Bursts of high intensity activity mixed with aerobic, steady state activity are proven to enhance calorie burn and extend the length of time the body sustains elevated metabolic activity after a workout. With repetition this translates into an increase in whole body capacity for carbohydrate and fat metabolism – more permanently tuning metabolism towards a lean build. These biologic realities are the basis for dozens of fitness books and articles asserting you can achieve superior results in a shorter work-out.
Fitness fad or fact? It is fact that if you have a fixed amount of time set aside and you add higher intensity intervals to your current routine you will achieve a more demanding workout and better results in the same length of time. For a particular person and form of exercise it is possible to calculate when shorter bouts of training deliver similar effects. The simplest approach is using your heart rate monitor to time when you reach the same calorie burn with intervals added compared to a steady state workout. But why short yourself? If you have the time, give it all and get even more in return.
No special recipe is required – just mix in intervals. Running or biking? Add sprints aiming for maximal intensity along with longer intervals that bump up your effort to 80 to 90% of maximum. To maximize results you want to increase intensity both within individual training sessions and across all sessions you plan in a week. Some workouts should be hard and some easy. Evaluation of total training time and intensity for elite endurance athletes reveals they average 80% low to moderate intensity and 20% high intensity. If you are just starting out work up slowly: 20 to 60 seconds at high intensity followed by 6 to 8 minutes at moderate. Allow your heart rate to come back into the middle of your aerobic zone and aim to shorten recovery time between intervals every week or so. If you are fit, high intensity work-outs can be as demanding as one-to-one ratios for shorter sessions and 20 minutes or more out of an hour for longer session. Remember too much of a good thing is still too much. Aim across the week to follow the 80:20 rule meaning about 20% of total effort at high intensity.
References:
Perry CG and colleagues. High-intensity aerobic interval training increases fat and carbohydrate metabolic capacities in human skeletal muscle in Applied Physiology, Nutrition and Metabolism 2008 Dec;33(6):1112-23.
Seiler S and Tonnessen E. Intervals, thresholds, and long slow distance: the role of intensity and duration in endurance training in Sportscience 2009 (13):32-53.
K. Hartmann, MD, PhD