If life is too busy for you to remember what days to do strength training and which to do cardio, or if your metabolism is slowing down and you really want the most out of your  workout, say "I."

“I” is for interval. 

Interval training comes in different forms, like when you alternate jogging and sprinting when you run in the park. But what I teach at Crunch fitness in Manhattan, and in my fitness DVDs, is take this a step further. By combining fast and slow cardio with multitasking muscle moves in one workout, you won't have to worry about doing legs one day, arms the next, and cardio the third. You really do get everything in one workout, and five very special benefits.

Appetite: Recent studies in the Journal of Sports Science and Medicine show that subjects who do cardio and muscle toning intervals eat 500 less calories per day after four months than those who did cardio one day and all muscle toning the next. They say the combination can cause a change in hormones that control your hunger.

Stress: The same study showed that being active reduces cortisol levels that are caused by stress.

Endurance: In biking or “ride” classes, the study showed that short high-intensity sessions, sprinkled with recovery instead of steady state pace, boosted endurance.

Metabolism: With interval cardio and muscle training sessions, you can also get the benefits of  increased metabolism. If you up the intensity of the workout through alternating cardio and muscle moves, and you increase excess post-exercise oxygen consumption. This is what we trainers call “after burn.” Resistance training builds metabolism-boosting muscle while cardio makes the fat over the muscle disappear. Then after your workout you will continue to burn more calories while reading, watching TV and sleeping.

Better toning: According to a 2008 study published in the Journal of Strength and Conditioning Research, women whose muscle routines only included weightlifting gained less muscle strength, added less muscle endurance and lost less body fat compared with those who ran for a minute before each strength set.

Fight osteoporosis: We all know those people, women especially, who only diet and run, bike and do cardio machines. They think skinny is better than toned, sleek, healthy and strong. A hump on your back or a broken hip doesn't look good at any age.

Young women are more prone to stress fractures because of reduced calorie intake and tons of cardio. They don't realize that it's not about being skinny as the Olson twins is not sexy. It's about looking toned, and being strong for everyday life. God forbid you fall into a situation where you need physical strength to save your life, you want to be ready for it. Lifting weights builds strong bones. Use these exercises to improve your posture, defend yourself and walk without a limp. Now that's sexy. 

So overcome workout boredom and make the hour fly by. Alternate between fast and slow cardio and multitasking muscle moves that work arms, abs and legs at the same time.

Working out is the most important thing you can do for yourself, and there should always be an "I" in fitness.

To read more NikkiFitness tips, read her blog on Military.com's Fitness channel.