June 11, 2020 – UPDATE As an update into my current thinking, HR is simply too variable to be a reliable metric for internal training load near VO2max. Both the strength and weakness of HR is that it is sensitive to many internal and external factors. This limits it’s correlation to any one single factor, like VO2 or power. Power is exclusively a measurement of external work and doesn’t tell us how the body is producing that power. There are many helpful models that can estimate internal intensity from external workload (a distinction for another time), but I’ve written previously about some issues I have with over-extrapolating metabolic information from power. I still believe RPE – relative perceived exertion – is a better metric for simple tracking of internal workload. Hard workouts should be hard, and easy workouts should be easy. Your brain knows how to take care of the details. Jan 24, 2019 – UPDATE I’ve been able to test the VO2 Master Pro wearable VO2 analyzer, a new portable VO2 mask. I’ve compared actual VO2 values on various workouts and will be writing up my results. The first few articles compare VO2 for continuous VO2max intervals and for the Hard-start 30/15s protocol recommended here. June 26, 2018 – UPDATE I’ve written an UPDATE to this post where I reconsider a few assumptions given below. One major change is that 90% HRmax seems to be a better target than 88% HRmax. This post has been updated to reflect the changes.

Optimizing VO2max Interval Prescription

I left my conceptual post on VO2max with a tease for some practical application of VO2max intervals. Here’s where I’ll discuss some of the workouts I’ve been doing and the research that led me to them.

First, recall my definitions for T90VO2max (just call it time >90%VO2max) and MAP (Max Aerobic Power).

T90VO2max is used to refer to the duration or time (T) spent at or above 90% VO2max, ie. duration at near-maximal rate of Oxygen consumption as measured in ml/kg/min via lab analysis. MAP is maximal aerobic power, which I use to refer to the maximum power/workload, and therefore shortest duration that will elicit VO2max before anaerobic energy system becomes the limiting factor to fatigue (read the last post for details).

Recall also that it appears that adaptations to VO2max are elicited by maximizing time spent above >90% VO2max (Rønnestad & Hansen, 2013; Seiler, 2010; Thevenet et al, 2007; I could go on…) hence why I think duration >90%VO2max is a valuable parameter of aerobic/VO2max adaptations.

This is the value I’m trying to maximize for my VO2max workouts.

Intermittent Short Intervals

It has been shown that intermittent exercise can elicit greater total exercise duration at high intensities (Rønnestad & Hansen, 2013; Rønnestad & Hansen, 2015; Thevenet et al, 2007).

Conceptually this makes sense: having brief rest intervals should allow you to maintain the target workload for a longer cumulative duration than continuous exercise. This is basically just restating the concept of interval training.

The 2013 Rønnestad & Hansen study referred to above found that 30sec work intervals with 2:1 work:rest ratio (30/15s) seemed to be most effective ‘short interval duration’ to maximizing duration above >90% VO2max.

I wonder if 30sec as a familiar ’round’ number allowed the athlete to better pace their effort? Compared to an individualized percentage of Tmax that might give some random integer duration (30/15s sounds better than 33/17s).

They followed up this study in 2015 by comparing this 30/15 protocol with a standard 4x5min 2:1 work:rest protocol. They found the 30/15 protocol was more effective at improving VO2max and performance metrics after 10 weeks training.

Training with the Rønnestad 30/15s Protocol

The exact protocol Rønnestad & Hansen used was 3 sets of 13x 30/15s, with 3min rest intervals between sets. They prescribed a workload target of >88% HRmax as a proxy for 90% VO2max (update: I now recommend just going with RPE: start at 9/10 effort and try to keep that relative effort just below the point of blowing up. Easy!). The instructions given to subjects was to “perform at their maximal sustainable work intensity, aiming to achieve the highest possible average power output”

Power was used only descriptively as an outcome measure of performance, rather than as a prescriptive target: therefore if average power was higher for workouts later in the training program, this would indicate that the subject had improved their performance.

