Your lungs can scream during squats, but that doesn’t mean your heart is getting the kind of training it needs.
Quick Take
- High-rep squats can drive heart rate near maximum and push oxygen use into vigorous territory, especially in stronger lifters.
- “Cardio” isn’t just breathing hard; sustained effort matters, and long rest periods change the adaptation.
- Resistance training shines at strength and muscle, but it usually doesn’t replace aerobic conditioning on its own.
- Research keeps pointing to a simple conclusion: combining cardio and strength delivers the broadest health payoff.
The squat “cardio” claim comes from hard numbers, not gym folklore
The recent squat protocol that lit up the debate looked simple: trained adults performed five sets of 10 reps at about 65% of their one-rep max with three-minute rests. Researchers tracked heart rate and oxygen consumption, the same metrics used to judge aerobic intensity. Peaks landed in the neighborhood of 166–184 beats per minute, and average oxygen use across sets hovered around the low-90% range of VO2 max—uncomfortably high for anyone who has ever “just lifted.”
The twist came from who hit the biggest oxygen numbers: stronger participants, likely carrying more working muscle, sometimes registered oxygen demand that rivaled or exceeded their measured VO2 max. That sounds like cardio on paper, and it explains why so many lifters leave the rack feeling like they just ran a hill. Acute stress, though, is only the opening act. The outcome you care about at 45, 55, or 65 is adaptation: what repeated training does to your heart, vessels, and endurance.
Why the rest periods quietly disqualify it from “cardio” in the strict sense
Cardio training works because the body stays under steady oxygen demand long enough to force durable changes: stroke volume, mitochondrial density, capillary growth, and better efficiency at using fat and glucose. Many cardio definitions also rely on sustained bouts, not repeated spikes. Three-minute rest periods chop the session into fragments, letting heart rate and breathing partially recover, then spiking again. That pattern can feel brutal while still failing to deliver the uninterrupted aerobic “signal” your physiology needs.
This is where coach Louisa Sondergaard’s critique lands with common sense. A brief oxygen spike during heavy sets proves the effort was intense, not that you trained the aerobic system the way a continuous run, bike, row, or brisk incline walk does. A conservative way to think about it: don’t confuse being exhausted with building the specific capability you’re missing. If your blood pressure, cholesterol, or family history puts you on the wrong side of risk, you want the training effect, not the workout story.
The principle of specificity still wins, even when workouts overlap
Exercise science has argued for decades about “interference” and “overlap,” but the guiding rule keeps holding: you get good at what you repeatedly practice. Large reviews comparing resistance training and aerobic training show some crossover—lifting can improve cardiorespiratory fitness modestly, and cardio can nudge strength a bit—yet each mode dominates its home turf. Resistance training reliably builds strength and muscle; aerobic training more reliably drives VO2 max and endurance performance. Effort matters, but modality still steers the result.
That doesn’t mean high-rep lifting is pointless for the heart. It can raise heart rate, improve glucose handling, and build the muscle mass that protects aging adults from frailty. The error comes when people use the “my heart rate was high” argument as a loophole to skip all aerobic work. That’s like saying you don’t need to balance your checkbook because you once found a ten-dollar bill in your pocket. One lucky moment doesn’t replace a system.
The real win is hybrid training, and the health math is hard to ignore
The most practical evidence for busy adults doesn’t come from gym debates; it comes from outcomes. Iowa State researchers reported that a split approach—roughly half aerobic, half resistance work—reduced cardiovascular disease risks comparably to aerobic-only training. That should grab the attention of anyone who doesn’t have time for two separate lifestyles. Strength training alone did not produce the same VO2 improvements, while combo training captured broad benefits without asking people to choose a team.
Clinical guidance echoes the same theme with different emphasis. Cardio often shows an edge for stress reduction and direct cardiorespiratory conditioning; strength training often boosts self-efficacy, posture, joint support, and day-to-day resilience. Adults over 40 don’t need a tribal identity; they need coverage. Heart health, independence, and metabolic control respond best when you treat exercise like retirement planning: diversify, keep contributing, and don’t bet your future on a single asset class.
How to use this without falling for the “shortcut” temptation
People chasing efficiency can still respect the science. Lift heavy with adequate rest when strength is the goal. Add truly aerobic work that stays steady long enough to matter: brisk walking, cycling, swimming, or intervals structured to keep the heart working rather than constantly resetting. If you love the burn of high-rep squats, keep them—but stop calling them your cardio insurance policy. Call them what they are: hard work that supports fitness, not a complete replacement for endurance training.
The conservative, reality-based conclusion is almost boring, which is why it’s easy to ignore: the body doesn’t negotiate with slogans. High-rep squats can push vigorous numbers, but the stop-and-go nature changes what you build. The smartest adults treat “cardio versus strength” like “brakes versus seatbelts.” You can argue which matters more, but you’ll sleep better when you have both.
Sources:
Sorry, But Your Strength Training Isn’t Cardio — Here’s Why It Matters
Concurrent training: a meta-analysis examining interference of aerobic and resistance exercises
New research finds half cardio, half strength training reduces cardiovascular disease risks
Resistance training and cardiovascular disease: a review of evidence and recommendations
Cardio vs. Strength Training: Which One Is Better for You?
The Great Debate: Cardio vs. Strength Training
