Thoughts on responses to this Twitter post: Stephen Seiler @StephenSeiler Stephen.Seiler@uia.no So, what does that word Demanding mean? adjective 1. requiring or claiming more than is generally felt by others to be due: a demanding teacher. 2. calling for intensive effort or attention; taxing: a demanding job. In keeping with this dual definition, a demanding workout contains both an external component, the work and the external numbers like pace and power, and an internal component, the cognitive focus and effort to maintain work rate in the face of evergrowing sensations of pain and exertion. Eight-time gold medalist Norwegian XC skier Bjørn Dæhlie wrote that he needed two days to mentally prepare for his hardest interval sessions. You cannot mentally detach yourself from the task at hand during these workouts. They cannot be run while on auto-pilot with your headphones on, or while observing the turtles lined up on a log in order of size as you row by. You enter the HIIT tunnel. The focus turns inward, not outward. At some level, all HIIT sessions are demanding by definition. Untrained people can be taken totally unaware when confronted with the discomfort of exercising in the red zone. But, athletes are used to it, thrive on it even. So, a really demanding HIIT session for well-accustomed, welltrained athletes feels more like a manifestation of hell than health. But, it is a journey they choose in hopes of coming across to the other side. Along these lines, one of my favorite responses to this post on Twitter came from Dr. Michael Joyner. He tweeted that perhaps really hard HIIT sessions were a form of meditation. I liked the cognitive juxtaposition between two seemingly unrelated situations. like sitting in a dark room with your legs crossed staring at a candle flame
versus running 400m repeats 20 times on a hot day at a local high school track. Meditation or not, mind and body engage in a deep sensory conversation during very demanding HIIT sessions. Another important comment was related to the word regularly in the question. There was some discussion about interval workouts that were doable (survivable) as one-off sessions versus sessions that can be sustainably incorporated into a training plan. This is an important distinction when considering the long-term progression of training. Personally, I always found that it was hard to do the same demanding interval prescription more than 4 weeks in a row without stagnating psychologically and physiologically. Our recent HIT periodization research has also quantified the disproportionate adaptive impact of the first 4 weeks in a period of training intensification. So, prescriptive variation is likely important for the mind and the body, making any single optimal HIIT prescription extremely unlikely. But, various HIIT fads come and go, and will continue to do so. So, what are the ingredients of those exceptionally demanding HIT sessions? The candidate variables are the usual suspects, but there are two that stand out and really help put the D in demanding: 1. Work intensity- as some relative % of a reproducible, individual calibration point, like MAP, FTP, or MLSS. No surprise here. But, intensity is like poison; the effect does not just depend on how intrinsically strong the poison is, but also how much you consume in one dose. I would argue that there are no demanding intensities, only demanding doses of these intensities. Which brings us to the second candidate variable. 2. Accumulated Work Duration- Yes, the specific work bout duration in the prescription matters, but the response to a maximal session effort 8 x 4 min HIIT session ends up being very similar to 4 x 8 min session when we add up the heart beats and perceived exertion at the end. Accumulated work duration regulates intensity responses to a maximal session effort prescription and contributes to the overall perception of demand at any given intensity. Perceived exertion (RPE, Borg scale) increases linearly with successive work bouts in all the studies we have published. It also tracks well with heart rate as a % of max. So, if HR is drifting upward, it is a safe bet that RPE is also, even if power or pace are constant. Of course, interval training would not be interval training without the recovery periods between work bouts, so these play a big role.. 3. Work:rest ratio and other recovery characteristics- recovery duration and recovery work intensity contribute to the demand of the session, and should also be contextualized as a work:rest ratio, at least up to a point. Way back in the 60s, Per-Olof Åstrand and colleagues at Karolinska Institute and the Sports College of Stockholm did studies on intermittent exercise that remain elegant and instructive today. Given that Dr. Åstrand is no longer with us, and the
