I just listened to the S2E162 of Koopcast https://www.jasonkoop.com/podcast and the Extramilest #56 https://extramilest.com/blog/gareth-king-extramilest-show-56/ podcast. I respect a lot Jason Koop and what he does for our sport and have been doing a very long time.On the other hand, I often totally disagree with his views, critiqued his book a multiple times and raise eyebrows about his simplified avoidance of certain scientific researches and anecdotal tales, while totally accepting and kneeling before others. Anyways, he is great and the more is talked about our sport the better it is for us !
So while in one pcast we can hear about laughing at the Zone 2 training and calling it a craze, in the other one we meet a guy Gareth King who went from 3:34 to 2:27 in the marathon, extended his endurance and represented Northern Ireland in the 100km world champs and have the national record for 50k and 100k. We talk about a regular guy and not a talented runner, who just stuck out the bad times, did not listen to the naysayers and feel absolutely blasted during training, never showing a sign of fatigue !
So no, there is no limit to Zone 2 training. Why ? Because, you do not build up just workout on workout, or month on month, but actually decade on decade !
First of all a Zone 2 based system or Maffetone Method are not exclusive to Zone 2 heart rate ! There is Zone 1 work too, there is walking, diet and strength and conditioning involved constantly. When testing is going well and the aerobic system is ready, higher heart rate training can be added in, even multiple times a week. This will lift up your Zone 2 pace the new heights. So now, your regular daily training is faster at the same Z2 heart rate, and if you ran the same hours like usual, you will get better stimulus and do longer mileage. Or, if you felt that it is too fast to run like that all the time, now you can drop your heart rate and run slower, to get even faster at lower heart rates and increase your mileage drastically. You can work on uphill Zone 2 capacity, downhill running, trail running and so on.
Actually Gareth says nicely that his major high intensity work drops between 80 and 85% of HRMAX. Guess what, for a 40year old that is 143 to 154 BPM heart rate, what is over the aerobic threshold but under the lactate threshold, just like Marius Bakken prescribes in his procedure that all the Norwegians are using, like the Ingebrigtsens and Kristian Blummenfelt too. That is around 1.8 to 3mmol/l of blood lactate . Once in a while he pushes over or prescribes a week of VO2Max development, where overall running volume drops.
I mean, using RPE as Koop does to prescribe intensity, when we can quite precisely dial in with heart rate, is very strange to me. The thing is with heart rate, that it not only does guidance, but foresees recovery already in the meantime. As an example, on a good day, I might run 3:45/km pace on 150bpm heart rate, on a bad day maybe 4:00/km and on a great day 3:38/km. Guess what, the moment my HR stays 150, my recovery will be the same all the time, no matter that I covered 15km or 16.5km an hour. There are times, when you are preparing for a race and you must dial in a certain pace, yeah sure. But this is like sub 1% of total training volume. The 99.5% rest is pulling up abilities ! You want to be able to produce, eliminate, recycle and resist lactate, for instance. 1h @ 150bpm is 1h @ 150bpm ! Next day you are out the door again and keep on building that aerobic engine.
Except if you ran 3:38/km on the 150bpm heart rate and you gave a meaning to that pace, that from now on, that is your LT pace, you can be in trouble. On a bad day, that speed can push you up to 160bpm heart rate, then the next day will look like differently, but most probably the days after too. You can get a fantastic stimulus from 1h @ 160bpm, except that now you’re jogging around in low zone one for 5 workouts, while the MAF runner who completed the 150bpm 1h run can crank out several workouts at high Zone 2 and can get way further !
Then we can talk about the neuromuscular benefits and imprinting good biomechanical habits. Where do you have better form ? In low zone 1, that you would need a lot after hard workouts, or in Zone 2, where you fly and breathe well ? Doing 2 hard workouts a week and running like pre is superb, but then the 6 extra shuffle workouts look like crap. Doing 2 controlled workouts a week and then sticking out the rest in Zone 2, or maybe running as well in Zone 1 but much more efficiently, will have long term better results !
In addition the Garmin Elevate 4th generation is a great sensor at the back of your watch that is actually superbly accurate at low heart rates. Just pull the watch a little higher. I got the Instinct 2 Solar for over a year now and I am freakin’ satisfied with it. Then you can also get a cheap optical heart rate strap to use it on your forearm or biceps. I use that to dial in my threshold work.