There are a couple running jokes in the CrossFit world about lifting weights and cardio. For example: “anything over 5 reps is cardio.” And “my cardio is just lifting weights faster.”
At first glance, it makes sense. You’re breathing hard. Your heart is pounding. You’re sweating. So… that means it’s cardio, right? Not exactly.
Dr. Peter Lemon said it best, “Just because something sounds logical doesn’t mean it’s physio-logical.”
Understanding how your body actually adapts to different types of training—especially at the level of your heart—can completely change how you approach your workouts.
Let’s break it down in a way that actually makes sense.
The Role of the Heart: More Than Just Beating Faster
Your heart’s main job is simple: pump blood efficiently throughout your body.
The key player here is the left ventricle—the chamber responsible for pumping oxygenated blood out to your muscles.
Two major things determine how well your heart does this:
- Stroke Volume: How much blood your heart pumps with each beat
- Heart Rate: How often your heart beats
Most people think improving cardiovascular fitness means increasing heart rate. But the real magic happens when you improve stroke volume.
And this is where cardio and strength training start to differ.
Cardio Training: Building an Efficient Pump
Traditional cardio—like running, biking, or rowing—creates what’s called eccentric hypertrophy of the left ventricle.
In simple terms:
- The chamber gets bigger
- The heart can hold more blood
- Stroke volume increases significantly
Think of it like upgrading from a small cup to a large bucket.
With each beat, your heart pushes more blood out, which means it doesn’t have to beat as often to do the same work.
Result:
- Lower resting heart rate
- Greater endurance
- Improved oxygen delivery
This is why endurance athletes often have resting heart rates in the 40s or 50s.
Strength Training: A Different Kind of Cardiac Stress
Now let’s talk about lifting weights.
During heavy strength training—especially movements like squats, deadlifts, or carries—you’re often:
- Holding your breath (even briefly)
- Creating high internal pressure
- Compressing blood vessels under load
This leads to a different kind of demand on the heart.
Instead of pumping large volumes of blood continuously, your heart is working against increased resistance—almost like pushing against a kinked hose.
This results in concentric hypertrophy of the left ventricle:
- The walls of the heart get thicker
- The chamber size doesn’t increase as much
- The heart becomes better at generating force, not volume
Result:
- Improved ability to handle high pressure
- Stronger cardiac muscle
- Better support for short bursts of effort
Why “Out of Breath” Doesn’t Always Mean Cardio
Here’s where things get misunderstood – you finish a heavy set. You’re gasping for air. It feels like cardio.
But what you’re experiencing is often a response to:
- Mechanical compression of blood vessels
- Temporary restriction of blood flow
- Rapid changes in pressure inside your body
Your heart is working hard—but not in the same way it does during steady-state cardio.
Being out of breath doesn’t automatically mean you’re improving your aerobic system in the same way as a brisk walk, jog, or bike ride would. If it did, then experiencing anxiety would make us all into endurance athletes, and we could improve our aerobic system by watching scaring movies. If only!
The Key Difference: Volume vs. Pressure
Here’s the simplest way to understand it:
- Cardio = Volume Load
Your heart adapts by pumping more blood per beat - Strength Training = Pressure Load
Your heart adapts by becoming stronger and thicker
Both are valuable—but they serve different purposes.
Why You Need Both for Long-Term Health
If your goal is heart health or longevity, relying on just one is leaving results on the table.
At Viking Athletics, we coach our members to combine:
- Strength Training (2–4x/week)
Build muscle, bone density, and resilience - Zone 2 Cardio (2–3x/week)
Improve stroke volume, endurance, and recovery
This combination gives you:
- A heart that can pump more blood efficiently
- The strength to handle real-world demands
- Better recovery between workouts
- Reduced risk of cardiovascular disease
Neglecting cardio and ONLY strength training can lead to thicker ventricular walls, which reduces chamber volume. That’s the opposite of what we want. So, it’s not an either or, it’s a both situation.
The Bottom Line
Lifting weights and doing cardio both “work your heart”—but they do so in fundamentally different ways.
- Cardio makes your heart more efficient
- Strength training makes your heart more forceful
And no—getting out of breath during a heavy lift doesn’t replace true cardiovascular training.
If anything, it highlights why you need both.