Most cyclists are familiar with the difference between “aerobic” and “anaerobic” exercise. Aerobic exercises is primarily supported via the oxidative energy system. “Oxidative” means that oxygen is required to produce energy during the break down of energy (from either carbs, fats, or protein).

“Anaerobic” exercise is fueled primarily by the ATP-PCr and glycolytic energy systems. These energy systems differ from the oxidative energy system in that they don’t require oxygen to produce energy [1].

Short on time? Check out our quick tips for ATP-PCr & glycolytic workouts.

While most power based systems group the ATP-PCr and glycolytic energy systems into a single “anaerobic” Zone 6, I think your training will be better served by a deeper understanding about the energy system contribution to your most intense efforts on the bike. 

To start with, all exercise is fueled by a combination of the different energy systems [1].

In a practical sense, this means there’s no on/off switch that flips between aerobic and anaerobic workouts. 

In fact, “anaerobic” workouts have a significant aerobic component depending on the combinations of work and rest within the interval series. 

This graphic highlights the estimated energy system contribution to an all-out 10s effort

Under the hood of a 10s all-out effort

Even 10s all-out efforts accept a contribution from the oxidative energy system.

In other words, the way in which your Zone 6 workouts are programmed will largely determine the degree to which your efforts target your anaerobic energy systems.

Here’s are a few key points to help conceptualize the aerobic/anaerobic relationship for zone 6 intervals [2].

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1. The longer the zone 6 effort, the more “aerobic” (greater reliance on the oxidative energy system) the workout becomes.

2. The shorter the zone 6 effort, the more “anaerobic” (greater reliance on the ATP-PCr & glycolytic energy systems) the workout becomes.


4. The shorter the rest intervals between zone 6 efforts, the more “aerobic” (greater reliance on the oxidative energy system) the workout becomes.

5. The longer the rest interval between zone 6 efforts, the more “anaerobic” your workout becomes (greater reliance on the ATP-PCr & glycolytic energy systems). 

Highlighting the energy systems responsible for Zone 6 workouts rather than grouping them together into a single “anaerobic” bin might sound like a semantic distinction but when it comes to cycling, semantics often matter. Here’s why.

Constructing a training plan that gives you the best chance to deliver a race winning attack in the last 60s of a criterium should never be built around the perceived “anaerobic” nature of that race-winning effort. 

Like all other areas of cycling performance, even short, explosive, “anaerobic”, efforts are underwritten by your oxidative energy system, to say nothing of the aerobic fitness required to get in a position to win a bike race (i.e., you have to be really fit to fight for, and maintain a position that gets you close enough to the line to even think about launching a race-winning attack). 

That’s a long way of saying false distinctions between “aerobic” and “anaerobic” in a training zone structure are more harmful than helpful to your progress on the bike.. 

Under the hood of a 60s all-out effort

Most cyclists are surprised to see that nearly 1/2 of the energy required for an all-out 60s sprint effort is supplied by aerobic (oxidative energy system) metabolism.

Conceptualizing your training zones as combinations of different energy systems is not only a more accurate way to describe your training, it’s also a more helpful way to conceptualize the building blocks of cycling success.  

In short, understanding the combined contribution (and ever-present foundation of the oxidative energy system) of your energy systems will give you a broader understanding of why you’re doing a specific workout, and how exactly to get it done. 

Warming up

With training  zones one through three we don’t pay much attention to  warming up before our workouts. In general I recommend athletes getting in whatever type of warmup (long, short, easy, or hard) that feels right. Once we cross over the threshold barrier the specificity of our warmup becomes more important.

In short, for higher intensity (≥zone 4) workouts, research suggests that utilizing a short, higher intensity ramp before your primary efforts can help to “prime the aerobic pump” of exercise, resulting in improved performance for subsequent hard riding. To read more about the science and theory behind a great warmup, check out this post over at our blog.


Warmup Template

If you prefer to jump straight to the goods, here’s a template for a science-backed warmup to execute before all (training or racing) your high-intensity efforts. 

This template is for a 30m warmup with a bit more recovery between your ramp and main set to allow for greater reconstitution of your “anaerobic” battery before your first high intensity efforts. As always, fine-tune your intensity and rest duration before the start of the main set to see what works best for you.

