As an applied physiologist and past rowing coach, that was my question. Recommendations typically encourage the separation of high-intensity interval training (HIIT) sessions by at least 48 h (11). But given there are so many ways to skin the cat with HIIT, and so many HIIT weapons (formats) and physiological target types (1), is it fair to think that each is going to require the same length of recovery time following? Adding to the puzzle might be how we actually quantify this recovery.  How do we know when an athlete is ready to perform another HIIT session given there are so many things that can be measured? As Dan Plews will describe for us in the HIIT Science rowing chapter and course, notwithstanding the importance of rowing technique and race tactics, the ability to generate absolute raw horsepower makes the lion’s share of the contribution towards a rower’s performance (Figure 1). So in the context of solving the puzzle that best builds a rower’s engine, figuring out the interplay between the HIIT weapon, target type, and subsequent recovery needed following is going to be critical.   Heart rate variability (HRV) has been gaining traction as a useful marker of recovery. HRV is a measure of the heart’s parasympathetic activity (think ‘feed and breed’), the branch of the nervous system responsible for telling the body to rest as opposed to being prepared to deal with stress (think sympathetic, ‘fight or flight’). We can use HRV as one way to assess recovery after exercise and measure the time it takes for HRV to return to pre-exercise levels. Typically, we see that HRV takes about 24 h to recover following low-intensity (< 70 % VO_2max) sessions,  24-48 h following threshold-intensity (70-82 % VO_2max) work and >48 h following HIIT (>82 % VO_2max) (11). Generally speaking then, it seems the higher the exercise intensity, the longer the HRV recovery time usually is. However, research has established this trend across passive recovery periods without other training content, thereby limiting the practical relevance for high-performance programs. That’s not how our athletes train, with 2-4 sessions per day being more typical in our elite. With this background, we undertook a study in well-trained rowers (3) to determine the duration likely needed between HIIT sessions where a rower would be considered ‘recovered’, defined as how long it took for heart rate measures and performance to return to the pre-exercise state.

The study

To achieve this, a group of 13 highly-trained rowers (Figure 2) each performed three different training sessions on a rowing ergometer on separate weeks as follows:

• 5 x 10 min, 4 min rest (threshold session)
• 5 x 3.5 min, 4 min rest (long interval HIIT)
• 10 x 30 s, 5 min rest (sprint interval training)

  We measured the intensity of each of these sessions as a percentage of the rowers’ power at VO_2peak and their blood lactate response on completion of the session. The sessions were also assessed using the training impulse (a product of the rowers’ session rating of perceived exertion and the duration of the session) and the duration of the session spent at high heart rates. A summary of the session demands are shown in Table 1. Table 1. Demands of each of the three HIIT sessions performed.

	Threshold Session	Long Interval HIIT	Sprint Interval Training
% VO2peak power	79.6 ± 6.8	97.0 ± 5.4	156.4 ± 15.6
TRIMP	16.7 ± 7.9	10.6 ± 0.4	12.1 ± 1.3
sRPE	18.3 ± 0.9	19.0 ± 0.7	16.2 ± 1.7
Blood lactate (mmol·L-1)	8.2 ± 2.9	11.6 ± 2.5	11.8 ± 3.8
Time >90% HRmax (min)	30.5 ± 11.8	14.8 ± 2.1	2.7 ± 3.6
Time 80-90% HRmax (min)	18.1 ± 9.7	7.6 ± 3.7	7.2 ± 2.0

Mean (± standard deviation [SD]) session power as a percentage of rower’s power at VO_2peak (% VO_2peak power); Training impulse (TRIMP; calculated using RPE from a 15 point Borg’s scale with time in hours, not minute); session rating of perceived exertion (sRPE); duration of session spent above 90 % of maximal heart rate (time >90% HR_max) and between 80-90 % of maximal heart rate (time 80-90% HR_max). Every 10-14 h for 72 h following the HIIT session, we measured: peak and mean power from a 30 s maximal test on the rowing ergometer, HRV immediately post HIIT, at rest, and following 5 min of submaximal rowing (HRVrec), heart rate (HRex) during and heart rate recovery (HRR) post-5 min of submaximal rowing, as well as perceived recovery. To ensure findings were relevant to athletes and coaches, we assessed recovery throughout regular low-intensity and resistance training following the HIIT session. The training schedule was kept the same for each of the three key training sessions.

