If you're looking to rev up your fitness routine, interval training may be your ticket. You can get more out of that spin class, run workout, or lap swim by alternating short bouts of high-intensity exercise (yes, you have to work hard!) followed by a few minutes or seconds of rest. By doing this, you stress out your cardiovascular system and build up lactic acid in the muscles—which boosts body fuel, strength, and stamina—while then letting yourself recover and prepare for the next tough interval.
The trend toward interval exercise to gain cardiovascular benefits isn't new for seasoned athletes, but the technique is gaining popularity among general fitness buffs looking for big gains in a short amount of time. And for good reason.
Evidence shows that with high-intensity interval training, participants can increase their maximum aerobic capacity—how well their body uses oxygen for energy at their greatest heart rate—higher than those who participate in a continuous exercise program, such as going for more than a 20-minute run, bike, or swim at a steady, moderate pace. The more oxygen your body can convert to energy, the stronger and faster you become. As your body adapts to the stress of interval training, your fitness level improves along with your muscle function.
But before taking your routine up a notch and risking injury, it's crucial to build a base fitness level first, notes Ed Coyle, director of the Human Performance Laboratory at the University of Texas at Austin. For healthy people, this means typically doing 40 minutes of exercise three to four times a week for eight weeks, gradually working up to a higher heart rate level (220 minus your age is the predicted maximum heart rate, beats per minute).
Once you're ready to begin intervals, you should only do them every other day, says Coyle: "It takes 24 to 48 hours to recover from intense interval workouts." And because you'll be burning a lot more carbohydrates than you typically would, be sure to refuel after your interval workout by eating an extra 300 to 400 grams of carbs, in additional to your regular meals and snacks.
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Mary Bradbury, USA Triathlon certified coach and founder of Illinois-based Bradbury Fitness, recommends her clients work on their technique when trying out any form of exercise—especially interval training. "Any sort of muscle imbalance will accelerate and be cumulative and increase chance of injury if you do speed work without having the proper form," says Bradbury.
For example when doing a running interval, Bradbury suggests starting out with short 20 to 30 second stride runs (gradually picking up of your pace to 90 percent effort) once a week. This helps maintain good technique, since you typically run better as you run faster. It can take a few months to get used to doing these and for your body to adapt.
Then when you are ready to add intervals, run for one minute at a fast, intense pace (trying not to sprint), then rest for a minute, repeating the cycle five times. The following week, either increase the length of each interval to 70 seconds, or repeat the cycle six times instead of five. Continue building up weekly. Rather than sprinting all out during the intense phases, do them at a strong pace you can sustain throughout. "The magic formula is pacing on all intervals. The last thing you want to do is go several seconds slower on the last few because you went too fast on the first ones," says Bradbury. You can apply this type of interval workout to any form of exercise you do, be it running, biking, swimming, or cross-country skiing.
How soon will you see and feel results? It will depend on if you are doing a single sport, such as running, or if you are doing more than one. If you're running with correct form, you will see results fairly quickly, rather than if you are doing multiple sports. And you don't have to go all out either. One study, for example, found that moderate-intensity intervals are just as effective as high-intensity intervals.