Sex, HIIT & Perceived Readiness: Any News on the Optimal Rest Times for Self-Paced HIIT Regimen in Men & Women?

Surrender bro, women are tougher than we'll ever be... and let's not talk about the other tactics by the means of which they trick us into doing whatever they want without us even noticing :-o

In the world of search engines for scientific papers on training and exercise science the acronym "HIIT" is currently what the word "sex" has always been on Google & co. Against that background it is actually surprising that no one else but me has taken notice of a paper on the "Sex specific  responses  to  self-paced,  high-intensity  interval  training  with  variable  recovery periods". The corresponding research was conducted by C. Matthew Laurent et al. from the School of Human Movement, Sport and Leisure Studies at the Bowling Green State University in Ohio and the paper is about to be published in an upcoming issue of the Journal of Strength and Conditioning Research (Laurent. 2013)

Men are different, women too

The results of previous experiments suggest that women are tougher than men, when it comes to steady state high intensity exercise at a self-selected pace. In their most recent study that involved 16 subjects (8 men and 8 women) between 19 and 30 years of age who had been participating in at least one session of interval training per week within the past months, Laurent et al. set out to test whether this would apply to HIIT sessions with fixed rest periods, but also variable, self-selected intensities. To this end, they had their subjects perform three bouts of HIIT.

"Each session consisted of 6, 4-minute intervals interspersed with either 1, 2, or 4 minutes of recovery. The recovery duration was counterbalanced and subjects  were  informed  of  the  specific  work-to-rest ratio  prior  to  performing  each  session.   Each  trial began  with  a  5-minute  warm-up  that  consisted  of  walking  4.8  km/h at  5%  incline." 

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The subjects were told to set the treadmill to the highest possible speed they felt they could maintain for 4 minutes knowing they were to perform 6 intervals.

The treadmill remained elevated at 5% incline for the duration of the whole session. Prior to each interval, subjects estimated their level of readiness using a perceived readiness scale.

Throughout and at the end of each interval, VO2(ml/kg/min), heart rate (bpm), and rate of perceived exertion (RPE) were measured, recorded and statistically processed.

At the conclusion of the fourth minute, the treadmill was slowed to 4.8 km/h for an active recovery.

"These procedures were followed identically for each of the 6 intervals and across all 3 trials. At the conclusion of the final interval of each session, subjects were disconnected from the metabolic system and sat quietly in a chair in the laboratory for approximately 15-20 minutes whereupon they provided a session RPE (SRPE) using the OMNI scale. "

All subjects were given at least 72 hours but no more than 10 days of rest between HIIT sessions, at the end of which the scientists had made the following observations:

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  Men ran at significantly higher relative velocities (i.e., %VO2peak) during the  1-minute  recovery  trial  with  the  effect size suggesting a large difference.
• The same velocity and effect size advantage for the strong sex was evident during the trial with 2 min rest times, as well, but in this case the difference was no longer significant.
• Even during the 4-minute recovery trial, men ran at higher velocities but the values were not significantly different and the effect size was considerably low.

Interestingly the women still had the higher %VO2 peak values during all conditions - in other words, relative to their physical constitution, they were performing at a higher intensity than the men irrespective of the fact the latter were running faster. Interestingly the difference was only significant during the 4-min rest condition (if you look at the data in figure 1 you will realize they had to pay for that dearly).

"Apropos active rest - what's the best?"
If you take a closer look at the data in figure 1, you will realize that the 4-min-condition with a work-to-rest ratio of 4:2 (in other words, the 2min rest-condition) appears to have a slight advantage over the other conditions. While the VO2 max may be ~1% higher in the short rest condition (VO2 data not shown), this is not worth the increased exertion both men and women experienced when they ran their 4-min intervals with only 1min of active rest in-between.

Figure 1: Lactate levels, perceived readiness before the HIIT bout, rate of perceived exertion right after and 15min the self-paced HIIT bout with 1, 2 and 4min active rest between sets in male and female subjects; all data expressed relative to the mean value for the respective parameter, data calculated for men and women separately (Laurent. 2013)

As far as the 4:4 condition is concerned it is certainly remarkable that the female participants appear to totally exhaust themselves during that condition. It is difficult to determine, whether this is a result of "getting out of the groove" due to the long rest period (if you are jogging you may know that once you stop for more than a minute it's very difficult to get into the groove again), or whether that may be a result of the fact that they were pushing themselves harder when they knew there would be a long recovery period. Personally I tend to believe that it's the latter effect. Otherwise, the male participants of the study at hand should have experienced a similar negative effect of resting too long. Now, whether that's a sign of toughness or rather one of hubris is a question I'd rather not answer ;-)

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(Re)read the SuppVersity HIIT Series and learn about the optimal interval:rest ratios for your personal training goals (click here)

Bottom line: Whether you can truly argue, that 4:2 is the optimal ratio is at least in my humble opinion still open - regardless of your sex by the way. So, if you are not sure what to do, try a couple of different interval:rest ratios and see how you feel. Meanwhile, I'd suggest you remember that the word "training" comes from "to train" and refers to the "sustained practice [...] in an art, profession, occupation, or procedure, with a view to proficiency in it" (Oxford English Dictionary). Proficiency in this context means that you achieve performance increases and those are not a necessary (and in most cases not even a likely) consequence of feeling like you have been run over by a train.

Once you've figured out what works best for you, stick to it! I don't care if it's 2:1 or 30s:1min, as long as it works for you and you don't have to drag yourself to the track or the gym, whenever your HIIT sessions are due, that's your personal optimum. You should still keep in mind that this optimum may change with your current performance / weight loss / hypertrophy goals and the corresponding amount of energy you consume. Previous research, for example, suggests that long(er) intervals (in the 4min range) could have a slight edge over very short ultra-intense ones, especially when your primary goal is to shed body fat (learn more in the HIIT Special Part I & Part II)

References: 

• Laurent CM, Vervaecke LS, Kutz MR, Green JM. Sex specific responses to self-paced, high-intensity interval training with variable recovery periods. J Strength Cond Res. 2013 Jul 8. [Epub ahead of print]