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  Last updated: 1/25/2021


High Intensity Training

When physical stress is applied to biologic systems, physical and metabolic changes adapt to the stress being applied. And this adaptation occurs with any type of physical activity. Interval training consists of repeated sets of intense physical activity (the stress) followed by periods of recovery.

The intermittent stress/recovery approach allows longer total training time at one's peak level of performance for that day's session. A study (in runners) found continuous, maximal performance could be sustained for just 0.8 miles (to exhaustion) while a similar level of peak exertion could be maintained for a total of over 4 miles when the training session consisted of shorter sprint intervals.

The Physiology

When you push your aerobic limits, adaptive changes occur in the cardiovascular system as well as within the muscle and muscle cells themselves. These then let you to push even further into your anaerobic zone the next time out. These include:

In 2015 Place et al came a step closer to understanding the stimulus to these mitochondrial changes. Interestingly antioxidants blunted that anaerobic stimulus, implying these compounds could also affect training benefits. Interestingly highly trained athletes derived less benefit from their anaerobic interval training, suggesting this group might find intervals more useful as a maintenance strategy that purely for VO2max improvement.

This study suggested that resistance exercise (weight training) also produced mitochondrial changes similar to those seen with interval training. This raises the possibility that resistance training might be used to supplement and/or maintain the benefits of interval training.

Subjectively anaerobic stress at the muscle cell is the subjective "burn" associated with intense, anaerobic exercise. Originally the burn was thought to be a build up of lactic acid , but further studies in subjects who, because of a genetic defect, do not produce lactic acid also demonstrated the same anaerobic "burn" as normal riders. Rather than lactic acid, the culprit is more likely to be another acidic intermediary metabolic byproduct, or perhaps related to cellular changes in the muscle fiber itself.

Aerobic training - the mitochondria and gut bacteria.

I'm a firm believer that training programs based on a clear understanding of human physiology will provide better results. Although trial and error will ultimately achieve the same ends, the data from scientific articles should get you there faster while occasionally providing unexpected surprises that might put you a step ahead of the competition.

The traditional teaching has been that the stress of aerobic training leads to adaptive changes in the muscle cell that improve performance. This article suggests that the physiology behind training improvements is slightly more complex than previously suspected.

Mitochondria are the powerhouses of our muscle cells. They contain the enzymes that convert glycogen and fats into the ATP needed to power muscle contraction. With aerobic training, we know the mitochondria increase in both size and number increasing the amount of ATP that can be produced per minute (assuming there is sufficient fuel (carbohydrates) and oxygen delivered to the exercising cell.)

We also knew that regular aerobic training changes the types and numbers of bacteria in our colon (the microbiome) I never understood why exercise might have this effect. Was it the fact that athletes see diet as part of a training program and eat healthier with more fiber and vegetables (which we also know changes the composition of the microbiome)?

The bacteria of the microbiome metabolize unabsorbed food material from our diet (generally fiber), in turn manufacture short chain fatty acids (which are absorbed and provide a modest amount of additional energy for our cells) as well as producing various small molecules that can affect energy metabolism.

This article puts forth a good argument that our mitochondria in some way affect colon bacterial composition which in return produces molecules that will positively influence mitochondrial metabolism and growth. A positive feedback loop which magnifies the benefits of aerobic training.

So what, you might ask., is the point? There are no clear training take-aways......yet. But if we can isolate the bacterial products that stimulate mitochondrial growth, we may have a cue as to how we might magnify the benefits of aerobic training. And as we know our diet can directly affect the microbiome, perhaps we will find helpful or harmful diets that should be included/avoided to maximize a training benefit.

So no recommendations yet. Just a nice example demonstrating how complex physiology can be, and illustrate how a clear understanding could give us clues to provide a competitive edge.

Intervals are the most efficient way to improve VO2max.

This study demonstrates the efficiency of using intervals to provide a training boost. The study compared two exercise routines - 10 minutes including 3 intervals and 45 minutes of steady riding with a 3rd control group on no specific exercise program. The findings: "Twelve weeks of brief intense interval exercise improved indices of cardiometabolic health to the same extent as traditional endurance training in sedentary men, despite one fifth the exercise volume and time commitment."

With limited time, intervals were not only more effective at improving VO2max but also did it more efficiently. More training benefit with less time commitment.

Your base level of fitness impacts how much additional benefit could be gained from interval training. In 2015, Place et al showed that the cell changes to adapt to interval stress were minimized in the highly trained athlete.

Each individual interval is made up of intense pedaling (the active interval) followed by a short recovery to pay back the oxygen debt and flush metabolic byproducts from the muscle. There are three parameters for the active phase.

