Are You Doing Cardio or Improving It? The Zone 2 + Zone 5 Truth

If you've spent years hopping on a treadmill or elliptical at a moderate-hard pace, breaking a sweat, and calling it a day, you've been doing cardio. But there's a difference between doing cardio and actually improving it — and understanding that difference is one of the most valuable things you can learn about training.

The zone 2 training benefits you've probably heard mentioned by longevity researchers and endurance coaches aren't hype. They're rooted in real physiology, and knowing how to pair zone 2 with its opposite — high-intensity zone 5 work — is what separates people who maintain their fitness with age from people who quietly lose it.

What "Zones" Actually Mean

Heart rate training zones break exercise intensity into five bands, from very light (zone 1) to all-out effort (zone 5). Zone 2 sits at roughly 60–70% of max heart rate — a pace where you can still hold a conversation, just barely. It feels almost too easy, which is exactly why so many people skip past it in favor of harder workouts.

Zone 5, by contrast, is 90–100% of max heart rate: short, hard bursts that leave you breathless within seconds.

(Cleveland Clinic: https://health.clevelandclinic.org/zone-2-cardio).

Most people default to the middle — moderately hard efforts that feel productive but don't fully develop either end of the system (zones 3 and 4). That's "doing" cardio. Training deliberately in zone 2 and zone 5 is "improving" it.

The Case for Zone 2: Building the Engine

Zone 2 training is having a moment in longevity science, largely thanks to researchers like Dr. Iñigo San-Millán, who has spent decades studying how this specific intensity affects mitochondria — the energy-producing structures inside your cells.

At zone 2 intensity, your body relies primarily on fat oxidation for fuel, and lactate produced during the effort acts as a signaling molecule that promotes mitochondrial biogenesis, essentially prompting your cells to build more energy-producing capacity.

(The Proof, with Dr. San-Millán: https://theproof.com/lactate-the-key-to-metabolic-health-mitochondria-and-longevity-dr-inigo-san-millan/; Peter Attia MD: https://peterattiamd.com/inigosanmillan2/).

Mayo Clinic Press notes that this kind of steady, moderate effort builds the aerobic base that supports everything else you do physically — from a hike on the Na Pali coast to recovering well between strength sessions — while minimizing the injury and overtraining risk that comes with going hard every time you train.

(Mayo Clinic Press: https://mcpress.mayoclinic.org/nutrition-fitness/zone-2-cardio-what-is-it-and-why-is-it-trending-online/).

It's worth noting that some recent research, including a 2025 narrative review in Sports Medicine, has pushed back on how strong the mitochondrial evidence really is — a healthy reminder that zone 2 is one valuable tool, not a magic bullet. But even the skeptics agree it builds a durable aerobic foundation with very low orthopedic cost, which matters enormously for anyone managing joints, tendons, or a busy schedule.

The Case for Zone 5: Building the Ceiling

If zone 2 builds your engine, zone 5 raises its ceiling.

Short, intense intervals train your fast-twitch muscle fibers, improve mechanical efficiency, and — critically — drive up VO2 max, your body's maximum capacity to use oxygen.

VO2 max isn't just an endurance-athlete stat. A landmark study of over 122,000 patients at Cleveland Clinic, published in JAMA Network Open, found that cardiorespiratory fitness was inversely associated with mortality with no upper limit to the benefit — people in the highest fitness categories had the lowest risk of death from any cause, more than any single medication or risk-factor modification could offer.

(NCBI/PMC: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8319417/).

Other research going back to Steven Blair's foundational work in JAMA found that low cardiorespiratory fitness roughly doubled mortality risk, independent of weight or smoking status.

The good news for anyone over 40 worried about "doing too much": high-intensity interval training doesn't require long sessions. Research shows HIIT protocols can deliver similar — sometimes superior — cardiometabolic benefits compared to longer moderate-intensity sessions, in a fraction of the time, and studies on older adults specifically show meaningful improvements in fitness, blood pressure, and cholesterol from structured interval work.

(ScienceDaily: https://www.sciencedaily.com/releases/2010/03/100311123639.htm; NCBI/PMC — HIIT and older adults: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9238013/).

Training Both Ends Without Wrecking Your Joints

Here's where the "doing vs. improving" distinction gets practical. Most people's cardio routines drift into a gray zone — not easy enough to build the aerobic base, not hard enough to meaningfully raise VO2 max. That's a comfortable place to train, but it's the least efficient one physiologically.

This is exactly the gap the CAROL AI Bike was designed to close for members at Strength Studio Kauai.

CAROL runs on Reduced Exertion High-Intensity Interval Training (REHIT) — a protocol built around just two 20-second all-out sprints inside a sub-10-minute session, with the bike's AI calibrating resistance to your real-time power output. In an 8-week study out of Western Colorado University, a REHIT group training on CAROL improved VO2 max by 12.3%, versus 6.9% in a group doing five 30-minute moderate-intensity sessions a week — nearly double the gain, in roughly 90% less time (CAROL Bike).

The underlying mechanism is rapid glycogen depletion: foundational research by Metcalfe and Vollaard found this ultra-short, maximal-effort format drives real gains in aerobic capacity and insulin sensitivity, despite the tiny time investment (PubMed).

It's genuine zone 5 stimulus, without the joint impact of sprint running or the time cost of a full interval workout.

And on the recovery side, a Sunlighten infrared sauna session supports the circulatory demands that both zone 2 and zone 5 training place on your cardiovascular system, an especially relevant complement given the growing research interest in heat therapy and vascular health.

The Takeaway

You don't need more hours in the gym — you need more intention in how those hours are spent.

A smart weekly rhythm includes real zone 2 work (steady, conversational effort, several times a week) and real zone 5 work (brief, hard intervals, once or twice a week), rather than a week of moderately-hard sessions that build neither base nor ceiling efficiently.

That's the difference between doing cardio and improving it, and after 40, it's a difference worth training for deliberately.

Ready to find out where your training actually sits — and build a program that closes the gap? Visit Strength Studio Kauai to schedule a session and see how ARX, Vasper, and infrared recovery fit into your plan. Give us a text/call at 808-346-4668 or use the scheduling tool on our website to come and visit!