experimental results presented are nearly 60 years old, I will take the liberty reproducing some data from Åstrand & Rodahl s Textbook of Work Physiology (1970) without permission here. I have converted the original data to SI units where appropriate. I have also added some color coding of the work:rest conditions. Performing fixed amount of work on cycle ergometer in 1 hour with different work and rest durations Type of exercise Oxygen uptake Heart Rate (b. min -1 ) Pulmonary Ventilation (L. min -1 ) L. hr -1 L. min -1 Continuous 175W 146 2.44 49 134 1.3 350* 4.6 124 190 16.7 Intermittant 350W/0W Blood Lactate (mm) Work Rest 30s 30s 154 2.9 63# 150 2.2 60s 60s 152 2.93 65# 167 5.0 2min 2min 160 4.4 95 178 10.5 3min 3min 163 4.6 107 188 13.3 #Measured over 30s. * Time to exhaustion was 9 min. Source: I Åstrand et al. Acta Physiol. Scand. 48:448, 1960. The example below suggests the following: if you perform at a power or pace that will take you to exhaustion within 10min or so when performed as a time trial, but break the work up into work:rest blocks with constant 1:1 ratio but different durations, you identify a breakpoint between 60s and 120s work duration where sustaining the on:off work for an hour goes from pretty easy to very hard! Inspired by Åstrand, but with a different design, we asked 9 well-trained runners to perform HIT sessions of 24x1 min, 12x2 min, 6x4 min, or 4x6min in mixed order on successive weeks (accumulated work duration = 24 min for all sessions) with 1:1 work:rest ratio, all at maximum session effort. Here again, we also found that for work bouts from 2 to 6 minutes and a constant total work duration, the physiological responses and perception of effort were essentially the same, but were achieved at different running velocities, depending on the prescription.
That is all well and good, but holding a 1:1 work-to-rest ratio is not normal for longer intervals. We knew that, but at the time we did not have data to support a fixed rest duration, and assumed using HR recovery would be too fuzzy (it is). So, we went back and specifically studied the impact of varying rest duration between 1 and 4 minutes during a standard 6x4 minute HIT session at maximal session effort. Compared to 1min rest, 2 min rest allowed for slightly higher higher running velocity during the work bouts, but increasing the rest periods to 4 minutes had no additional effect on work velocity. The rest duration did not impact the perceived exertion at the end of each work bout either. That is, the sessions did not feel easier with longer recoveries. RPE responses during a 6 x 4 min HIT sessions, with 1, 2, or 4 minute rest periods When we asked runners to self-select the minimal rest duration that would allow them to reproduce their best HIT session, the average was 118 s, and this stayed constant across the session, even though they were blinded to the elapsed time.
In contrast, small variations in rest duration and work:rest ratio can dramatically impact physiological and perceptual responses during micro-interval prescriptions (e.g. 3 x 10min of 30s:30s versus 30s:15s). Once again, Åstrand and colleagues established this years ago: One subject running for 30 minutes on a treadmill at 20km. h -1 using different intermittent work rest combinations. Running continuously, subject could maintain this speed for 4 min to exhaustion, VO2max 5.6 L. min -1, ventilation 156 L. min -1, Blood lactate concentration 16.7mM. Periods, work-rest, seconds Oxygen uptake (L. min -1 ) Peak Average Average Pulmonary Ventilation (L. min -1 ) 5-5.. 4.3 101 2.6 5-10 10-5 10-10 15-10 15-15 15-30. 5.6 4.7 5.3 5.3 3.9 4.6 5.1 4.4 5.0 4.6 3.6 81 142 104 139 90 75 1.8 4.9 2.2 5.7 2.3 1.8 Blood Lactate (mm) Data from Christiensen et al. 1960. Acta Physiol. Scand. 50:269, 1960. Is there one type of HIIT session recipe that jumps out as most demanding? No, there is not. I categorized your feedback into 4 basic types of demanding session. Obviously, this is not a comprehensive study. But, I think your examples captured the main types of session being used by endurance athletes, from recreational to elite. 1. Sprint (Anaerobic) interval sessions at supra-maximal intensity (relative to MAP/PPO) with 4-6 30s-60s sprint bouts separated by relatively long rest periods (5-6min). Heart rate means nothing in this kind of session, but blood lactate concentration can easily exceed 15mM. Power output declines steadily despite maximal effort. These sessions will even leave a unique metallic taste in your mouth and make your jaw muscles tighten up (or maybe that is just me).