  1. Intensity
  2. Duration
  3. Frequency

Interval Intensity

The intensity of intervals, not frequency or duration, is the key to a successful interval training program. The following study began with 3 groups of cyclists, all starting from equivalent interval training programs (in terms of frequency, duration, and intensity).

The VO2max of the first< two groups held constant, while that of the third decreased. Their conclusion: intensity is more important than either the duration or the frequency of intervals in maximizing their benefit.

Interval Duration

The active part of short intervals last less than 30 seconds each, while long intervals can be more than two minutes each. The most efficient, time-saving and health-benefiting way to interval train is to use short intervals.

The study nicely demonstrates that short intervals improved one-hour race times the most. The study compared 30-second all-out, 5-minute all-out, and 40--min all-out bouts of cycling. In 10 weeks of twice-a-week, effort-matched trials, they found that short interval training was more effective than long intervals in improving mean power output and VO2max (the maximal amount of oxygen you can take in and use per minute and the best laboratory measure of how fast a cyclist can ride or a runner can run over a distance.

The more intense the interval, the greater the gain in the ability to take in and use oxygen (VO2max), and the faster you will ride or run. The length of time you can hold an interval pace depends on the intensity of the interval. The more intense your interval pace, the shorter the time you can maintain it. It appears that even competitive athletes can maintain maximum exertion intervals for only 30 seconds before they gradually slow.

You can do many more short intervals than long intervals because they do not cause as much muscle damage and do not lead to a build up of significant amounts of lactic acid in less than 30 seconds. Long intervals cause more muscle damage and such high levels of lactic acid that you can do very few of them in a single workout. They will also require a longer recovery time before your next interval workout.

Recreational athletes can train purely with short intervals, but competitive athletes need long intervals as well to train them to "ride through" the lactic acid build up that occurs with intervals over 30 seconds in length.

Interval Frequency

And the intensity and duration of your intervals will impact the third aspect of an interval training program, the frequency (times per week) of interval training days. The longer your intervals, the more minor muscle damage and the more likely it is you may need to add an easier riding day before your next interval session.

I could not find much data on interval frequency in the scientific literature, but from all I've read, Dr. Mirkin's article on interval training makes the most sense to me. He is a proponent of incorporating some training stress into every riding day (even on a slow easy day). He also advocates listening to your body as you start each day's ride and getting off the bike if your legs are telling you that it is not a day to ride. How did we arrive at the magic number of twice a week intervals? Why not do intervals more frequently? The medical literature is interesting in how standards develop. A study is done, in this case looking at interval training twice a week. Subsequent investigators use the same frequency for their studies. And without further investigation twice a week becomes the defacto "optimum". The demands of a balanced training program reinforce this frequency. You need a long day at some point during the week to get use to longer times on the saddle, an occasional day of restful spinning to minimize the risk of overtraining and burnout, maybe a ride during the week with friends, a day or two off the bike with bad weather or to take care of family or work responsibilities, and soon an ideal training week has room for just 2 (or perhaps 3) focused interval days.

But Dr. Mirkin suggests you incorporate periods of increased exertion (intervals really) into every ride. He came to this conclusion based on his personal observation that the more traditional approach was not working for him. In his words: "....every time that you exercise intensely, you damage your muscles. You know this has happened when your muscles feel tight, heavy or sore on the next day. To deal with this soreness, we (he rides a tandem) followed a program of racing as fast as we could three times a week (Tuesdays, Thursdays and Saturdays). On the other four days we would recover by riding 20 to 30 miles slowly, at about 10 to 11 miles per hour. But something was wrong with this program because we were gradually losing our ability to ride as fast as we had in a previous year. We were doing too many junk miles on our four recovery days each week."

He decided that fewer rest days were actually better for them, and found that when he eliminated rest days (at least a regimented number per week) he actually had less overall muscle discomfort. He also speculated that every ride should include some stress to provide the stimulus to maintain or improve his speed, that the only reason to do extra easy miles was to acclimate the riders' bodies (butts and shoulders) to prolonged time in the saddle. That "....Slow riding or running does not increase your ability to take in and use oxygen and it does not make your muscles stronger."

So they changed their training - not more rest, but more intervals "...riding a short distance fast enough to make you very short of breath. Then you slow down until you recover your breath, and keep on alternating short fast bursts with slow recoveries until your legs start to feel stiff and heavy. Then you stop the workout for that day." Intervals were worked into every riding day. Maybe 50 - 100 pedal strokes (which at a normal cadence is about a minute). And this number was based on how the legs felt. Not an arbitrary number to be mindlessly achieved. "On some interval days, we would do 50 pedal-stroke repeats, resting between each long enough to get our breath back. Other days we would do 100 or 150 pedal stroke repeats. We never plan to do a fixed number of intervals. Instead we would stop the intervals as soon as our legs started to feel heavy or stiff, or when our legs did not recover and continued to feel tired a minute after finishing a fast interval."

