Is an elevated heart rate always “cardio”? Not quite

December 5, 2025

Here’s a confession.

When I first got into training, I thought if my heart rate monitor said I was in the “cardio zone”, then boom—I was doing cardio. Didn’t matter if I was lifting, sprinting, or sweating it out in the sauna. So long as my watch hit 150 bpm, I was “getting my cardio in”.

Turns out I was a little misguided.

The reality is, many things will spike your heart rate. But that doesn’t mean they actually train your heart, blood vessels, and mitochondria the way true aerobic work does.

That’s a critical distinction. Especially if you’re chasing better health and longevity and not just a burn. You need to stress the right systems if you want a bigger aerobic engine.

So let’s cut through the noise. I’ll unpack what I’ve learned—both in the research and my own self-experiments—about why not all cardio is created equal.

Many things can spike your heart rate. Only one thing can boost your endurance.

Raising your heart rate doesn’t mean building your aerobic engine.

One summer in my twenties, I decided to “hack” conditioning by parking myself in the sauna every evening. Before long, my HR would easily climb to 130-140 bpm.

I thought I’d found the ultimate cardio cheat code.

Two months later I went for a run. It felt like my shoes were full of concrete. Zero change in my endurance. My VO2max hadn’t shifted, either.

That’s because your heart rate spikes in the sauna from your body dumping heat, not from your muscles demanding oxygen.

There are two key pieces to anything that truly counts as “cardio”.

It has to challenge both oxygen delivery and oxygen use at the muscle. If not, it’s just a heart rate spike.

That’s why training adaptations are so specific. Your body doesn’t just respond to “general” stress—it adapts to the specific type of stimulus placed on it.

Steady-state cardio is the foundation.

Look, I love lifting and I love intensity. But nothing beats plain-old steady cardio for building your aerobic base.

When I string together a decent block of 30-40 min runs at an easy, conversational pace, good things happen. My VO2max jumps, my resting HR drops, HRV goes up, and I recover faster between gym sets.

That’s no accident. Because this kind of “boring” work stresses the right system. And that drives the gains that actually boost your aerobic engine:

You pump more blood per beat (stroke volume)
You grow new capillaries to deliver oxygen to muscle
You build more mitochondria to use that oxygen for fuel

Sauna or lifting might spike your heart rate, but they don’t cause these changes. They only come from consistent, oxygen-demanding work.

That’s why “Zone 2” matters. Jogging or riding at a pace where you can still talk in full sentences, but know you’re working, is enough for measurable aerobic gains.

The difference is your heart is working harder to serve your muscle—not to drop body heat.

If you can talk in full sentences, you’re probably in the aerobic sweet spot.

Strength training raises heart rate, but not for the reason you think.

I remember the first time I tracked my HR during a heavy lifting session.

I was surprised to see it climb into the 160s. It’d stay elevated for minutes between sets too. I thought I had a two-for-one strength and cardio deal.

How wrong I was.

What actually happens when you lift weights is your muscles contract with enough force to restrict blood flow in local vessels. Your heart compensates by working harder to push blood through the resistance—not just because your muscles demand more oxygen.

Other factors—like the pressor reflex and increased sympathetic drive—also make your heart and your lungs work harder when lifting. But it’s not the type of activity where your muscles demand more O2 for extended periods.

That’s why lifting weights drives strength and muscle gains. Not better endurance.

Make no mistake, resistance training provides other essential healthspan components. It builds muscle, bone, strength, power—things you won’t get from cardio alone. Just don’t fool yourself into thinking your VO2max will climb because your watch says 150 bpm after some heavy deadlifts.

Think of strength and cardio as complementary. Both are important. But for totally different reasons.

HIIT provides an edge—at a cost.

There’s nothing like hard intervals to send your heart rate sky high. But it’s more than that. Intense HIIT torches your muscles, makes your lungs burn, and leaves you on the floor.