2. Short, (about 60s, often with 1:1 work to rest ratio) high intensity work bouts repeated an unusually high number of times in succession. The classic workout in this category has to be the 20x400m on the track, made famous by Jim Ryan, and suggested as a rite of passage meditative experience by Mike Joyner. 3. Repeated blocks of micro-intervals like 15s on, 15s off, 30:30, or 40:20. Sets are typically 10-15 minutes long, separated by several minutes of low intensity recovery. From a cardiovascular standpoint, these look very much like long interval sessions. But, depending on the modality, the force demands of repeatedly (re)accelerating to goal power or pace can change the motor unit recruitment profile. Accumulating 30 minutes of work in this way is very demanding. Also, doing a 15s:15s session on a treadmill running (standing beside the belt and jumping on a moving belt) will be very different than rowing (bringing boat back up to speed against high water resistance), due to the acceleration cost differences. 4. Long HIT interval bouts- like 6x4 min, 8x5 min, 4x8 min, and accumulating 20 to 40 minutes of work well above FTP/MLSS. The sprint interval sessions are different from the others. These sessions mobilize a very high percentage of total anaerobic capacity and even with long recovery time, they drive the athlete towards peak blood lactate concentrations. The other 3 types are all variations on the same theme in that the athlete collects a lot of minutes at intensity between FTP/MLSS and power/velocity at VO2 max and heart rate around 90-95% of HRmax. These longer HIIT sessions are wars of attrition that wear down the athlete both cognitively (sustaining high effort over many minutes is cognitively demanding) and physiologically. Physiological demand drifts steadily upward with increasing fatigue, central drive increases, and perceived exertion follows. In these sessions, you throw down all our chips on the classic premise of interval training; by breaking up the work with brief rest, we can accumulate more minutes of high intensity work and produce a strong(er) signal for adaptation. Can you compare sessions and call one or the other HIT session the most demanding? I don t think so, or at least I would not go on record favoring one or the other. However, I do think that individually, we can use a single distance effort as our calibration, and compare minutes of work at that power or pace. For example, if I can hold 400 watts for a 10 min cycling time trial, then HIT sessions can be calibrated against that reference. The classic 20 x 400m running session made famous by high school sensation Jim Ryan exemplifies this time-trial calibration approach. Just before becoming the first high school athlete to run under 4 minutes for the English mile, he completed a track session consisting of 20 x 400m at ~60s pace with short jog recoveries. That is, he accumulated 5x his time to exhaustion for that speed during the HIT session. Currently, in my own hardest ski poling session (Concept 2 ski ergometer, see below), I accumulated 19 minutes of work at the maximum power that I can sustain for 7.5 min, for a ratio of 2.5. This ratio approach might be useful when planning and evaluating demanding HIIT sessions in preparation for race durations in the 2-20 min range. I suspect the math falls apart a bit for longer time trial, but we can all agree that the integration of work
intensity and accumulated work duration is at the heart of whatever makes some HIIT sessions worse than others. The acute, relative intensity of work, and the accumulated duration of high intensity work both contribute to the perception of exertion during a HIIT session. But what matters most to the demanding label a specific HIIT session is given, intensity or accumulated duration? These two variables are connected because of an overall fatigue induced relative intensity drift, so teasing them apart is difficult. But, recently we did a large multi-center training intervention study comparing different periodization schemes for high intensity aerobic interval training. A fortunate by-product of that study was that we accumulated very good physiological and perceptual data on over 1,400 individual HIIT session performed by 63 subjects over 12 weeks, and published a separate article from that data. These sessions were always one of three prescriptions: 4 x 4 minutes, 4x8 minutes, or 4 x 16 minutes, prescribed as maximal session effort workouts. We collected RPE (Borg) and sessionrpe (Foster) data on all these sessions. Each subject performed 22-24 HIIT sessions, so we could average individual responses to the different prescriptions. Physiological and Perceptual responses to 3 HIIT prescriptions varying in work bout duration and total accumulated duration, but at maximum session effort Our data lead us to conclude that effort and exertion are different constructs. Acute exertion seems to be strongly driven by acute work intensity. Effort can be seen as the integral of exertion and time. Go to that full article if you are interested in more discussion around the interpretation of these data, in light of other key theories from the psychobiologists among us. To finish off, I have pasted in some visual examples provided by Twitter followers. I have removed the names, but tried to contextualize the data as well as possible. Most hard sessions show gradual cardiovascular drift during the session, but not all. Stepdown sessions, where power or pace stay constant but work time decreases are one way to even out the heart rate response despite increasing fatigue. Even though these were touted as your most demanding sessions, along with others not visualized, most report
an RPE of 18-19 (or 8-9), but rarely 20, or 10. I think most athletes leave the last number on the scale for a place they only go during a race, or perhaps an ergometer test in front of a coach! (rowing). Demanding HIT session examples from Twitter followers: 1 min @ about 125% Functional Threshold Power, 1 min off, for 60 min.