DDA workouts that focus in on the Glycolytic & ATP-PCr energy systems utilize two primary forms of HIIT training: Repeated Sprint Training (HIIT-rst) and Sprint Interval Training (HIIT-sit) [2]. When we draw on our understanding ATP-PCr and glycolytic energy systems the rationale for how these two different types of HIIT intervals becomes clearer. 

HIIT-Repeated Sprint Training

RST utilizes short sprint efforts of three to ten seconds bundled together in a series of sprints with fifteen to sixty seconds of rest between each sprint. The short nature of these sprints, coupled with their repeated nature places a high demand on the glycolytic energy system (in addition to providing stress to the oxidative system) [2]. 

HIIT-Sprint Interval Training

SIT utilizes all-out sprint efforts of around 20 to 30s, interspersed with longer rest periods of one to four minutes. These efforts place a greater emphasis on the ATP-PCr and glycolytic energy systems. 

Energy System Target

Zone 6/ATP-PCr & glycolytic workouts target the primarily “anaerobic” end of your power curve, utilizing nearly exclusively carbohydrate as a fuel source.

Nutritional Considerations

Because Zone 6 workouts often generate a high degree of RPE, it makes sense to make nutritional choices that stack the deck in your favor to reduce RPE. The three nutrition choices you can make that are likely to have the most significant impact on lowering your exercise RPE are:

  1. Following the principles of high carbohydrate availability in the hours before your ride. In other words, come to each of your Zone 6 rides with a full tank of glycogen.
  2. Strategically utilize caffeine to in the hour before your workout (unless of course your workout is in the evening, in which case disrupting your sleep isn’t worth it)
  3. Ensure you arrive at the start of your workout well hydrated.


Pacing for these efforts might be the easiest of all training zones. In short, you should be going all-out. This is an area where RPE guidelines are generally much more helpful than specific power targets. Since workout power targets are scaled to a percentage of your threshold power, there is often a high degree of variability between “accurate” power targets in ATP-PCr & glycolytic workouts between cyclists.. 

In simple terms, this means that for some riders the listed power targets might be insufficient to maximize the intended stress to your “anaerobic” energy systems. If your Zone 6 power targets are feeling “comfortable”, then you’re not going hard enough.

More times than not, dialing in accurate targets for Zone 6 intervals requires a bit of trial and error using cross-calibration between RPE and power.  

Screen for ATP-PCr & glycolytic rides

If you do use power to pace your Zone 6 efforts, keep things simple and only display your average lap power. Additionally it can be a big help to show the number of laps on your computer screen, this way you’re not having to keep track of the number of efforts you’ve completed over the course of the workout.

Reducing RPE

Like VO2max workouts, part of the magic in Zone 6 workouts is that for a relatively short investment in time, you can reap a significant training benefit. Not only do Zone 6 efforts help to increase your ATP-PCr and glycolytic power production, they can improve other “down-stream” fitness variables like endurance and power at threshold [3]. 

This makes Zone 6 workouts a useful tool for not only improving your readiness to race, but also to maintain gains in your fitness even if you’re severely limited on training time. 

So what’s the catch? When done properly, Zone 6 workouts generate a ton of RPE. That’s why outlining a specific strategy that seeks to minimize RPE is essential. Here are a few suggestions. 

  1. Start with suggestions for Zone 5 RPE reduction.
  2. Prioritize more rest between efforts if needed. 
  3. Do your best to focus on the present effort at hand, avoiding the tendency to get lost in ruminating about how many additional efforts you have to do to complete the interval. One, effort, at, a, time.


  1. Kenney, W.L., J.H. Wilmore, and D.L. Costill, Physiology of sport and exercise. 2020, Champaign, IL: Human Kinetics.
  2. Laursen, P. and M. Buchheit, Science and application of high-intensity interval training : solutions to the programming puzzle. 2019.
  3. Gibala, M.J., et al., Short-term sprint interval versus traditional endurance training: similar initial adaptations in human skeletal muscle and exercise performance. The Journal of Physiology, 2006. 575: p. 901-911.