No doubt HIIT satisfies both the supply and demand sides of the oxygen equation.

And it’s legit for building aerobic capacity. When I throw 4-min intervals into the mix once or twice per week, my VO2max climbs faster than with steady-state work alone.

The advantage of interval work is you can operate near your aerobic ceiling for longer (relative to training time). That’s a powerful trigger for aerobic gains.

The trade off is recovery. The gains may be fast, but too much intensity and you’re cooked. Your sleep tanks, cortisol spikes, and motivation dies. I’ve been there.

That’s why I view HIIT as a complement, not a replacement. Most weeks I’ll add in 1 interval session, maybe 2 if I’m fresh.

It’s a finishing touch, not a substitute for the groundwork.

Sauna is a cardio supplement, not a substitute.

I still hit the sauna 3-4 times on most weeks. Sure, my HR spikes and I sweat buckets, but the benefits are real. My blood pressure is lower. I recover faster. And I just feel better.

Don’t just take my word for it. The data says sauna is linked to:

Lower cardiovascular mortality
Improved heat tolerance
Better vascular function
Reduced inflammation

Sauna delivers real longevity benefits, but it’s not a cardio replacement. It helps you squeeze out more juice if you’re already hitting the right systems.

Science agrees too. A 2022 study found adding a 15-min sauna to a 50-min workout (20 min resistance circuit + 30 min cycling) for 8 weeks boosted VO2max and lowered blood pressure more than training alone.

Sauna might supplement your cardio, but don’t fool yourself into thinking sitting in a hot room replaces the benefits of running a few miles.

Sauna delivers real health perks. But aerobic gains aren’t one of them.

So what actually counts as cardio?

For me it boils down to this:

Does the activity repeatedly challenge my body to deliver and use oxygen more efficiently?

That’s the litmus test. Jogging, cycling, or swimming deliver both—whether it’s easy Zone 2, sprints, or intervals.

Lifting or sitting in the sauna only satisfy one part of the equation. These things have many upsides, but true cardiorespiratory fitness—the kind that lowers your risk of heart disease, dementia, and early death—isn’t one of them.

If your muscles aren’t actively working to demand more oxygen to burn extra fuel, don’t expect aerobic gains from a HR spike alone.

How I structure my own week.

Not all HR spikes are equal. But that doesn’t mean different activity types don’t have their benefits.

That’s why I try to combine all pillars most weeks:

Strength training 3x/week: big lifts, accessory work, gradual progression.
Zone 2 cardio 2–3x/week: 30–40 min runs, cycling, or rowing (at a conversational pace).
HIIT 1x/week: short sprints or longer intervals if I’m feeling good.
Sauna 3–4x/week: 20 min after workouts or in the evening.

To know if it’s working, I measure what matters most. If my VO2max, resting HR, HRV, blood pressure, and strength are trending in the right way, I’m on track.

Simple, sustainable, and driven by data.

What this means for you.

Next time you’re in the sauna or lifting and thinking you’re “getting your cardio in”, think again.

Those things are valuable. But they won’t give you the aerobic base you need for optimal health. You don’t have to run marathons (even though I am). You just need a mix: easy aerobic work, some intensity, and strength and sauna for important accessory benefits.

The key is to build a routine that works for you. This is exactly how we coach people at Rebel Health. Our team works with you to develop an individualized plan suited to your goals, body, and medical profile. The goal is long-term health, not short-term fitness gains.

The bottom line is lifting, sauna, and cardio all make your heart work, but for different reasons.

They are complementary tools with distinct benefits. Not things you can lump together in the same “cardio” bucket.

So go build a body (and engine) that lasts.

Just make sure you’re stressing the right system.

— John

Keen to map out a “cardio” longevity plan that works for you?

→ Book a consult with the RHA team and we’ll make it happen.

Disclaimer: This reflects my personal approach and data; RHA is a medical practice—talk to our concierge doctors for individualized care.