Twice a week? Why not do intervals more frequently? The medical literature is interesting. A study is done, in this case, looking at interval training twice a week. Subsequent investigators use the same frequency. 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 your training week has room for just 2 (or perhaps 3) interval days.
But this personal observation from Dr. Mirkin suggests that there may be another way, that you should consider incorporating periods of increased exertion (intervals really) into every ride.
He changed his mind based on personal observations that the more traditional approach was not working for he and his tandem partner. "However, 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 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 and when he eliminated the rest days (at least a regimented number per week) he actually had less overall muscle pain. He also speculated that every ride should include some stress to provide the stimulus to maintain or improve his speed. And finally the only reason to do extra easy miles was to acclimate the riders butt and shoulders to prolonged time in the saddle. Basically 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 a 100 pedal strokes (which at a normal cadence is about a minute). And this number based on how the legs felt. Not an arbitrary number to be mindlessly finished. "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."
So instead of a mandatory one or two rest days every week, they rested 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 interval riding with intervals that might be longer in duration. And the total riding time often 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. And finally, even on a rest day of easy spinning there would be mild changes in tempo throughout the ride.
But along with adding the physiologic stress of interval training to every ride was the concept of backing off, or stopping completely, if the legs were tired after the warm up. Not an 'I must ride' approach to training.
My guess is that a lot of us take this approach already, varying our tempo. How is Dr. Mirkin's approach different?
1) First, if you have the time, you can benefit from daily riding (or 6 days a week). You may feel better than if you were focused on taking 2 days off the bike each week.
2) Rest is important. But you incorporate it into your riding program by listening to your body – and being disciplined about it. If you are tired after your warm up, stop. Get off the bike. You have had your daily ride and will benefit more from the rest than the additional miles.
3) The traditional 2 interval days can still be part of using physiologic stress to improve cardiovascular and muscle systems. But now you are adding, within the limits of how you feel, some stress to every ride. And a focused stress within each ride – counted revolutions, a sprint up a hill, a race to the next light post or telephone poll.
My impression is that cyclists are a pretty focused group. This suggests we need to change that focus from the weekly schedule of riding days of different types and instead look at each day as it comes – rest if tired, push a bit every time we are on the bike, and still keep a day or two of focused interval training.