Intervals are a popular method of performing cardio. In addition to being an effective way of developing anaerobic metabolism, which is the major reason people perform brief intervals, many people do intervals as a means of saving time. Because of the increased energy demands of an all-out sprint, exercise doesn’t need to be performed very long to at least feel like you’ve gotten a great workout.
But for overall effectiveness, there might still be reasonable doubts, not so much about the ability of intervals to work the anaerobic system, but instead about their effects on the aerobic system.Researchers looked into this topic in a recent study published in the Journal of Strength and Conditioning Research. They wanted to find out to what extent cardiovascular factors outside of just anaerobic contributions were worked by high intensity sprinting intervals.
One good reason for a study looking into these factors is the time savings itself, since this is one big reason to perform sprint intervals. But when thinking about this, you also have to consider what you’d be doing instead that might take longer. Since the anaerobic systems of the body have the biggest kick at the beginning of exercise, they don’t take long to push hard and develop progressively over time. The aerobic system, on the other hand, can sustain heightened energy outputs for hours on end, so this would be where sprints would save time. If sprinting interval programs work well for developing the aerobic system over a brief workout, then it may be worth it for the time-crunched among us.
In the study, the participants performed four all-out sprinting bursts on an exercise bike, each separated by four minutes of active rest. Active rest is typical in these types of maximum-intensity intervals. The down time is occupied by light activity without much or any resistance. In this case, the light activity was performed on the exercise bike with the resistance turned down to zero.
As mentioned before, sprinting is good for developing anaerobic metabolism, so the researchers assumed this as uncontroversial. Instead they looked at a few other performance factors associated more with aerobic work, such as VO2 max (the ability of your body to absorb, transport, and use oxygen), heart rate, and ventilation. Sure enough, these factors were all taxed by the sprinting intervals as well. The participants achieved 80% of their maximal values despite the small amount of actual hard work.
The results of this study point to the importance of the aerobic systems, even in such intense and brief sprints. The researchers hypothesized that the levels of cardiorespiratory stress achieved by this protocol were sufficient to potentially elicit aerobic benefits, which is pretty cool.
However, I’ll warn against misinterpreting this information. First, the tested program totaled eighteen minutes in length, including the four-minute rest periods. With the warm up included, the time goes up to about 23 minutes for this protocol. This isn’t all that much of a time savings compared to a brief warm up and two mile run that most people could achieve in the same amount of time. Since cardiorespiratory values were hit only briefly in this study, it’s possible that a two-mile run might be just as time-efficient, but a greater stimulus for other aspects of athletic improvement. Intervals are great for anaerobic work, and may even help a little bit with your aerobic work too, but ultimately, variety is key.