Whether your clients are lawyers, soccer moms or dads or businessmen or women, they all want to make the best use of their workout time. Although organizations like the American Heart Association, the Centers for Disease Control and Prevention and the American College of Sports Medicine recommend at least 30 minutes per day of moderate- to high-intensity aerobic exercise, many people lack the time or the inclination to meet these recommendations. So, how can you make your clients' cardio workouts more efficient and obtain the greatest benefit in the least amount of time?

    If your clients only have time for a few cardio workouts per week, 15 or 20 minutes of steady-state cardio on the elliptical trainer or stationary bike at an intensity easy enough to allow you to read a magazine while exercising isn't going to cut it. The fewer workouts your clients do, the greater each workout's quality should be.

    In order of intensity, here are the most effective cardio workouts for improving cardiovascular fitness and burning fat. Have your clients do all three of these cardio workouts each week (see Sample Cardio Program).

    Long, Slow Distance (LSD) Workouts

    What: As its name implies, LSD workouts are long, continuous aerobic workouts at a low intensity during which you cover a great distance.

    Why: LSD workouts cause many adaptations that increase your clients' aerobic fitness, including an increase in the number of red blood cells and hemoglobin. Since hemoglobin in red blood cells transports oxygen through blood vessels, the more red blood cells and hemoglobin your clients have, the greater their blood vessels' oxygen-carrying capability. LSD workouts also stimulate the storage of more fuel (glycogen) in your clients' muscles, create a greater capillary network for a more rapid diffusion of oxygen into the muscles and, through the complex activation of gene expression, increase the muscles' mitochondrial density and the number of aerobic enzymes contained within them, increasing the muscles' aerobic metabolic capacity. These cellular adaptations following LSD workouts enhance your clients' muscles' ability to conserve carbohydrates and rely on fat as fuel so they become better fat-burning machines.

    How: Clients can use almost any piece of cardio equipment to do LSD workouts, including a treadmill, bike or rowing machine. They can also run or cycle outside or swim in the pool. The more muscles involved and the more weight-bearing the exercise, the more calories they'll burn during the workout. Whichever mode of exercise they choose, they should do these workouts at about 65-70% maximum heart rate for at least one hour, increasing the length of time as their training progresses.

    Tempo Workouts

    What: Tempo workouts are performed at the intensity corresponding to the lactate threshold, an important physiological variable that demarcates the transition between exercise that generates energy almost purely aerobically and exercise that includes energy generated from both aerobic and oxygen-independent (anaerobic) metabolism. While your clients use both fat and carbohydrates for energy during exercise, these two fuels provide that energy on a sliding scale -- as they increase their intensity up to their lactate threshold, the contribution from fat decreases, while the contribution from carbohydrates increases. When they exercise at an intensity above their lactate threshold, they use only carbohydrates.

    Why: Tempo workouts raise the lactate threshold to a higher percentage of maximum heart rate, which delays fatigue during aerobic exercise. Since tempo workouts are performed at the highest intensity your clients can sustain aerobically, these workouts are great for burning fat. Although the percentage of calories from fat is small when exercising at or slightly lower than the lactate threshold intensity, the total number of calories being burned per minute is much higher than at a lower intensity. Research has shown that exercising at or slightly below the lactate threshold elicits the highest rate of fat oxidation.

    How: Like with LSD workouts, your clients can use any piece of cardio equipment for tempo workouts. Lactate threshold intensity is approximately 75-80% maximum heart rate for unfit people and about 80-85% for those more aerobically fit. Subjectively, these workouts should feel "comfortably hard." Your clients can do tempo workouts continuously for about 15 to 20 minutes or as shorter segments (e.g., 5 minutes) with short recovery periods (1 minute).

    Interval Workouts

    What: In the 1960s, famous Swedish physiologist Per-Olaf™strand discovered that by breaking a set amount of work up into smaller segments or intervals, you can perform the whole set of work at a higher intensity. Interval workouts alternate high-intensity work periods with low-intensity recovery periods. There are four variables that can be manipulated within an interval workout: time (or distance) of each work period, intensity of each work period, time of each recovery period, and number of repetitions.

    Why: Long, aerobic interval workouts target your clients' cardiovascular systems and increase their VO2max by increasing their maximum stroke volume (volume of blood the heart pumps with each beat) and cardiac output (volume of blood the heart pumps each minute). Short, intense interval workouts increase your clients' anaerobic power and capacity by calling on their anaerobic metabolic pathways that don't use oxygen. Since short, intense intervals recruit fast-twitch muscle fibers, they also complement your clients' strength training workouts by adding to their muscle definition and size. Although all interval workouts keep metabolism elevated for a few hours after the workout compared to LSD workouts, the more intense the workout, the greater and longer the post-workout elevation in metabolism, which burns more calories the rest of the day.

    How: Your clients can do interval workouts on a treadmill, bike, rowing machine, or outside on a running track. For long intervals, run, cycle or row for three to five minutes at 95-100% max heart rate with recovery periods equal to or slightly less than the time of the work period. For short interval workouts, run, cycle or row at slightly less than an all-out sprint for 20 seconds to a minute with recovery periods two to three times as long as the time of the work period.
    Time is a valuable commodity. If your clients don't have the time for cardio every day but still want to improve their cardiovascular fitness and lose fat, have them try these three fitness-inducing, fat-burning cardio workouts. With all of the time they'll save, they'll even be able to watch their kids at soccer practice.

    Dr. Jason Karp is a nationally-recognized speaker, writer, exercise physiologist and owner of RunCoachJason.com, a state-of-the-science running coaching and personal training company. He holds a Ph.D. in exercise physiology and is founder and coach of REVO2LT Running Team and Dr. Karp's Run-Fit Boot Camp. He writes for international magazines and scientific journals, is the author of several books, including the forthcoming Women's Running Bible (Human Kinetics, 2012) and is a frequent presenter at national fitness and coaching conferences.