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  Latest update: 11/13/2021



Apply a physical stress to a biologic systems and it will adapt and get stronger. Interval training, repeated cycles of intense physical activity, is an ideal example.

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 spports 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 funcion. 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.

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."

Interval variables

An interval is one cycle 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 4 variables that will impact an interval training program:
  1. Intensity of a single interval's active phase
  2. Duration of a single interval's active phase
  3. Recovery within an interval cycle and between interval sessions.
  4. Number of interval 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.