One of the best ways to cross-train is to use a different cardio machine each time you go to the gym. Unless you're on a specific type of program, such as walking or bicycling, then changing machines is a great way to work your heart, burn calories, and keep yourself interested in your cardio training. More variety means less boredom!

In terms of calorie burning and heart-pumping effectiveness, the elliptical trainer is your best bet. It is low-impact, but it allows you to burn more calories and work your heart harder than a walk would, be it on the road or on a treadmill. Most people find the elliptical trainer fun and easier to use, and they don't feel the intensity of their workout as much as they do on other machines, especially rowers and stationary cycles.

Most personal trainers will also tell you that another great machine is the rower, which works all of your body efficiently and strengthens the heart. Unfortunately, most people can actually feel how hard the rower makes you work, unlike with the elliptical trainer, which allows you to work hard but not feel your effort as intensely.

You can switch machines during one workout, or alternate machines each time you head to a gym. For example, here are two sample cardio cross-training exercise sessions:

• 5-minute warm-up: Start the treadmill at 3.0 mph and go up to 3.5 mph.

• 20-minute interval program: Use the elliptical trainer at an RPE between 5 and 8.

• 5-minute steady state: Use the rower at an RPE of 6.

• 10-minute cooldown: Use the stationary cycle, going down to an RPE of 2–3.

• 5-minute warm-up: Stationary cycle at an RPE of 2–5.

• 5-minute steady state: Use the stationary cycle at an RPE of 6.

• 15-minute hill-climbing program: Use the treadmill with the ramp going up at an RPE between 6 and 8.

• 10-minute steady state: Use the elliptical trainer at an RPE of 6.

• 5-minute cooldown: Use the treadmill, going down to a slow walk.

Don't worry about looking funny moving from machine to machine. Real gym rats practice cross-training programs more than they stick to one machine over time. It's much more effective to challenge your body with variety.