There are already plenty of apps that can help you tone your body and build a stronger core. If you need a portable machine to push your body harder, you may want to give the CoreXT46 a look. It is a 4 minute ab trainer that activates 46 core muscles in your body and offers up to 200 lbs of resistance.
The trainer comes with a smartphone holder, so you can keep up with your iPhone workouts. The CoreXT46 can give you a workout in standing, seated, and supine positions. Its upper cartridge has up to 60lbs resistance while the lower part has up to 70lbs resistance. The app has video instructions and 2 workout modes.[where to get it]