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  Latest update: 2/17/2023



Why is interval training an essential component of all aerobic training programs? Because intervals improve the athlete's VO2max, the gold standard measure predictor of success in competitive cycling and other aerobic events. Apply a physical stress (maximal aerobic exertion in this case) to a biologic systems and it will adapt and get stronger. Interval training increases a rider's VO2max.

Intervals are the most efficient way to improve VO2max.

This paper reveals how efficient intervals are in providing a training boost. It compared 3 groups of previously sedentary riders. 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."

The Physiology

Anaerobic stress is experienced as the "burn" associated with intense exercise. Originally it was thought to be the result of a build up of lactic acid in the muscle cell, but further studies in subjects with a genetic defect prevents the production of lactic acid, also experience the same anaerobic burn. Rather than lactic acid, the culprit is more likely another acidic intermediary metabolic byproduct, or related to other cellular changes in the muscle cells.

Pushing the athlete's aerobic limits results in changes in the cardiovascular system as well as the muscle cells. These allow pushing even further into the anaerobic zone the next time out.

In 2015 Place et al came a step closer to understanding the mitochondrial changes. Antioxidants will blunt those changes, implying an overuse of antioxidant supplements could negatively affect training effects.

This study suggested that resistance exercise (weight training) can lead to mitochondrial changes similar to those seen with interval training. This supports the idea of using resistance training to supplement the benefits of interval training.

This study suggests their is a limit to how fast we can improve mitochondrial function. This is a graphic representation of the relationship between exercise volume, performance, and mitochondrial function. We also know that regular aerobic training changes the types and numbers of bacteria in our colon (the microbiome). These bacteria of the microbiome metabolize unabsorbed food material from our diet (generally fiber) and as a byproduct of that metabolism manufacture short chain fatty acids (which can be absorbed from the colon and provide a very modest additional source of energy for the muscle cell). But more importantly they also produce other small molecules that can impact mitochondrial development and energy metabolism.

The traditional teaching has been that aerobic training stress leads to an anaerobic state in the muscle cell which then directly induces adaptive changes in the muscle cell mitochondrial to improve performance. This article suggests the physiology behind these improvements is a bit more convoluted than previously suspected. The article puts forth a good argument that part of the improvement in aerobic performance is indirect via the microbiome. Exercise -> changes in bacteria -> production of molecules will positively influence mitochondrial metabolism and growth.

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

Attributes of an Interval Training Program.

An interval is a cycle of intense pedaling (the active interval) followed by a short recovery period to "pay back" the oxygen debt and flush metabolic byproducts from the muscle. An interval training program is a series of individual intervals. The four variables define an interval training program:
  1. Intensity of a single interval's active phase
  2. Duration of a single interval's active phase
  3. Recovery period between active intervals, between sets of intervals, and between interval sessions.
  4. Number of intervals in a set and number of sessions per week

Non traditional intervals.

Fartleks. Some riders use (fartlek training) as an alternative to a more structured interval training session. It is a "mini-interval" and has the advantages of: Rolling Hill Intervals. This excerpt from suggests using rolling hills as an alternative to intervals. As hills are not always found spaced appropriately, it might actually be considered as a variation of fartlek training.

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:

Telephone Pole Sprints. Again from When we're training alone and feeling 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 that is used:

HIIT requires an all out effort for 30 seconds with a minute of recovery. Repeat five sets - three times a week. can maintain your aerobic fitness with just 3 ten minute sessions a week."

Heart Rate Intervals. If you use 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 total riding time on that long endurance ride by adding some interval training. Here are two tricks to consider:

As a 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 body) a little lift.

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 provide a return or benefit benefit from 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 80% 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.

Takeaways - An Interval Program.

  • If you don't have the time for the complete set of intervals you had planned for the day, consider using the HIIT approach.

    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, just got an urgent call, 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%.

    Intervals will improve endurance performance.

    Studies on the benefits of interval training for endurance performance show a wide range of results as the studied groups are so diverse - from untrained riders to elite athletes. In regular riders, a 5 to 10% improvement in total time to exhaustion at an endurance pace (60-80$VO2max)was not unusual after adding intervals to a training program. And studying sedentary individuals produced even more impressive results.

    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.

    A third study, a review, supported an 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).

    And finally, In this study, the group assigned to 30 minutes of steady riding 3 times a week increased their time to exhaustion by 64%. But training with intervals, 3 times a week, doubled the benefit with a 129% improvement in time to exhaustion! Twice as much physiologic improvement for the same amount of training time.

    For those interested, this is a great summary article on the physiology of interval training with a short section on the physiology behind improving endurance performance.

    And for more on improving endurance riding.

    Can you train too much?

    The answer depends on whether you are asking about your personal performance or harming your health, and whether we are asking about interval training or total training time (training volumes).

    Personal performance. Here the answer is a definite "YES", especially for interval training. The risks of too much training (over reaching and over training) are well established in the training literature. This study suggests the reason is mitochondrial (the ATP producing powerhouse of the cell) dysfunction.

    This is a graphic representation of the relationship between the amount of intense interval type training, performance, and mitochondrial function.

    Interestingly, this degree of excessive training also affected the athlete's metabolic health, impairing their glucose tolerance. They went on to study glucose metabolism in world class endurance athletes and found it was also impaired compared with a matched control group.

    Cardiovascular. If you are worried about your heart or cardiovascular system, the answer is fuzzier.

    This study using accelerometer data rather than self reported interviews and questionnaires (which risk being biased.) The authors found no evidence of a threshold or plateau in an inverse association between both moderate and vigorous physical activity and a lower incidence of cardiovascular disease. Their findings suggest that long term physical activity is not only associated with a lower risk for of CVD, but the greatest benefit is seen for those who are active at the highest level.

    This article in the NYT specifically stated that without a 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 for interval type training, there is evidence that when we are talking about exercise volume (endurance training and competing) there is an upper limit. The curve of benefits versus exercise volume doesn't just plateau but is likely to drop off as the extremes are reached . A few examples:

    Although there is less evidence to support acute cardiac injury from excess interval training, cardiac muscle injury with leakage of cardiac muscle enzymes into the blood where they can be measured, this apparently heals within a few days and only with long term repeated injury/healing/injury does scarring appear to be a risk.

    How do you reconcile these findings? Assuming you have no family or personal history of cardiac disease or conditions, the cardiovascular risks appear to be related to repeated stress at the ultra-endurance event level. There is little evidence that short term, high level exertion such as 30 to 60 second intervals is harmful, but also no sound evidence that intervals longer than this add any cardiovascular fitness benefits. Pushing your training (within reason) is not harmful with cardiovascular risk appearing only as you move to the ultra event level.

    All cause mortality. Findings here mirror to a degree the findings with CV disease. Benefit at lower training volumes but evidence for harm at the upper extremes. One study of Harvard alumni found a lower death rate (mortality) among men expending as few as 200 Calories per week in exercise versus those leading sedentary lifestyles, but when they regularly pushed their limits with over 4000 exercise Calories per week the death rate began to rise again.

    Immune system.Several studies have suggested a decrease in immune system competence with intense training (cycling 300 miles per week for 6 months or 2 intensive sessions of running per day for 6 days). This has to be balanced with the overwhelming evidence that a moderate cycling program actually stimulates and improves the immune system.

    Musculoskeletal. 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.

    It seems wisest to chart a course with a balanced approach. Let your body tell you when it is time for an extra recovery day.


    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

    All questions and suggestions are appreciated and will be answered.

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