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Training: Optimizing interval workouts using oxygen deficit

  • By Trevor Connor
  • Published Mar. 25, 2014
To make sure you have enough left in the tank for the finishing sprint, it's important to understand interval training. Photo: Tim De Waele | TDWsport.com

My friend didn’t know what to expect. He had just arrived in California, and while the weather was great, it was the company that scared him. It was his first ride with Levi Leipheimer and he prayed he’d be able to keep up. They were soon riding the hilly Northern California landscape. My friend, an accomplished mountain biker, was happy to see that he could hang with Leipheimer on the climbs, even if he was at his limit. For two hours they explored back roads while he wondered why their workout was so undirected. Eventually they stopped. With a smile, Leipheimer said, “Thanks for the ride, I have to go do my intervals now.” Just one sentence and my friend learned what being a Tour de France contender meant. The workout hadn’t been undirected. It was just the warm-up.

For those of us looking to optimize our performance, the days of purely undirected rides are behind us. As Robert Pickels, an exercise physiologist at the renowned Boulder Center for Sports Medicine, explains it, “By focusing on specific workouts, you have a much better sense of what you have done, what you need to do and how to recover.”

While a quick Google search will bring up hundreds of interval options, they too often just list times and zones. There’s a lot more to it if you want to make your intervals truly effective. So, let’s look at how to optimize your intervals.

Back to basics: Aerobic vs. anaerobic

Vince Lombardi was famous for starting team talks with the line, “This is a football.” The message was clear: never forget the basics. The same philosophy applies to cycling. We have two ways of producing energy: aerobic work requires oxygen and doesn’t fatigue easily; anaerobic work is fast, strong, 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 optimally stressing one of our energy pathways. As Pickels puts it, “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.”

Aerobic power: Be mindful of the deficit

You’re out for a ride with a few friends. You hit that steep climb and suddenly the competitive fire has you setting all new power records. Yet minutes pass before your heart rate catches up with how you feel. 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. This effect is called oxygen deficit; with an increase in intensity we initially rely on anaerobic metabolism to meet the increased energy demands. According to Pickels, “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.”

Oxygen deficit can be critical in effectively targeting your intervals. Being in oxygen deficit generally means your aerobic system is not being fully stressed. If you are doing purely aerobic-focused work such as threshold intervals and you spend the majority 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.

Anaerobic power: The big spender

Think of the fastest anaerobic animal on the planet, the cheetah, capable of running at 70 mph. Now think about what it’s doing when it’s not hunting down some slower prey — it’s laying around. We can produce a lot of anaerobic power, very fast, but it doesn’t last long and it takes time to recharge. While you have to do work to activate aerobic pathways, the best way to prepare for anaerobic work is to do nothing.

The box below gives suggestions on how to approach the different types of intervals, but in selecting which ones you should do, remember, if you want to achieve optimal performance you have to start with an understanding of yourself. As Pickels said, “The interval workout really needs to be designed based on what your goals are, what your weaknesses are, and what you need to achieve with your upcoming performances.”

Maximizing your intervals

Aerobic intervals
Intervals that improve your sustainable aerobic power, such as threshold intervals, are some of the most important work a cyclist can do. These intervals are generally five to 15 minutes at your threshold heart rate/power. Also popular are VO2 max intervals, which are one to four minutes at slightly above threshold. They force normally anaerobic muscle fibers to work more aerobically. The key point: aerobic intervals are most effective when oxygen deficit is minimized. Here are tips on how to do that:

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 1-3 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 at least 20 to 30 minutes of easy to tempo riding before starting your threshold work.

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

Same length/intensity
I’ve seen many riders start strong and get slower with each interval. That’s because their anaerobic reserves are depleting and they never fully utilize their aerobic system. Make sure your intervals are a consistent intensity. Target the exact same wattage or pace (on a flat road) for each. Done correctly, the first interval should feel hard, but not unbearable, and your heart rate will be a little below your threshold. By the final interval you should struggle to maintain pace.

What should you do in the recovery periods?
Keep the legs spinning between threshold intervals. This will keep the aerobic system primed. Active recovery will also aid lactate clearance.

Anaerobic strengthening
While cycling is an aerobic sport, anaerobic power can still be the difference between a podium and middle of the pack at the end of a race. Sprint intervals are the best means of training anaerobic power.

Long recoveries
Says Pickels: “If you’re trying to achieve those high powers, you need long recoveries. Accumulating oxygen debt is going to destroy that high power. There’s no point doing a sprint workout if you hit 1,000 watts and then 800 watts, and then 600 watts.” Recoveries should be as long as four or five minutes between sprints to recharge your anaerobic pathways.

Recovery means rest
Watch track sprinters train and you’ll see them spend lots of time sitting around. 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 to stay anaerobically focused
Limit 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.

The secret weapon: Training the deficit
There’s 20 minutes left in the race. Someone just attacked and you have to sprint to catch on. But then a precious few seconds later another attack goes up the road. We’ve all been there. Spend too much of that last 20 minutes in deficit and you’re in trouble. Fortunately oxygen deficit, like most systems, can be trained if it’s stressed. “Training oxygen deficit is the thing that a lot of people are going for now and seeing huge increases not only in VO2 max, but in submaximal performance as well,” said Pickels.

Short efforts and short recoveries
To maximize the oxygen deficit, efforts need to be short and all-out, generally 20 to 30 seconds. However, the recoveries need to be equally short to prevent anaerobic pathways from recharging. Common intervals are six to 12 repetitions of 30-30s (thirty seconds on, thirty seconds off) and 20-10s.

Trevor Connor is a long-time cycling coach and researches both exercise physiology and nutrition at Colorado State University.

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