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All day endurance, say trail running, has four interconnected training components: aerobic base, anaerobic base, technique, and durability. By interconnected, I mean each component can’t be optimized without training the others, but at the same time training the others may be paradoxically detrimental to optimizing a specific component.
AEROBIC BASE may be further split into its two components: cardiovascular endurance (CV, “the delivery”, ~VO2max) and muscular endurance (ME, “the uptake”, ~lactate threshold).
ANAEROBIC BASE can also be split into two components: muscular endurance (the same ME as for aerobic base above), and power/strength.
ME functions on a spectrum: one end of the spectrum is more aerobic (lungs are the rate limiters), while the other end is more dependent on power/strength (legs are the rate limiters).
Z1/Z2 training develops the CV component of aerobic base, and to a lesser degree the ME component. Only training Z1/Z2, however, even for a prolonged period of time, will leave the ME component relatively underdeveloped, and therefore one will remain ‘aerobically deficient’ until this is corrected.
Muscle and/or sport specific interval training (eg, hill sprints and hill fartleks) develops the ME component of aerobic base, and to a lesser degree the CV component (it probably develops both equally per unit time). An extreme, nonspecific example of this type of training would be the metabolic circuits done in Crossfit.
One can independently develop either the CV or ME component of aerobic base to an extreme degree (looking at you DC). Extensive CV development does not require ME development per se, and vice versa. But to fully develop ones aerobic base you need both, and preferably both from the start.
The catch is dosing.
Mans’ ability to perform Z1/Z2 training is almost limitless; we are meant to be moving all day, everyday. Improvements over time can be slow but at the same time regression is also slow.
ME training, on the other hand, can only be tolerated in relatively small doses, and when the dosage is HIGH we can only tolerate it for a short periods of time (weeks) before plateauing. Persistent and significant ME training (eg, >20% of total training time—this varies), regardless if its on the aerobic end of the ME spectrum (UA training), or more on the power/strength end of the ME spectrum (metabolic Crossfit circuits), can also lead to hormonal burnout, especially if you have other stressors in your life.
That said, ME training can be healthy to perform year round, and, in fact, is required to build a strong and complete aerobic base. Small amounts of ME training from the get go probably also speeds up your aerobic development–nevertheless, a stout aerobic base still takes forever to develop. ME training should be low dosage, however, especially in the transition phase. For example, <5-10% total training time, or say 1-8x10sec hill sprints and/or 1-8x40sec hill fartleks. This seems like a homeopathic dose, but due to its potency, it can fulfill the requirement for full aerobic base development, while at the same time not causing burnout. It can periodized (ramped up) as you progress from transition, base, and ME phases of your training.
The downside to not performing at least some ME work from the start may be twofold:
Your aerobic base, as defined above with its CV and ME components, may be underdeveloped going into an ME phase, and although you will still benefit highly from the ME workouts, your overall improvement during an ME phase may be handicapped because you started with such a poor ME base. This is often manifested as feeling too weak, and/or with a HR too high, to complete the initial 45-60min ME workouts properly.
You may also be more prone to injury if you go, for example, from only Z1/Z2 workouts, with or without supplemental strength (gym) training, to a cycle with workouts dedicated solely to ME (often 45-90min long), ie, going from 0% to 20% of your workout volume being ME, this often is just too big of a jump, even with a year or more of Z1/Z2 workouts and a well developed CV system. These injuries may manifest as muscle strains and cramps, often in the calf or groin. Alternatively, going from, say, 5% to 20% gradually over time, may be not only better tolerated, but more fruitful.
DC, it’s my opinion that you should incorporate some Z4 into your schedule, even though there’s a >10% difference between your AeT HR and your AnT HR. Start very small, volume wise, and slowly increase it to about 10% of your training volume over the next few weeks—let this become your new baseline. Then feel free to kick into a full ME peak phase whenever you want.
The easy stuff should remain easy. Continue the bulk of your training below a HR of 152, as you’ve been doing for the past year, whether it’s Z1 or Z2, it doesn’t matter much. I wouldn’t suggest doing much in the 152-170 zone, even though it may still be Z2 based on your AeT drift test. Keep in mind easy doesn’t necessarily mean slow, but it should definitely be easy, perhaps too easy.
(My thoughts on these topics are evolving so any comments or corrections would be appreciated.)