What's the Optimal HIIT Protocol for Trained Individuals? 48 x 10s or 8 x 60s for Conditioning + Improved Body Comp.?

Cycling and sprinting (running) are not your only options when doing HIIT. An intense plyometric or other body-weight based workouts can be done in an "80-95% for 10-60s" vs. "running on the spot for 20-120s" or with kettle-bells workout, as well. Be creative...

HIIT is "the hit" in the fitness world. And though it has been a hit for years, now, nobody seems to know how it is done "correctly", or, should I say, optimally!? How's that? Well, as in so many areas of this field, there's simply too little scientific data to make science-based recommendations that are "bulletproof" in the literal, not the new nutrition-and-fitness-craze sense of the word.

With their latest paper, scientists from China are trying to change just that (Chia-Lun. 2016). They set out to compare the effects of a matched-volume HIIT protocols w/ 10s or 60s all-out exercise and identical work-to-rest ratios (1:2) on - a comparison that has, believe it or not, not yet been done!
What did they want to do? Well, let's recap: The idea is that the researchers from the National Sun YatSen University and the National Taiwan Normal University will have 42 young, healthy, recreationally active male university students (at least 3x training per week), who are HIIT beginners, though, do work-equated HIIT protocols. And while the authors confirmed that both groups allegedly burned the same amount of energy, (only 213.7 and 226.5 kJ in the HIIT60s and HIIT10s group, respectively), it seems a bit odd that the 60s sprints were supposed to be done as intensely as the 10s sprints. The same goes for the (likewise measured) power during the sprints on the cycle ergometer, i.e. 232 ± 37 W and 249 ± 39 W for 85% VO2max and 90% VO2max, respectively, for HIIT 60s and 237 ± 45 W and 255 ± 47 W for 85% VO2max and 90% VO2max, respectively, for the HIIT10s group, where we likewise do not see a statistical significant difference, even though one may expect an advantage for the shorter intervals, too.

• HIIT10s - consisted of 48 × 10-s sprint intervals with 20-s recovery and 
• HIIT60s - consisted of 8 × 60-s intervals with 120-s recovery
• CON - just maintained their regular workout routines

Two things that were undoubtedly identical, though, were (a) the work-to-rest ratio of 1:2, and (b) the heart rate during the sprints (HIIT60s 182 ± 12 bpm and HIIT10s 184 ± 9 bpm). Similarly, all subjects trained three times a week (on Mondays/Wednesdays/Fridays or on Tuesdays / Thursdays / Saturdays) for four weeks. All workouts were done under supervision and at workloads equal to the subjects' predetermined 85%(week 1-2)–90%(week 3-4) VO2max after a 5-min warm-up and before 3-min cool-down period at 30% VO2max.

Figure 1: Relative changes in interesting markers of conditioning, hormone levels and skinfold (body fat); * indicates sign. pre- / post-difference; # indicates sign. difference to control group (Chiah-Lun. 2016).

Pre- and post-testing, which also included blood draws to analyse the concentration of lactate, blood glucose, lipids and testosterone and cortisol levels (body composition was determined by employing the sum of three skinfolds; the performance tests were identical for both groups and designed in a way that would allow for an unbiased (by training) comparison of the two regimens) yielded the following results:

• Cardiorespiratory Parameters and Performance - compared with CON, the HIIT60s and HIIT10s interventions resulted in 18.4% (d = 0.5) and 17.9% (d = 0.5) increases in VO2max, and non-significant increases in an endurance test, namely 7.7% (P = 0.01, d = 0.3) and 8.5% (P < 0.01, d = 0.3) in the HIIT60s and HIIT10s group, respectively.
  
  Similar increases were observed for performance markers at the aerobic threshold (AT), namely the O2 pulse (P < 0.01), the absolute VO2 at AT (P < 0.01), and the relative VO2 at AT (P < 0.01), as well as the AT as percentage of VO2max (indicating a sign. increase in the ability to use fat as fuel at high(er) intensities).
  
  As far as the question in the headline is concerned, the study did yet produce NULL results - a significant inter-group difference between the workload-equated HIIT protocols was not observed.
• Body composition - While there were no sign. group x time interactions, in the case of skinfold thicknesses in the chest (P = 0.86) or thigh (P = 0.31), the effect measured for time was significant: skinfold thickness decreased at posttraining (relative to baseline) in both the chest (P = 0.04, d at HIIT60s and HIIT10s were 0.3 and 0.2) and the thigh (P < 0.01, d at HIIT60s and HIIT10s was 0.9).
  
  What many of you will consider most interesting, though, is that skinfold thickness of the abdomen showed a significant interaction (P = 0.04), too. (Un-)fortunately (?) again without significant inter-group difference, just like the total percent body fat was significantly lower at posttraining, which decreased in both HIIT groups similarly, yet, again, without inter-group difference (P = 0.63, d = 0.2).
• Glucose and insulin management: In contrast to studies in untrained and/or obese individuals, significant improvements in markers of glucose or lipid management (even compared to control) were not observed - the levels of  insulin (d = 0.3), HbAlc (d = 0.4), HOMA-IR (d = 0.2), blood glucose (d = 0.2), TG (d = 0.3), total Chol (d = 0.2), HDL-C (d = 0.2), and LCL-C (d = 0.2), simply didn't change significantly from pre- to post in any group.

• Recent Study: HIIT training sheds significantly more belly fat over (-10% vs. 0%) and under (-25% vs. +10%) female abs than isocaloric medium intensity steady state exercise aka MICT | read more.

  Testosterone and cortisol: What did change, though, were the levels of testosterone and cortisol in the subjects' blood, where the authors detected (a) significant main effects of training for both testosterone (P = 0.01, d = 0.5) and cortisol (P = 0.04, d = 0.6), and (b) a significant T-increase only in the HIIT10s group.
  
  As you know from previous articles, though, we have to be careful not to overestimate the purported benefits of the 13.6% increase in testosterone the scientists recorded (even if it's not measured directly after the workout) - and that's not just because the cortisol levels in that group likewise increased (by 16%), but rather because that's too little of a temporary increase (levels in the control group increased by 9.7% as well) to have any physiological effect, even if the testosterone increases did matter (learn why they probably don't)

For the sake of completeness, it should also be mentioned that there were no differences in blood lactate levels, and blood pressure RPE in either of the group (pre-/post-testing and inter-group comparison yielded no differences that were worth mentioning, let alone statistically significant).