Interval training is a personal favorite of mine so I’m excited to finally get around to writing a post on the science of it. I’ll be giving a summary of this article published in the Exercise Sport Science Review (2008). In this piece, researchers put participants on a two week High Intensity Interval Training (HIIT) regiment in which they would exercise three times per week.
The basic idea behind HIIT is performing short bursts of maximal effort cardiovascular work. It’s different from traditional cardio work in that the focus is on going as hard as you can rather than as far as you can. As a personal trainer of my favorite HIIT patterns to run with my clients is a 30-seconds on 30-seconds off sled push drill. In this case I’ll typically run 8-12 rounds of maximum effort on the 30-30 pattern. Off the bat, you can see a major difference between HIIT and traditional cardio. If you add up the work time, there is actually only 5 minutes of activity in a 10 round set. Other models do even less, a 30s work-4min rest pattern done 6-8 times is common in the represented studies as well. For people with busy schedules, this sounds like a miracle drug. 5 minutes of work to get the benefits of 30 minutes of cardio? It almost sounds too good to be true.
This study found that after the two week exercise intervention (2 weeks/6 HIIT sessions), participants nearly doubled their capacity to maintain a cycling pace at 80% of maximum effort (from 26 to 51 minutes). Maximum effort was calculated using the pre-intervention oxygen capacity of the participant. The control group which did not exercise at all saw no change in the two weeks between endurance tests. In this test, researchers did not see an improvement in oxygen uptake which suggests the improved efficiency came from secondary systems. They do note however that other studies with more intense HIT protocols did see increased oxygen capacity in their participants.
In a separate study conducted by these researchers, participants on a two week HIIT program were compared against participants on a traditional cardio program, where they would cycle at 65% max effort between 90 and 120 minutes daily. In this study, the HIIT group spent 2.5 hours exercising over the two weeks while the endurance group spent 10.5 hours in the gym. At the end of the study, researchers observed “remarkably similar adaptations” in the muscle size and efficiency of the participants. Additionally participants in the the HIIT program saw positive metabolic changes typically associated with traditional cardiovascular work.
As a personal trainer from my experience, HIIT has a lot of mental benefits as well. When I work with new clients, I’ll ask them about their fitness history. What exercises have you done in the past? Have you ever worked to exhaustion? Have you ever pushed as hard as you can? Nine times out of ten, the answer to those last two is a “no.” Maximum effort is uncomfortable and people don’t like that. HIIT training pushes people outside the bounds of comfort and unlocks potential they didn’t realize they had before. Talk to some of my more experienced clients and they’ll tell you that sprinting just feels good. It’s empowering, it’s explosive, it makes them feel strong. Maximum effort work shows people a new side of their body’s capabilites. For this reason alone, I’ll continue to incorporate HIIT into my programming.
All this being said, HIIT is another tool in the belt and isn’t the be all, end all for fitness. Steady state, long distance cardio work has its place in the fitness world and has clearly shown benefits for heart health, hormone levels, metabolic function, and overall well being. Resistance training improve neurological function and muscle adaptations more linearly than HIIT will ever be able to. When it comes down to time limitations (such as a one hour session with a trainer) getting the most bang for your buck is essential and HIIT is about the biggest bang you’re going to get.