Why You Should Only Sprint for 15 Seconds - Understanding The Physiology Supporting the HIRT Protocol

(Third article in a three-part series.)

In mainstream fitness, we often misunderstand and misappropriate the term "sprinting." Per exercise physiology data, the human body is only capable of a true all-out sprint for around eight seconds. This is how much pure ATP-creatine phosphate energy is stored directly in the muscle cell to fuel a maximum explosive effort. The phosphate system can generate maximum power but is quickly exhausted. That's right, even Olympic 100-meter sprinters cannot generate full speed for the entire race; they are decelerating to the finish line after racing for around 10 seconds.

We often hear the term sprint used incorrectly to describe the work intervals of a HIIT workout, as in: "At spin class, we did 10 times 30-second sprints, with 30 seconds recovery." These "sprints" are actually a set of sub-maximal efforts of diminishing quality thanks to insufficient rest. Granted, HIIT workouts deliver many fitness benefits, but this is not sprinting. The terms High Intensity Repeat Training (HIRT) and Sprint Interval Training (SIT) are more accurate descriptions for workouts featuring near-maximum efforts followed by extensive rest.

While a true, completely anaerobic (without oxygen) sprint is only around eight seconds, there is good support for the idea that you can do near-maximum efforts of up to 15 seconds and obtain an excellent training effect emphasizing the anaerobic system. The key is that these near-maximum efforts are always paired with lengthy rest intervals. A long rest period after a short sprint enables you to repeat an effort of similar quality, a critical distinction from HIIT workouts that purposely limit the rest interval and extend the work period to prompt a different fitness adaptation.

Sprinting for 8-15 seconds feels great and allows you to generate near-maximum power and explosiveness. If you were to try to continue an all-out effort for longer than 15 seconds, you would experience diminishing returns as the stress impact of the workout skyrockets for minimal extra fitness adaptation. According to Dr. Craig Marker (the author of the HIIT versus HIRT article discussed in the previous posts in this series) and others, prolonged sprinting triggers the cellular processes of disassembly and deamination in order to supply more ATP for maximum energy output. Dr. Marker describes this disassembly and deamination process as "breaking down the A-frames of your cells." The vaunted benefit of mitochondrial biogenesis that you get from sprinting gets put on hold, ammonia builds up to toxic levels, and you essentially fry your cells to get to the finish line.

Granted, this is the object of the game in a competitive event like the 400 meters: you sequentially exhaust the anaerobic energy systems of the body and try to decelerate less than the competition. But it's best to save these types of efforts for serious racing, and train in a manner where you can recover quickly from workouts and steadily progress without the interruptions caused by exhaustive workouts.

Here's what's happening physiologically over the duration of a near-maximum-intensity sprint of any kind (running, cycling, rowing, or kettlebell swings). During the first five seconds of your sprint, lactic acid starts to accumulate in the bloodstream. Levels double between five and ten seconds, then double again from 10 to 20 seconds, up to what Marker calls the highest acceptable level. As you increasingly feel the burn, levels double again from 20 to 30 seconds, and double again from 30 to 60 seconds. This results in the aforementioned cellular destruction, ammonia toxicity, and extended recovery time.

As Marker explains, "The amount of lactic acid produced up to 20 seconds [of sprinting] is still manageable, but the next doubling is over the top. Even a single 30-second sprint spikes ammonia levels almost five times! Why trash the body for no good reason? Rebuilding broken down cells is a costly and time-consuming process. And while it's taking place, you feel tired and run down, with your ATP short of a full stack."

Marker observes with irony that the catabolic nature of stressful HIIT patterns featuring this type of cellular breakdown can result in significant weight loss: along with a bit of fat loss, you break down lean tissue too, leaving you looking emaciated as a consequence of the cellular destruction. You may get some props at the gym for your slimmer physique, but Marker straight up calls this "sick!"

To transition into a more effective, less stressful high-intensity workout pattern, pick the sweet spot of 8-15 seconds for your explosive efforts. Observe a minimum 6:1 recovery-to-work ratio. This ensures that your cells have a chance to partially or fully regenerate ATP (takes around three minutes) and minimizes the disassembly and deamination that occur when you ask your body to perform again and again with rapidly depleting cellular energy. Finally, only do 4-8 sprints. You should be able to manage four short sprints even if you are a novice. If you claim you can complete more than eight and feel great, you are better off going faster and doing fewer, more explosive reps.

A properly conducted HIRT workout is going to feel different than a HIIT sufferfest, and may require an adjustment in your mindset to feel confident and satisfied that you are training with maximum efficiency and minimal suffering, like a real athlete. If you're a focused, driven, goal-oriented type, you must be vigilant to resist the addictive allure of the endorphin rush that happens after a sufferfest. Remember, the blissful feeling of powerful painkilling chemicals flooding your bloodstream after a sufferfest is a fight-or-flight reaction to the extreme stress of the workout.

Realize that the genetic purpose of the endorphin response is to help you continue to run for your life instead of lying down in exhaustion! If you abuse this delicate mechanism with a chronic pattern of extreme workouts, you are going to fall apart. Dr. Tommy Wood calls this overactivation of the fight-or-flight response "liquidating your assets." It feels great to drive off the showroom floor in your new Tesla, but when you prioritize the Tesla purchase over your high-interest credit card debt, you pay the price later.

Go out there and try some HIRT workouts and let us know how they feel. Many people discover that they feel much better in the days following a properly conducted sprint session: more energy, and less soreness and stiffness than after a sufferfest HIIT session.