HIIT vs SIT Intervals: How To Find Your Best Mix

There are 1001 different intervals, and each athlete and coach has their favourite. Does a 4×4 interval get you the same fitness changes as Tabata-style sprint intervals, or is it a case of horses for courses?

Interval Hodge-Podge

Whether you ascribe to polarized training, pyramidal training, or sweet-spot training, ultimately all different training philosophies come down to mixing up easy training with hard training in various combinations and percentages. Regardless of the style of training you go with, this means that there will come a time where you put in a really hard effort, typically involving a race or an interval workout.

Interval workouts, as the name suggests, involve going hard for a certain period of time, interspersed with recovery periods at lower intensities. By doing so, you can accumulate much more volume of work at high intensities than you can if you just did a single hard effort.

In turn, two common styles of intervals are longer duration efforts at an intensity slightly above (e.g. 110-120%) above your threshold, or else Tabata-style sprint intervals consisting of many brief periods (10-30 s) of very intense effort (150-200% threshold) with equally brief (10-40 s) of recovery.

An example of a HIIT (High-intensity interval training) workout involving 4×4 min at 117% of threshold power.

An example of a sprint interval training (SIT) workout consisting of 3x13x30 s at 115% threshold power. Note that this works out to 19.5 min of intense effort compared to 16 min for the same overall workout time, such that proponents for SIT often tout the greater volume of intensity as a benefit of this interval style.

HIIT vs SIT

Every athlete is interested in what works best, so a natural question is whether there is an optimal interval workout. Comparing HIIT to two different variations of SIT on overall fitness changes was the focus of a Norwegian study (Hov et al. 2022). Here’s the basic experimental setup:

• 48 healthy non-smoking males.  This study excluded females to enhance physiological homogeneity and training status. They were aerobically-trained (VO2max > 50 mL/kg/min) who did endurance training at least once/week, but were not specialized or competitive runners.
• Pre/post tests included VO2max, running economy, lactate threshold, maximal accumulated oxygen deficit (a measure of anaerobic capacity), hematological measures, a 300-m sprint, and a 3,000-m time trial
• The participants were split into three types of intervals, completing the following 3x/week for 8 weeks. The number of finishers for each training group is in brackets).
• HIIT (n=10). 4×4 min at 95% of Maximal Aerobic Speed or MAS (Think of MAS as analogous to the power output at VO2max), with 3 min active recovery at 70% of HRmax.
• SIT 8x20s (n=12). Target of 8×20 s intervals at 150% of MAS with 10 s of passive rest. The aim here is to reach exhaustion during the 8^th or 9^th If a 9^th interval was completed, the speed was increased at the next session.
• SIT 10x30s (n=9). Target of 10×30 s with 3.5 min active rest at <70% HRmax. The starting workload for day one was set at the average speed from their initial 300 m run test. This worked out to roughly 175% MAS.

Three Sides of the Same Coin?

All three interval types were extremely tough. It was good that the participants were already aerobically fit and used to treadmill running, because the 3x/week of intensity would’ve caused much greater dropout in lower-fit individuals due to fatigue or injury. What were some of the major findings?

• HIIT and SIT8x20 groups increased VO2max, with a higher VO2max increase in HIIT. No changes were seen in SIT10x30.
• SIT8x20 had an 11.6% increase in maximal accumulated oxygen deficit, with no changes pre/post in HIIT or SIT10x30.
• In blood measures, bicarbonate concentration (a marker of lactate buffering capacity) increased with HIIT by 6.9%, with no changes in SIT8x20 or SIT10x30. No changes in red blood cell number, hemoglobin or hematocrit were seen with any of the groups.
• All three groups improved 3,000-m TT performance, but the improvement was higher in HIIT (5.9%) than with either SIT8x20 (4.1%) or SIT10x30 (2.2%).
• SIT8x20 (4.4%) and SIT 10×30(3.3%) improved 300-m performance but not HIIT.

Horses for Courses

Ultimately, what this study suggests is that the optimal interval program depends ultimately on what you’re looking to improve, the classic “horses for courses.” If your goal is to maximize aerobic gains and VO2max, then it’s likely that the longer-duration intervals like 4×4 is the way to go. There is, overall, consensus that the 4-8 min duration at maximal intensity is right at about your VO2max, so it is logical that this would be the interval program with the greatest stimulus to VO2max.

However, if you’re targeting anaerobic capacity, then it is likely that SIT-type efforts are the way to go. The much higher intensity places greater stimulus on anaerobic metabolism.

Remember also that you don’t have to blindly stick to one type of interval. Your choice might depend on the limiter you’re addressing, the race you’re targeting, or the time of the year. This is where sound planning is required, and where block periodization might be used to systematically target one type of interval for a period of time.

Have fun and ride fast!

References

Hov H, Wang E, Lim YR, et al (2022) Aerobic high-intensity intervals are superior to improve V̇O2max compared with sprint intervals in well-trained men. Scandinavian Journal of Medicine & Science in Sports n/a: https://doi.org/10.1111/sms.14251