Invite MMA World Champion Georges St-Pierre into your living room, by way of his iPhone app Touchfit: GSP (free; subscription $3.99 per month or $9.99 per year), and there's little chance you won't wake up sore tomorrow morning. This total body workout app paces you through rather intense 20-, 40-, and 60-minute workouts. As you use it, you mark off whether the exercises were too tough or too easy, and the app customizes your future routines so that you never hit a plateau, but also never have to do moves that are impossible. I wouldn't recommend it to beginners, but anyone looking for a challenge in their living room or gym will find it pushes just the right buttons.

In testing the app, I stuck to 20-minute workouts. Each one includes a few minutes of warm up and cool down, sandwiching the main workout, which includes moves that tested my strength, flexibility, cardio stamina, and balance. I squatted on one leg, shadow boxed, crunched my abs, and much more, until I was ready to hit the floor.

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After each exercise, you tell the app whether the move was easy, tough, impossible, or difficult but worth learning. The app uses that feedback to create better routines that challenge you without leaving you dead in a pool of your own sweat.

If you try to complete two workouts too closely together, the app warns you that it's not advisable, saying your body needs time to recover. It will even tell you how many hours you should wait since your last workout. Those details really make this app worthwhile.

I really love that Georges St-Pierre, in all the videos that show you how to do each move (they play as you workout), is totally barefoot. You don't even need sneakers for his workouts. It helps to have a mat for some of the floor exercises, and a couple of moves require a resistance band, but you can always mark those exercises "impossible" if you want to leave them out in the future.

The app includes very detailed videos of each exercise so that you can learn exactly how to do them. It also has an included music player so you can stream in your own playlist while working out.

Considering so much video content is included, the 27.7MB size of the app seems pretty reasonable. Some of the videos are stored offline until you need them, though. In comparison, GAIN Fitness is a whopping 64MB download. If you need something lighter, try Sworkit Pro (99 cents), which is only 14.6MB.

Touchfit: GSP doesn't seem suitable for beginners or people who are exercising for the first time in their lives. The workouts are hard. One app that I think is much better for beginners is The Johnson & Johnson Official 7 Minute Workout App (free). You can tell the app that you're a beginner and that it should be gentle with you for those seven minutes—or longer if you choose another workout from its options.

I like Touchfit nearly as much as our Editors' Choice GAIN Fitness (free; specialty workout packs available as in-app purchases). If you like to try before you buy, go for GAIN Fitness and see if you ever tire of the total body workouts that come included with the free app, and spend a little extra if you want to add a specialty workout, such as yoga or prenatal Pilates.