I’ve used a modified Rønnestad 30/15 Protocol on and off for the past year. It’s always a super tough workout, and I usually run it with 5min recoveries between sets, rather than 3min. I’ve still had to drop work intervals from the later sets, had to turn down the resistance, or just completely failed a set plenty of times!

When I’ve done this workout on my Wahoo Kickr I’ve prioritized a power target in Erg mode and used HR as a descriptive measure of time >90%VO2max, aiming to maximize duration >88% HRmax as per Rønnestad & Hansen’s prescription (as above, I now recommend going on subjective effort. Just make it hard!).

Prescribing Intervals via HR RPE or Power If my intervals are prescribed via target HR RPE of 9/10, I will naturally put out greater power at the beginning of the workout when the effort feels easier. As fatigue begins to accumulate, my power will end up decreasing through the workout in order to keep the relative effort at 9/10, or “not quite blowing up!”. This naturally produces a Hard-Start interval, which I talk further about below. If my intervals are prescribed via target Power (eg. work intervals at 400 W) I’m more likely to hold a constant power output through the workout, especially if I’m locked in to Erg mode. My HR and RPE will be slower to rise than it would in the RPE-prescribed workout, since even as I fatigue the power will remain constant and unrelenting. Or I might even fail the workout at the given power target if I fatigue too rapidly. Hard-start Faster rise in HR and power when fresh

Plateau of HR and gradual decrease in power as fatigue accumulates

Constant 8-9/10 effort through the full interval, eg. just under the verge of blowing up. Evenly-paced Steady power output through the entire interval

RPE starts lower when fresh, and drifts higher with fatigue

Slower primary rise in HR and drift through the interval

Higher HRpeak and potential inability to finish

Hard-Start Intervals

I was very satisfied following the Rønnestad 30/15 protocol as my go-to VO2max workout. I felt like I was able to reach a good balance of output workload (power) and input workload (HR) in order to maximize duration accumulated >90% VO2max.

Then I encountered some studies looking at Hard-Start intervals.

Recall above that when prescribing workouts via HR, power will naturally start higher and decrease through the workout as fatigue increases. The body becomes less efficient at producing power during a hard interval, so for the same perceived effort and HR, power output will decrease.

Hard-start intervals are where power is manually modulated to maintain VO2 above target as long as possible – ie. to maximize time >90VO2max. This is done with an initial extreme intensity workload to rapidly raise VO2 and HR, then reducing power to match rising fatigue and maintain an elevated VO2.

Billat et al, 2013 found that by carefully manipulating power workload in real-time responding to subjects’ measured VO2 (via gas exchange analysis), they could maintain >90%VO2max continuously in excess of 15 minutes!

One subject was able to last fully 26 minutes at >90% VO2max!!

This is obviously very compelling. But it would be difficult to manually manipulate target power in real-time for my athletes, or myself outside of the lab environment. So I would need a more generalized prescriptive protocol to plug into a custom Erg mode workout to be able to follow at home.

Practical Prescription for Hard-Start Intervals

Another study, Lisbôa et al, 2015 looked at a similar Hard-start interval protocol. They used a standard method of starting the interval at the maximum workload to elicit VO2max; what they called Ihigh, and what I call MAP, and finishing the decreasing-power interval at 110% FTP.

The interval duration was determined individually as the longest duration @ MAP (what they called Tlow, and is often called Tmax… sorry for the acronym soup) This duration was then divided into 4 equal intervals of decreasing-power, from MAP to 110% FTP.

They found the sustainable duration above >90% VO2max was over 50% greater, and total O2 consumption was over 70% greater in these decreasing-power intervals compared to constant power intervals at MAP @ Tmax.

Combining Intermittent Short Intervals and Hard-Start

My immediate interest after seeing these studies was to combine the two workout protocol to rapidly raise VO2 toward >90% VO2max (which can’t be measured at home… or can it?) and HR toward >90% HRmax 9/10 RPE (which can be measured at home), and to sustain that duration for as long as possible.

