HIIT vs Zone 2: The Case for High-Intensity Training

The Case for Consistent High-Intensity Training

By Coach Charles, PPP

Zone 2 training is having a moment. Podcasts and longevity books call it the best way to build your mitochondria and burn fat. Low-intensity aerobic work does matter. But for in-season athletes and busy adults, the research points somewhere else: consistent high-intensity training gets more done in less time.

Here’s what two studies show, and how to use them.

The Problem: Time

Elite endurance athletes do huge volumes of Zone 2, often more than 20 hours a week. They also do plenty of hard intervals. Most of the people I coach, from high school and college athletes to working parents, have three or four hours a week, tops.

In my experience, teams lose conditioning as the season goes on. Practice turns tactical and games pile up. Athletes who take time off come back with less capacity, a pattern researchers call detraining (Mujika & Padilla, 2000). The fix isn’t more hours. It’s a small, consistent dose of intensity.

In-season study: less time, better conditioning

Hostrup and colleagues (2019) studied 20 sub-elite Danish soccer players over 10 weeks in-season. Practice was cut by about 20 minutes per session, roughly 20% less training time. The last 14-23 minutes of each session became one of two interval protocols, three times a week:

  • 10-20-30 training: 30 s jog, 20 s moderate, 10 s all-out, five reps per set, 2-3 sets
  • Sprint interval training (SIT): 6 s all-out change-of-direction sprints with 54 s rest
Outcome 10-20-30 group Sprint interval group
Yo-Yo IR1 (repeat-effort conditioning) +330 m (+18%) No change
30-m sprint and T-test agility No change About 1% faster
Mitochondrial electron transport chain proteins +51% +31%
Aerobic enzymes HADHA / PDH-E1α +24% / +40% No change

The takeaway: both protocols worked with less total practice time, and each built a different quality. The 10-20-30 format, with short sprints on top of a high aerobic load, built match-style conditioning. Pure sprints built speed. Most athletes need both, all season long.

Zone 2 review: not the magic intensity

In 2025, Storoschuk, Gibala and colleagues published “Much Ado About Zone 2” in Sports Medicine. They tested the popular claim that Zone 2 is the best intensity for mitochondria and fat burning. Their conclusion: current evidence doesn’t support it.

The signaling story is overstated. Muscles build mitochondria in response to signals like AMPK (an energy sensor) and CaMKII (a calcium sensor). The idea that Zone 2 works uniquely through calcium signaling is, in the authors’ words, “largely based on an unsubstantiated theory.” Zone 2 bouts of 65-70 minutes didn’t raise CaMKII activation. High-intensity exercise activates both pathways.

Intensity drives mitochondrial gains. A meta-analysis cited in the review found exercise below about 60% of max work rate isn’t expected to increase mitochondrial content. Very high-intensity and sprint intervals were most effective for mitochondrial function.

VO2max is the health number that matters. Cardiorespiratory fitness is one of the strongest predictors of disease and mortality risk. For the same amount of exercise, higher intensity produces bigger VO2max gains. ACSM notes that 150 minutes a week of moderate exercise may not be enough to improve it.

Zone 2 still has a place. It improved fat-oxidation capacity in untrained people, and moderate continuous training does improve mitochondria, just slightly less than intervals. It’s a useful tool, not the whole toolbox.

How to apply it

  1. Train intensity year-round. Two or three short, hard exposures a week, including in-season.
  2. Match the interval to the goal. Repeat-effort conditioning: 10-20-30 style work. Speed: short sprints with full recovery.
  3. Use easy aerobic work for base and recovery, not as your only conditioning.
  4. Progress your intensity. If you’re new to training, returning from injury, or in ACL rehab, build up with your coach or clinician first.

The bottom line

If you’re short on time, intensity is your lever. Consistent high-intensity training builds the aerobic engine that athletes need to perform and that everyone needs to stay healthy.

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References

  1. Hostrup M, Gunnarsson TP, Fiorenza M, et al. In-season adaptations to intense intermittent training and sprint interval training in sub-elite football players. Scand J Med Sci Sports. 2019;29:669-677. doi:10.1111/sms.13395
  2. Storoschuk KL, Moran-MacDonald A, Gibala MJ, Gurd BJ. Much ado about Zone 2: a narrative review assessing the efficacy of Zone 2 training for improving mitochondrial capacity and cardiorespiratory fitness in the general population. Sports Med. 2025;55:1611-1624. doi:10.1007/s40279-025-02261-y
  3. Mujika I, Padilla S. Detraining: loss of training-induced physiological and performance adaptations. Part I. Sports Med. 2000;30(2):79-87.