This Is an Interval

Trevor Connor revisits the fundamentals of aerobic versus anaerobic pathways, and helps us understand more advanced principles of interval training.

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Years ago, a friend of mine was invited to train in California, and while the weather was great, it was the company that scared him. He waited to take his first ride with then WorldTour rider Levi Leipheimer wondering if he’d be able to keep up. They soon set off, riding the hilly California landscape. My friend, an accomplished mountain biker, was happy to see that while he was at his limit, he could hang with Leipheimer on the climbs.  

For two hours they explored back roads while my friend wondered why their workout was so undirected. Finally they came to a stop. With a smile, Leipheimer said, “Thanks for the ride—I have to go do my intervals now.” With that one sentence my friend learned what being a Tour de France contender meant. The workout hadn’t been unstructured. It was just the warm-up. 

For those of you looking to optimize your performance, set the days of purely unstructured rides behind you. Instead, focus your attention on the value of interval work. It’s a ubiquitous term in cycling training, but there’s plenty to know when it comes to the science of interval training. 

As exercise physiologist Robert Pickels explains, “If you are not paying attention to structured intervals, you never know exactly what your training impulse is, you don’t know what that workload has been, you don’t know what your recovery is. Left to our own devices, we all have that innate desire to do more and work harder because we think that’s what makes us stronger. Situations like that lead to overtraining.” 

So, what intervals should you do, and what do they entail? A quick Google search will bring up hundreds of options. Talk to any friend at your next group ride and they can tell you their new secret interval weapon. The links at the bottom of this article offer a few good suggestions. Fortunately, the purpose of this article isn’t to add to the list. It’s to give you some of the background and tactics you need to pick your own intervals and make them effective.

Back to basics: aerobic vs. anaerobic 

“This is a football.” Vince Lombardi was famous for starting team talks with that line. The message was simple: never forget the basics. So let’s get back to the basics of what intervals are about. We have two ways of producing energy. The aerobic energy system requires oxygen and doesn’t fatigue easily. The anaerobic energy system is used to produce fast and strong efforts, doesn’t require oxygen, and doesn’t last very long.  

Almost all interval work targets one aspect of these two energy systems. Effective intervals aren’t about just going hard or riding in zone “X”. Effective intervals are about targeting one aspect of these two energy pathways and stressing it optimally.  

“There’s a difference between working hard and achieving a big number,” Pickels says. “If you’re working hard, but your power is going down, you’re not necessarily getting the benefit.” 

Let’s look at some of the important physiological concepts underlying these two systems, in order to help you maximize your interval training. 

“There’s a difference between working hard and achieving a big number. If you’re working hard, but your power is going down, you’re not necessarily getting the benefit.”

Exercise physiologist Robert Pickels

Aerobic power—be careful about the deficit 

You’re out for a ride with a few friends. You hit that steep climb and suddenly the competitive fire has you producing the best power numbers you’ve ever seen. Minutes later, your heart rate catches up with how you feel.

This effect is called oxygen deficit. Our aerobic system is sluggish. It takes time for aerobic pathways to respond to an increase in work. Until it does, we rely on anaerobic metabolism for our energy. The ability to reduce oxygen deficit is one of the primary training effects in elite cyclists.  

Oxygen deficit

“People might think of the oxygen deficit but not ever call it by name, or recognize that that’s the component they are trying to increase or decrease in a given workout,” Pickels says.  

Oxygen deficit is critical to effectively targeting your intervals. As long as you are in oxygen deficit, your aerobic system is not being optimally stressed. So, if you are doing aerobically focused intervals, such as threshold intervals, and you spend most of the time in deficit, your power numbers might be great, but your workout wasn’t. On the flip side, intervals targeting your anaerobic system should maximize oxygen deficit. 

At the bottom of this article we offer some suggestions on how to take advantage of oxygen deficit to make your intervals more effective. Just remember, while understanding that point is a critical, oxygen debt is only one piece of the puzzle, according to Pickels. 

Anaerobic power—the big spender 

Take the fastest anaerobic animal on the planet: the cheetah. It is capable of running up to 70mph. Now, consider what it’s doing when it’s not hunting dinner—it’s lying around. We can produce a lot of anaerobic power, very fast, but it doesn’t last long, and it takes time to recharge. While the best way to prepare the aerobic system is by doing work, the best way to prepare for anaerobic work is to do nothing. 

Maximizing your interval work 

Let’s turn our attention to selecting which intervals you should do. Ultimately, it comes down to your goals and knowing yourself.  