3 x 8mins - 1st 10s sprint 10s hold speed 10s recovery for 8 mins, 2nd set - 15/15/15, 3rd set- 20/20/20! 8 mins recovery between sets! Max HR was 177-181bpm dependant on the day 1st effort 411w (413w Normalized Power, Coggan method) 2nd effort 425w (417w NP) 3rd effort 407w (469w NP) 20:10 x8min 8min rec 30:15 x 8min 8min rec 40:20x8min 3x 3 min, then 4 x 1min at 140-150% FTP:1min at 90%FTP, Max HR 177-181 day to day. FTP=395W
3 sets; 10x30s on/off with 2-5min between-set light intensity, self-paced effort based on maximal session RPE. A research description of a very tough 7 day HIT block intensification study was given as the most demanding HIT session experienced by one of the respondents: Cyclists in the two experimental training groups completed seven consecutive days of HIT. The composition of the training sessions was designed to replicate the intensity and duration of efforts seen in real competition and was determined in conjunction with two elite level coaches, using power data collected from competitive cyclists during racing and on previous competition based performance analysis by Ebert et al., [17]. The training sessions, consisted of multiple sets of self-paced maximal intensity sprints and corresponding recovery periods. The work to rest ratio was matched for both groups at 1:5 (i.e. a 10-s effort would require a 50-s recovery period) and the total session time was 120 minutes including a self-selected 15 minute warm up and cool down period. Cyclists in the short training group completed 25 sets of sprints lasting 5, 10 and 20 seconds (each set) completed in sequence for a total work period of 14.6 minutes and corresponding recovery period of 73 minutes. Cyclists in the long training group completed 10 sets of sprints lasting 15, 30 and 45 seconds for a total work period of 15 minutes and corresponding recovery period of 75 minutes. Cyclists in both groups were asked to complete each effort at the highest possible intensity and in the recovery periods, maintain a work rate of 30 40% PPO. All training sessions were controlled using pre-recorded audio signals which indicated the duration of the exercise and recovery periods. https://doi.org/10.1371/journal.pone.0115308
Cycling hill repeats: (17 x 1.5km) 8x5 min (1 min rest) @ about 85% of Max 5min power and 92-99% cycling HRpeak
And, from a RUNNER: 1200m-800m-400m-longer rest-400m-800m-1200m. Blue is running pace (min. km -1 ). Red is heart rate. Athlete s max HR was about 181.
Finally, my own current old-boy killer workout, performed 7 times over the last 19 weeks on a ski ergometer (imitating the upper body dominant double-poling technique). I started using this HIIT session when relatively untrained for this skiing movement, so my potential for performance improvement was big. The workout is 10 x 500m at maximal session effort, work bout duration approx. 2 min, rest time 90s. RPE has been 18-19 at the end, blood lactate 12+mM. Average power output has increased 39% over 19 weeks (from 178 to 248 Watts), but HR responses and RPE are unchanged, within typical day-to-day heart rate variation (and small variations in autonomic balance and maximal heart rate). The power I manage for this session seems to correspond well with my sustainable 2000m time trial power (about 7m30s work duration). I left out the week 18 session because I collapsed on the 9 th interval and actually stopped pulling for about 20 seconds before resuming, and finishing the workout. That power pacing collapse was both painful and humiliating, but confirms to me that I am on my ragged edge for this session prescription. Most recent 10x500m ski double poling ergo session (Goal power was 250 watts)