I would start with a longer hard-start interval at MAP (highest workload over the shortest duration that elicit VO2max) then drop into a decreasing-power 30/15 protocol, finishing around 110% FTP.

My HR rapidly reached near-maximal and then plateaued as the power declined at roughly the same rate at which fatigue accumulated. This particular workout I pretty much nailed the balance and managed to accumulate >36min @ >88% HRmax! (update: the accumulated time was less at >90% HRmax, and literally the entire workout was at 9/10 RPE!)

I was using 88% HRmax as my target based on the Rønnestad & Hansen 2013 & 2015 studies, although I now believe a better target for most people is to simply use subjective sensations and target an RPE of ~9/10. The big assumption here is that this HR target correlates to 90% VO2max… an assumption that I’ve been able to test on similar workouts. (Update: I’ve recently been experimenting with new models using muscle oxygenation to monitor internal training load)

The good thing about using Erg mode in Zwift or TrainerRoad is that I can have my hand ‘on the trigger’ to turn up or down the resistance in real-time. If I saw my HR drifting away from my target, or if I felt like I was about to explode I would rapidly raise or drop the target power a few percentage, which would instantaneously adjust the resistance of my Wahoo Kickr and allow me to continue.

Prescribing a Hard-Start 30/15s VO2max Workout for Yourself First, the disclaimers. Ultimately, there are a lot of assumptions built into this VO2max training protocol. Since we are unable to easily measure actual VO2 at home or out on the road we need to use correlated metrics to estimate our VO2 at any given time. Namely, HR and Power. There is an interdependent relationship between HR, Power, and VO2 that I still don’t fully understand.. but they have to be in the right balance for optimal adaptations to occur. I believe aiming to sustain a near-maximal effort (RPE 9/10) and modulating power output to make sure you don’t blow up, will elicit the greatest time spent near VO2max. And that accumulated duration near VO2max is a critical parameter for enhancing aerobic fitness and endurance performance. Ideally before prescribing this workout for yourself you should have a good sense of your MAP (Max Aerobic Power; the maximal power that will elicit VO2max) and the maximum duration at this workload (MAP @ Tmax), and of course your FTP. MAP can be estimated at home with a Ramp Test (such as TrainerRoad, Zwift, or many others) or in a lab-based VO2max or lactate test. Unfortunately, there’s no easy way to prescribe this workout based on percentage of FTP, since everyone will have a different relationship between MAP and FTP (see the recent Fast Talk Podcast for a fantastic discussion on the hyperbolic “death of FTP”). Trainer Workout The workout protocol I’ve been using is 3 sets of 13x 30/15s, 5min RI between sets. This should accumulate around ~10min of work each set, for ~30min cumulative work duration. To do this workout on a smart/Erg trainer, the first hard-start interval should be 1 minute @ MAP, before going into the 30/15s work:rest short duration for 12-15 work intervals. Power will then decline in regular steps every ~3-4 work intervals toward ~110% FTP by the end of each set. Your target will be to maintain RPE at ~9/10. Start each set too hard and modulate power to keep yourself just under the edge of blowing up. The point of this workout is to accumulate the full duration, so you’re better served by dropping power and finishing the set, rather than reaching exhaustion too early. The second and third sets can have shorter hard-start intervals, as HR will more easily rise toward your target due to accumulated fatigue, and spending too long at MAP will over-fatigue you for the remaining work intervals. I found average power for the later sets will inevitably be lower than the first set, but this is fine as long as RPE remains high. If possible, have your hand near the trigger to adjust the power up or down as necessary in your workout app of choice, to avoid a death spiral. Outside Workout If you were to do this workout on the road the recommendation would still be simply go hard! Try to maintain a ‘near-maximal’ effort through all three sets, from start to finish. Modulating power to maintain the intensity without reaching exhaustion. Power will naturally decrease as fatigue accumulates, but the power of each 30sec microburst should remain well above FTP. I haven’t actually tried this workout out on the road, so if anyone does.. tell me how it goes! And if you changed anything to make it easier/more realistic to complete.