“If you want to achieve optimal performance, you have to start with an understanding of yourself,” Pickels says. “That understanding can come from lab-based testing or field-based testing. Learn your strengths and weaknesses. The interval workout really needs to be designed on what your goals are, what your weaknesses are, and what you need to achieve with your upcoming performances.” 

Now it’s time to take all of the above principles and apply it to your training—specifically your intervals training.

Aerobic intervals 

Intervals that improve your sustainable aerobic power are some of the most important work a cyclist can do. Threshold intervals are designed to stress the aerobic system. These intervals are generally 5-15 minutes in length at your threshold heart rate/power.  

Also popular are VO2max intervals, which are generally 1-4 minutes at slightly above threshold. They force normally anaerobic muscle fibers to work more aerobically. 

In either case, as long as you are in oxygen deficit, you are not effectively targeting your aerobic system. These intervals are most effective when time in oxygen deficit is minimized. Here are a few tips:

Short recoveries 

Fortunately, the aerobic system is as slow shutting down as it is starting up. As a result, prior intervals create a “priming” effect that reduces oxygen deficit in subsequent intervals—provided the recovery length is short enough. For threshold intervals, recoveries of one to three minutes are optimal to ensure some recovery while limiting oxygen deficit. 

Get a good warm-up 

Even getting the aerobic system primed at low intensities takes time. Give yourself 20 to 30 minutes of easy to tempo riding before you even think about starting your threshold work. 

Disregard the first interval 

While you can put out your best power in your first threshold interval, that’s because of your body’s heavy reliance on anaerobic energy. In terms of training, most of that interval is spent in deficit. It’s the later intervals that hep you produce your best training stimulus. So, don’t kill the first interval. All you’ll do is reduce the quality of the intervals that count. 

Consistency is key 

I’ve seen many riders start strong and get slower with each interval. That’s because their anaerobic reserves are depleting, and they are never fully utilizing their aerobic system. Make sure your intervals are done at a consistent intensity. If you have a power meter, target the exact same power for each. If not, use a climb. Start in the same spot and make sure your five-minute effort (as an example) ends at the same spot, as well. Done correctly, the first interval should feel hard, but not unbearable. By the final interval it should be a struggle to meet the same target. 

Call it a day if you can’t maintain intensity 

If you start to fade, it means you’re relying too much on anaerobic factors, and the aerobic pathways are not recovered. You aren’t effectively targeting your aerobic system and are just digging yourself into a hole. 

What to do during recovery periods 

Keep the legs spinning between threshold intervals. This will keep the aerobic system primed. Active recovery will also aid lactate clearance. 

Break it up 

Doing one long threshold interval would eliminate subsequent oxygen deficit, but outside of a weekly time trial, it’s hard to maintain the effort. Dividing the workout into 5- to 15-minute intervals allows for higher quality work. 

Anaerobic strengthening 

While cycling is an aerobic sport, anaerobic power can still make the difference between a podium finish and a mediocre result. Sprint Intervals are the best means of training anaerobic strength. When picking a sprint workout, remember the anaerobic system depletes quickly and takes time to recharge. 

Long recoveries 

Go to the track and watch a good track sprinter train. They spend a lot of time in the grass—up to four or five minutes between each sprint.  

“If you’re trying to achieve those high powers, you need the long recoveries,” Pickels says. “Accumulating oxygen debt is going to destroy that high power. There’s no point doing a sprint workout if you hit 1000 watts, and then 800 watts, and then 600 watts.” 

Recovery means rest! 

Maybe you’ve heard the expression: “Why stand when you can sit?” Letting your anaerobic system recharge for the next interval means doing as little as possible with your legs. Get off the bike and sit if you have to. 

Keep the workout short 

Keep your warm-up to 10-15 minutes, and go home as soon as you’re done with your intervals. The entire workout should be around an hour, at the most.

Training the deficit 

There are 20 minutes left in the race. Someone attacks and you need to sprint to catch on. But then a few seconds pass and there’s another attack.  

We’ve all been there. If you spend too much of that time in deficit, you’ll be in trouble. Fortunately oxygen deficit, like any system, can be trained if it’s stressed.  

“Oxygen deficit is the thing that a lot of people are going for, and you’ll see huge increases not only in VO2max but in submaximal performance as well,” Pickels says. 

Short efforts and short recoveries 

To improve oxygen deficit, your efforts need to be short and hard. Generally, 20 to 30 seconds is enough time. Likewise, the recoveries need to be equally short to prevent anaerobic pathways from recharging. Common intervals are 30-30s (30seconds on, 30 seconds off) and 20-10s.  

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