Some of you might know about these type of intervals. They are generally short effort performed at 105-130% of Functional Threshold Power (FTP) or 80-100% of Maximal Aerobic Power. There are plenty of duration possibilities but I usually go with 2 to 5 minutes intervals with equal rest time between efforts. Their general purpose is obvious: increase one's Vo2max, the maximal oxygen volume your body can process during exercise. I think they are essential in a training programme because they bring some adaptations required to raise the aerobic ceiling but they also provide some of the lower intensities adaptations as well. So, they are essential, but I hate em, and love em at the same time. Here's why.
Damn you, pain!
Yes, these are painful! They generally have a relatively high anaerobic contribution especially in the first effort when your anaerobic stores are fresh. Depending on intensity, duration and pacing strategy, you can manipulate the anaerobic contribution to these efforts and therefore make them more or less painful. Though they will remain very uncomfortable. The intensity means you will create a relatively important oxygen deficit in the first few minutes and depending on your particular physiology (Vo2 kinetics) you will reach the effort's oxygen requirements within a given time duration, usually between 60 and 180 seconds. Say you are very well trained and gifted, and reach the required O2 consumption within 60 seconds during a 4 minutes effort, this means you will theorically spend 3 minutes at your maximal O2 consumption, since the effort's intensity is supposed to ellicit Vo2max. Overall, it all means the same thing: pain in your legs, pain in your chest and soft arms.
They are hard to perform on a flat terrain at a certain intensity. Or at least I prefer doing them on climbs because the power is more consistant, more specific to my racing demand and aerodynamics play's less of a role. So they are perfect for hill repeats if you have some long enough hills nearby.
They generally dont create a large kilojoules expandure since the total work duration is usually between 15 and 25 minutes of effort. For those on a kJ's target, they might not be the best type of workout, even though this aspect is debateable.
Finally, I see them more as a complement, the icing on the aerobic cake (who once said that eh?) so you might not want to live on a steady diet of these intervals. Careful, it does not mean they should only be included in your programme when racing comes close, as is the general belief in the power training community. They do induce a significant fatigue so they should be planned carefully in your programme along with well thought out rest days. Indeed, these efforts rely heavily on your glycogen stores so they should generally be attempted in a well rested state unless you are doing some sort of block including multiple days of hard efforts
Yay, only one more to go!
Given the low total work duration, the workout is usually pretty short and all intervals can be done within a 60 minutes workout. Sure the first, second, third intervals are very hard but the mental pain eases off when you reach the second to last interval. I always see the last effort as a bonus. Easy done, only 4-5 minutes of pain and it's over! Ironically, my last effort is often my best one.
The logistic of such a workout is pretty easy to sort out: find a 4-5 minutes hill, climb it 4-5 times and use the descent as your rest time. The best type of hill is a steady one so you can settle in a certain rythm. Speaking of which...
I like these efforts because their duration allows you to settle into a steady, yet very high rythm and maintain it for a nice chunk of time. This is especially true when doing 4 or 5 minutes efforts. Oh, did I mentionned 5 minutes intervals are WAY harder then 4 minutes ones? That additionnal minute makes a significant difference. The feeling of these intervals is pretty close to race situations when you are digging deep on some medium duration efforts. In my opinion, they are very cross-country specific since XCO racing almost always involves a medium duration hill on the course which is repeated 4-6 times.
Finally, I feel they are very effective at inducing marked aerobic changes that will impact a lot of other durations like your FTP. People generally dont think of them when speaking of mitochondria adaptations but the High Intensity Training guru's seem think otherwise (see my post on Hill Climb Repeats). I think they are a wise training strategy when coupled with either long slow rides or more sustained tempo work.
Purely speculating about these intervals, I thought (and did read somewhere) they could bring significant skeletal muscle adaptations, namely mitochondria and capilary ones. I recall some high intensity training experts discussion on twitter relating the muscle recruitment patern of such high intensity efforts and targeted muscle fibers adaptations. The ''theory'' mainly speculated that these efforts recruited more motor units, therefore more muscle fibers, which would mean more fibers are working to produce power. More working fibers means more tissu needs to recover and adapt to the stress they ecountered during training. More targeted tissu for adaptations means more mitochondria density and size in more fibers. It also means more capillary density and more capillary arround more fibers too. Now this might be all broscience and pure speculation but the theory makes sense in some way.
What about doing these on hills? Hills generally mean lower cadence and more force applied. More force means more motor unit recruitment... Food for thoughts