Zone 5 Won’t Wreck Your Health. A Low VO2 Max Will.

If you read my post on Zone 2, you’ll know I spent a good few thousand words making the case for going easy, on purpose, the (vast) majority of the time.

This is my necessary companion piece, because the honest answer to “what cardio should I do?” was never just Zone 2.

It’s a lot of Zone 2, and a little of something that sits at the entirely other end of the dial: Zone 5.

What Zone 5 feels like

Zone 2 is the effort you can hold a conversation through, the one where a phone call while you’re on the treadmill only lightly sounds of effort.

Zone 5 is the opposite of that in every respect.

You’re working somewhere north of 90 percent of your maximum heart rate (ways to calculate that here, as the old “220 minus your age” is wayyyyy outdated), breathing too hard to string a sentence together, and counting down the seconds until it’s over.

It isn’t meant to feel sustainable, because it isn’t. Where Zone 2 asks your body to get efficient at a pace you could maintain for about an hour, Zone 5 asks it to briefly do more than it thinks it can, for a minute or two at a time, recover, then do it all again. Whew.

What Zone 5 is actually doing for your physiology

In case I haven’t made the distinction clear, Zone 2 and Zone 5 aren’t just different intensities of the same thing. They’re training different systems entirely.

Zone 2 builds your aerobic base: more mitochondria, better fat oxidation, a heart that pumps more efficiently at rest and at low effort. Useful, foundational, and essential.

Zone 5 is what actually raises your fitness ceiling. This is the intensity that pushes your VO2 max, which describes the maximum rate at which your body can take in and use oxygen, and is quite possibly the single best predictor we have of how long and how well you’ll live. Your VO2 max is also one of the four markers of metabolic health we screen for at LIFT Clinic, alongside handgrip and functional strength, body composition, and lipid/glucose/protein metabolism.

Zone 2 makes the engine you already have more efficient. Zone 5 makes the engine bigger.

You need both, but if you only have time or appetite for one hard thing this week, Zone 5 is the one with outsized returns.

The protocol I coach: the Norwegian 4×4

If you want one Zone 5 format to anchor down your workout week, the Norwegian 4×4 is it. Four rounds of four minutes at 90–95 percent of your max heart rate, with three minutes of easy recovery between each. It’s sixteen minutes of actual work, and takes roughly thirty minutes to complete, door to door.

The Norwegian 4×4 is the most heavily studied VO2 max protocol there is, and for good reason: the original 2007 trial found roughly a 7% improvement in VO2 max in eight weeks, three sessions a week, and later work has replicated gains of similar or greater size. This is a big deal, considering VO2 max typically drops 10% per decade if left untrained; up to 15% each decade after menopause. That is, genuinely, one of the better returns on thirty minutes you’ll find anywhere in exercise science.

You don’t need a heart rate strap and a stopwatch to get it right, though I heartily (get it) recommend both if you’re serious about tracking progress. The internal cue is simple: by the end of each four-minute block, you should be working hard enough that just finishing the fourth minute feels like a trophy-worthy achievement.

What Zone 5 might actually look like for you

Given that most people’s joints, schedules, and if we’re being honest, enthusiasm for sprinting on concrete are roughly zero, here’s where I’d point you:

The assault bike (accurately named, if you’ve ever tried one) is my first choice for most clients. It’s fully joint-supported, the resistance is entirely responsive and within your control, and there’s no eccentric loading to worry about the next day.

A rowing erg is a close second. It’s full body, low impact, and it teaches you something about pacing that running never quite can.

An incline treadmill walk, pushed up to 12–15 percent grade and done without holding onto the handles, can get you into genuine Zone 5 territory, which is a very underrated option if running has never agreed with you.

What I’d actively steer you away from, at this stage of life and without a specific reason to do otherwise, is the aforementioned repeated hard running sprints on pavement. If sprinting is your thing (and it is mine!), a rubber track or treadmill is your Zone 5 bestie. The injury cost-benefit definitely bears consideration when it comes to how you choose to Zone 5.

The cortisol conversation, because I know it’s coming

If you’ve spent any time in the wellness corner of the internet, you’ve almost certainly seen the claim that high-intensity training is secretly sabotaging your hormones. The trolls and influencers will tell you it spikes cortisol, wrecks your sleep, and stalls fat loss in perimenopause.

I want to push back firmly on this, because no research supports the blanket application of that warning.

Cortisol rising during a hard effort is a normal, healthy, entirely expected response to physical stress, whether in a forty-five-year-old or a twenty-five-year-old. The same is true of a hard conversation with your boss, which can have the same cortisol-spiking effect. What matters is whether it comes back down, as well as whether your body is able to handle and recover from its stressful moments.

Several trials in midlife adults have found that a structured course of properly-dosed HIIT actually lowered resting cortisol over several weeks, not raised it.

Real nuance worth holding on to, though: this only applies if you’re recovering properly around it.

One Zone 5 session a week (or one every other week), not five. Adequate sleep and food around the session, not a fasted 6 a.m. effort on four hours of sleep after a week of skipped meals and international work travel.

If a Zone 5 session leaves you wired at 11 p.m. or dreading the next one, that’s useful information about your current recovery capacity, not a referendum on whether hard training belongs in your life. It does.

Time to get in the Zone

For most of my clients, a complete training week has one Zone 5 sitting alongside two to three Zone 2 sessions and 90 combined minutes of progressively loaded strength work (broken up as they wish), with actual rest days and lots of mobility in between.

Zone 5 isn’t a large time commitment. It’s a demanding one, for about 10-20 minutes, which is honestly the appeal if you ask me.

One caveat before you kick into Zone 5 gear: if you’re newly returning to exercise, have any history of cardiac issues, or simply haven’t done anything harder than a brisk walk in some time, get medical clearance first and build your aerobic base for a few weeks before you consider touching this.

Zone 5 rewards a body that already has some fitness to draw on. It isn’t the place to begin. But once you’re ready for it, it’s one of the few things in the exercise world that gives you an ROI far more than the sixteen minutes it costs you. Go get yours.