What we found

As shown in Figure 3, there were no statistically clear differences in HRV recovery time between any of the sessions, but recovery times for HRV following the HIIT sessions were faster than the > 48 h duration reported in other studies (11). Interestingly, HRV took the longest absolute time to return to pre-HIIT levels following the 5 x 10 min threshold session, which occurred after 29 ± 12 h (mean ± SD). Time for HRV recovery from the 5 x 3.5 min long interval and 10 x 30 s sprint training sessions took 16 ± 11 and 18 ± 10 h, respectively.   When comparing the different measures of recovery, we found they did not all tell the same story. The recovery time of HRV was longer than that of 30 s mean power following both the 5 x 10 min threshold session and 10 x 30 s sprint interval session. Recovery time of HRVrec was also longer than 30 s mean power, peak power, HRex, and HRR recovery times following the 5 x 10 min threshold session. As well, perceived recovery did not return to pre-HIIT levels during the 72 h measurement period following any of the HIIT sessions. The recovery time for measures taken following the three HIIT sessions, including individual responses, are shown in Figure 4.  

What it means

Research comparing HIIT recovery time to that of low-intensity has consistently shown longer recovery times following higher intensity sessions (4,5). Given the 5 x 10 min threshold session was performed at a lower intensity (17 and 76 percentage points of VO_2max lower than the 5 x 3.5 min long interval and 10 x 30 s sprint interval sessions, respectively), with greater time spent at high heart rates (Table 1), these results suggest to us that recovery time following training can also be influenced substantially by the duration of time spent at moderate and high intensities, and not just simply the absolute intensity of the training session itself. Nevertheless, recovery durations we observed following the different HIIT sessions performed in this study do not necessarily conform with a 48 h separation recommendation between all types of HIIT sessions. We saw that even when training is continued throughout the post-HIIT recovery period, the time taken to recover to a pre-HIIT state can occur anywhere between 6-38 h depending on the HIIT session, the individual, and recovery measure assessed (Figure 4). This highlights a crucial point:

“If we truly want to optimize the recovery duration between HIIT, we must consider the individual”.

Another important outcome of our study was that recovery time following HIIT seemed to depend on the measure we assessed. Our findings indicate that the recovery of performance occurs before the recovery of cardiac parasympathetic activity. Indeed, perceived recovery did not return to pre-HIIT levels during the 72-h measurement period. As regular training was continued throughout the 72 h period assessed (to replicate a real-world training environment), and the first programmed rest day occurred after the 72 h measurement period, the accumulation of training load throughout this period may explain why perceived recovery remained elevated, as psychometric measures have been shown to be more sensitive to training load than physiological assessments (9). With this in mind, we believe it is important to not only assess performance but also cardiac parasympathetic activity and wellness measures when assessing recovery and readiness-to-train, as the short-term return of performance does not necessarily mean an athlete has completely recovered. This is particularly important in programming considerations, as the repeated performance of threshold and HIIT sessions without appropriate recovery may lead to the continuous suppression of cardiac parasympathetic activity and reduced physical performance (7).

Key practical outcomes:

• Recovery from HIIT does not always require 48 h, so subsequent HIIT can be programmed earlier depending on the recovery status of the individual.
• Both long duration moderate to high-intensity threshold sessions, as well as long interval HIIT and SIT, were equally taxing from a recovery standpoint.
• A combination of performance, cardiac parasympathetic (i.e., HR), and perceptual/wellness measures are recommended monitoring variables that can be used to assess recovery from HIIT.
• An individualised approach to assessing recovery is advised, given the numerous factors that influence recovery and wide variation shown between individual athlete recovery time to HIIT

  About the author: Ana Holt is currently completing a PhD in the application of power meters in rowing with Victoria University and the Victorian Institute of Sport, while working part-time as a sport scientist at the Victorian Institute of Sport. Follow her on Twitter: @Ana_C_Holt References:

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