They replaced a mandatory one or two rest days every week with resting based on how they felt. "...then as you continue to ride, your leg muscles usually start to feel better and you can ride fast after you have warmed up. However, if your legs do not feel fresh after you have warmed up for more than 15 minutes, you should just take the day off. So some weeks this might lead to more days off the bike, and other weeks riding everyday might happen."

With this approach it was the duration and intensity of intervals that would change from day to day. Not the traditional 2 days of focused interval riding with intervals that might be longer in duration. And the total riding time might end up being less than the average "preplanned" ride. Even on what would traditionally be a long slow distance ride, intervals (hills could be substituted) were done. Not as a focused period of time within the ride, but randomly throughout the ride (a fartlick or mini-interval). And finally, even on a rest day of easy spinning there would be mild changes in tempo throughout the ride.

But this does mean you have to listen to your legs. You get the benefit of adding the physiologic stress of interval training to every ride but add the additional step (complexity) of assessing your riding status each day and backing off, or stopping completely, if the legs are tired after the warm up. Not an 'I must ride' approach to interval training. My guess is that a lot of us take this approach already, varying our tempo. How is Dr. Mirkin's approach different?

How do you roll this approach into a more traditional training week? I agree with Dr. Mirkin's conclusion that a sound endurance program should include one or two focused interval workouts and possibly one additional day that includes a few long intervals. But then, instead of building up your weekly mileage with long easy rides, would have "mini-intervals" embedded on a random basis. These then build some metabolic stress into every ride. A short, perhaps 50 pedal revolutions in duration increase in cadence, and then back to base line. Speed up to the next telephone pole. Count pedal revolutions. Push up a short hill. All would qualify as a mini interval.

Interval Rest/Recovery Phase

To get maximum benefit from your interval training day, you need adequate recovery between each active interval. The benefit is directly related to the average intensity of the cumulative 20 minutes of active pedaling. If you don't allow adequate recovery, the intensity of each interval will gradually drop and bring down the average intensity of the total 20 minutes of cardiovascular stress.

The recovery phase is easy spinning with the duration related to the duration of the active phase. As you do not built up significant metabolic byproducts with a short interval, the recovery period can be shorter than with longer intervals. For example:

It is not necessary that your heart rate return entirely to normal before the next interval. If you are using a heart rate monitor, you need only wait for your heart rate to drop to 60 or 65% of your maximum heart rate. If you are using perceived exertion (i.e. how you feel), wait until your breathing has returned to it's normal depth and rate.

Non traditional intervals.

Fartleks. Some riders use a modification of interval training (fartlek training) with alternate periods of slow and fast riding to improve aerobic capacity. It is a "mini-interval" as described below and not as precise as interval training. But has the advantages of: Rolling Hill Intervals. Here is an excerpt from that suggests a way you might use rolling hills as an alternative to intervals. As these are not always found spaced appropriately, they might be considered as "fartleks".

Find a road where little hills come one after another. Attacking these humps can be a peak experience -- like riding a roller coaster. You fly up one side, blast down the other and use your momentum to conquer the next rise.

But if you use improper technique, you can get bogged down. Instead of grinning, you're grinding. You churn up, coast down to catch your breath, then bang against the next wall.

Rhythm is everything. Here's how to keep yours on successive climbs: --As you ride into a hill that takes just seconds to climb, shift one gear lower (next larger cog) than you might normally use. Stay seated and spin fast for about two thirds of the climb.

--If you're riding with others, they'll probably be standing, pedaling slower than you and maybe pulling a little ahead. Don't worry about getting dropped. Keep spinning. You're saving your legs.

-- In the final third of the hill, click to a bigger gear (next smaller cog), stand and apply the pressure. Your legs will still have snap, thanks to spinning to this point. When you hit it right, you'll know where the phrase "dancing up the hill" comes from.

You'll roll right by your laboring companions. Even better, your momentum will carry you over, down and well into the next rise. Then do it again.

