A 20-minute workout gives you enough time to push yourself as hard as you can without feeling like you just scaled Mount Kilimanjaro. When you only have a short amount of time, the trick is to increase your intensity to get results, says Adam Ernster, a personal trainer. The reason it works? You'll speed your metabolism so you can burn even more kilojoules after your workout.

OPTION 1: change your muscle circuit

Build muscle to burn kilojoules. "For every half kilo of muscle you gain, you burn on average about 250 kilojoules a day maintaining it," Ernster says. (Don't worry, though: those extra kilos of muscle look a lot smaller on you than fat.) To build muscle in less time, Ernster suggests changing the order of your exercises. "It will keep your body from adapting" and make it work harder, he says. You can also make exercises more complicated to make your muscles work harder.

For instance, perform standing moves on an unstable surface like a foam pad or a Bosu ball (sportstek.net) to challenge your core-stabilising muscles, or add complex exercises like squat shoulder presses to work more muscles (in this case, your whole body) at once. When you perform the shoulder press, use one arm at a time to make it even more intricate. "The added bonus with moves like these is that when you work large muscle groups, you burn more kilojoules in the same amount of time," says Dr Nancy Cummings, a research scientist at the USA Triathlon National Training Center in Florida.

OPTION 2: up your cardio intensity

By increasing the friction between the treadmill belt and your feet, you can make a cardiovascular workout much harder - and more effective. Try this killer butt-toning trek from fitness coach Gregory Joujon-Roche. Walk on a treadmill for two and a half minutes, gradually increasing your speed to around 5km/h. Next, raise the treadmill to its steepest incline and set your speed to a walking pace that's almost jogging (about 6km/h). Keep your body low and extend your stride, flicking your back foot up at the end. Keep your feet on the treadmill as long as possible to create friction, and focus on feeling your glutes. Hold the handles of the treadmill, and use your ab muscles to control your body. Keep this going for 15 minutes, then reduce your speed and incline to cool down for two and a half minutes.
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