The TRX device is basically a nylon parachute cord that you attach to a bar, doorjamb or tree limb. It is a simple training device that provides instability in a variety of motions that overload the core muscles. Swiss balls resemble large beach balls that are used to provide instability during exercise. Core muscle stability is critical for back health and spinal stabilization during exercise. Ronald Snarr and Michael Esco, from the University of Alabama and Auburn University, found that performing planks with the TRX device or Swiss balls provided greater loads to the core muscles than traditional stable planks. They measured loading with electromyography, which measures the electrical activation of muscles. TRX and Swiss balls are practical ways of increasing core fitness and spinal stability.
Source: Journal of Strength Conditioning Research, 28: 2298-3305, 2014