Telephone Pole Sprints. Or this suggestion from the webzine: Dial up some telephone pole sprints. When we're training alone, sprinting against imaginary opponents can be deadly dull. Next time you feel like some speed work, use telephone poles as sprint markers. After warming up, start by sprinting from one pole to the next and then spinning easily for 4 poles. Repeat 3-5 times. To vary the drill and increase the effective length of your sprint, go all out for 2 poles, spin easily for the next 4, and repeat 3 times. Of course, all telephone poles aren't the same distance apart. Use the varying spacing to simulate race conditions. After all, you never know how long you'll need to sprint. Go hard to the next pole, no matter how far it is, then spin for a minute or two to recover. Follow this with another sprint between poles. It's perfect for developing the ability to rev up in an instant and then hold your speed for the required distance.

High Intensity Training (HIT or HIIT). High Intensity Training (HIT) is an interval training adaptation for athletes already at a high level of training. Although elite athletes do not gain as much improvement in VO2max as the rest of us, it can help them maintain their edge.

And HIT is a great strategy to maintain CV fitness while on vacation, or during those long dark days of winter when you can't get out on the road. "...HIIT requires an all out effort for 30 to 60 seconds followed by a minute of recovery. Repeat five sets - three times a week. can maintain your aerobic fitness with just 3 ten minute sessions a week."

So you do have choices. A structured program of 2 to 3 dedicated interval training days per week. A fartlik program that varies riding intensity on all types of ride - adapting interval intensity and duration to your perceived level of fatigue. Or HIT to help you maintain cardiovascular gains while on vacation.

Heart Rate Intervals. If you have a heart rate monitor, you can key intervals to your maximum heart rate. Ride your intervals at 80 to 90% of your maximum heart rate and spin easily until your heart rate drops to 60 to 65% of maximum.

Endurance Ride Intervals. You not only get credit for some interval work but can decrease your riding time on a long endurance ride by adding some interval training. Here are two tricks to consider on your next long ride.

And as an added bonus, endurance ride intervals may provide a "pick me up" in energy. When you are on a long ride, a slow, meandering pace can make you feel sluggish or even bored. Before that happens, give your legs (and your bottom side) a little lift. Throw in a short "pickup" every once in a while.

Watch for opportunities. Get out of the saddle and accelerate away from stop signs, over short hills, out of turns or past the lair of a troublesome mutt. Don't script these pickups. Instead, do them when the terrain or situation asks for it. To do a pickup, choose a cog 2-3 teeth smaller (higher gear) than you'd normally use for the situation. So, if you'd usually roll over a rise in a 53x21-tooth, use the 53x19. Don't sprint all-out. That's not the purpose. Instead, simply stand and wind up the gear for 10-12 seconds. Effort should be about 80% of a flat-out sprint. You shouldn't be panting after you sit down. A few deep breaths should get you back to the ride's baseline effort. You'll be amazed at how much better you feel on longer rides when you relieve saddle pressure and treat your legs to these brisk efforts.

Pace line Intervals. These training techniques simulate what happens in road racing. They're great workouts and guaranteed monotony-busters as well. Warm up and settle into a single pace line moving at a moderate speed. Then try one of the following:

Training balance and adequate recovery

This well written article, based on this scientific paper, addresses the importance of recovery days. Although it focuses on running, the cardiovascular training benefits should be similar for cycling and swimming.

The science tells us that lack of recovery (riding at lower training intensities) not only fails to add any benefit from your extra riding time, but might be sacrificing overall performance.

A reasonable balance might be 20% of your riding time at a moderate to high intensity pace (perceived effort of 6 - 10 or > 77% MHR) and 80% at a PE of 4 or less. For most of us, the challenge will be to keep our slow and easy rides slow and easy. My guess if you will probably have to work to keep that HR down.

Recovery is important, but you do need to keep up those weekly miles. You cannot just focus on intervals and allow total total weekly exercise volumes (the total number of hours on the bike per week) fall off without impacting endurance performance (time to exhaustion riding at 75% VO2max). It is a combination of intensity of exercise (best achieved with intervals) and total time on the bike (from the long slow distance rides) that determines overall competitiveness or performance in an event or on a longer ride.

Bottom Line: An Interval Program.

To summarize:

You might consider

If you don't have the time for the complete set of intervals you had planned for the day, we know from weight-training studies that the first set or two provides the stimulus for most of the improvement gained during multi-set workouts. If you do five sets of bench presses, for instance, much of the benefit occurs during the first set. The second set stimulates most of the remaining improvement possible from the session. The final three sets do relatively little.

It's likely the same applies in interval training with the first set (or two) of intervals providing the bulk of the benefit and diminishing returns from additional sets. So if you're pressed for time, or just in a hurry to get off the bike, take a break after those first few intervals and come back fresh, ready to give 100% on your next interval day. You will have gotten a significant benefit to maintain past gains, and if you avoid listening to your body, you increase the odds of mental burnout and the inevitable decrease in energy that comes with overtraining.

