscles is sparse; however, research performed ar Dalhousie University’s School of Health and Human Performance compared the muscle activation of the upper abdomnal muscles during a series of abdominal exercises. They compared activation by performing electromyography. This is where a machine measures the electrical activity of muscles during exercises.
Kathryn Clark and her research team analysed the muscle activation during several abdominal exercises: the curl up, swiss ball curl up, ab roller curl up, leg-lower on back, ab roller curl up and Swiss ball roll-out to bridge.
The team found that the Swiss ball curl up resulted in the highest EMG readings when compared with the abdominal curl up from the floor.

Gym Balls for Core Exercises - Physio Clinic Naas