Don't short change yourself on a 20 to 30 minute warm up at the beginning of and a 15 minute cool down at the end of your session. And also assessing your overall state of readiness before launching off on a series of intervals. I like the approach used by Dr. Mirkin:

An interval training suggestion.

We know that The following is one possible scenario:

With these short intervals, the challenge is keeping track of where you are in the interval cycle, focusing on your maximum effort and not getting lost in keeping track of time or reps. Looking at total time for each of the 3 sets of intervals rather than timing each single interval might help.

Intervals will improve endurance performance as well.

Will interval training improve endurance riding. What is the science?

First article (abstract, full article) - approximately 15 min of intense exercise over 2 wk (a pretty short time) DOUBLED endurance capacity (exercise at ~ 80% VO2max) in active individuals.

Second study (abstract, full article) - in well trained competitive cyclists rather than recreationally active adults. Endurance performance in a 40 km time trial improved after interval training (a minute off of a 57 minute result). The improvement appeared to be from the muscles increased ability to buffer the acidic products of metabolism rather than any change in the riders' VO2max.

Finally, a third study, a review, supported the improvement in endurance performance without a change in VO2max. The authors indicated an increased "tolerance" for fatigue from an increase in the efficiency (using less energy per unit of distance covered).

There is solid evidence that intervals not only increase your VO2max for short sprint performance, but also your endurance performance (rides done at ~ 80% VO2max.)

Can too much exercise harm the heart?

I was asked: "How 'healthy' is the high intensity cycling that I've been doing (at over 50's)? By high intensity I mean continuous effort at 85-90% HRMax with touches of 100% HRMax. And then a few days later I ran across this article in the NYT that stated, assuming no family or personal history of heart disease, there was "... no evidence .there is a level of exercise that is dangerous or too much for a normal, healthy person." Although this is generally true, there is sound evidence that there is indeed an upper limit for cardiac healthy exercise. The curve of benefits versus exercise volume doesn't just plateau, it probably starts to drop off as the extremes are reached . A few examples:

So what is the answer to the reader's question? First, there is no solid data on the upper limit of beneficial exercise (where more is really less health wise). And my comments assume you have no family or personal history of cardiac disease or sudden death.

There are two potential health risks from high intensity cycling - Musculoskeletal (or overuse) injuries and cardiovascular. The musculoskeletal injuries are known to all of us who exercise and participate in aerobic sports. Overuse leads to injury. And the cure is to listen to your body, and if it hurts when you are using it, decrease your activity level. The cardiovascular risks appear to be from repeated stress at the ultraendurance event level. (Pushing through the pain, as it were.) Although acute stress might cause some modest cardiac muscle injury (and leakage of muscle enzymes into the blood where they can be measured) this heals within a few days, and only with repeated injury/healing/injury does scarring appear to be a risk.

There is no evidence that short term, high level exertion such as 30 to 60 second intervals is harmful and also no sound evidence that intervals longer than this add benefit to cardiovascular fitness. The answer and My Bottom Line? Pushing your limits (within reason) is not harmful to your health. But cardiovascular risk appears as you move to the ultra event level.


Do you ride a fast 50 miler or century but want to improve you time trialing or short distance performance?. Then use intervals. If you ride 50 miles (or 100) then you have the endurance base. Now you need to add speed work.

Add intervals of 3 minute duration - repeat 5 times - to your program (ridden at a pace a bit faster than your speed in a time trial and a lot more intense than your century pace). Then spin easy for 3 minutes between each effort. Twice a week. And never back to back days. For variety, longer intervals are also effective - perhaps 10 to 20 minutes at your time trial intensity with two or three repeats.

The following question from a reader suggest that riding with weight is another alternative (just like riding into the wind) to add intensity to your training, simulating an increased load and thus generating a training response. I've had similar comments from other readers.

Q.I was wondering whether you know something about the following: I am riding about 150 pretty fast miles a week, usually with a 12-15 pound backpack (because it is my commute and I carry a laptop and clothes etc.). Sometimes I ride without the backpack and noticed that I am considerably faster. While this is of course not surprising I was wondering whether 'riding with weights' could actually be a useful training technique -- I couldn't find any information about this. - MS

A. Mike, as I mentioned above, added weight (speed remaining the same) is a way to add a stress to your muscles and CV system, which will adapt, and then you will be able to perform more effectively when unburdened. It is the same reason one would weight train as part of a program, or train on a clunker and then get out a titanium frame for that important ride. Dick

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