Is Zone 2 Cardio a Waste of Time? The 10-Minute Workout That Beats It (Backed by Science!)

The search for the minimum effective dose of HIIT led to the discovery that just 40 seconds of maximal sprinting creates a powerful training stimulus. Olrich explains that during the sprints, phosphocreatine and ATP are heavily tapped; the 3‑minute recovery allows those fast energy stores to partially replenish so the second sprint can also be maximal. The subsequent 3‑minute cool‑down helps restore blood pressure and clear metabolic byproducts. A randomized controlled trial showed that doing REHIT three times per week (less than 30 minutes total weekly) increased VO₂max by 12% over 8 weeks, nearly twice the improvement of 150 minutes/week of moderate continuous training. The rate of non‑responders was also lower. Ben notes that he personally uses the Carol Bike for this protocol, sometimes even at health expos when he has only a few minutes.

Ben differentiates between zone 2 training (which should be around VT1, moderately uncomfortable effort) and true low‑intensity movement that falls into NEAT (non‑exercise activity thermogenesis). He observes that many people mistakenly label easy walking as zone 2, when it’s actually zone 1. He believes NEAT is highly valuable for blood glucose management and extra calorie burn, but it does not significantly boost mitochondrial biogenesis or VO₂max. Combining NEAT with a couple of brief, intense workouts per week is his preferred prescription.

Olrich explains that when you search PubMed for “zone 2 training,” you find almost no studies because scientists study clear endpoints like lactate threshold, ventilatory threshold, VO₂max, or all‑out performance. Because the term isn’t fixed, the recommendations floating around (60–90 minutes 3‑5x/week) are based more on anecdote and the schedules of elite athletes than on controlled trials. He notes that some researchers even stay out of the debate because the concept doesn’t match their work. Ben agrees, recalling that in his exercise physiology classes they used ventilatory thresholds, not zone numbers. The implication is that much of the popular conversation around zone 2 is built on shifting sands.

Olrich walks through the numbers: professionals on the Tour de France train 25–35 hours weekly. The maximum high‑intensity dose they can sustain is about 5–7 hours. Once those hard sessions are scheduled, they fill the rest of the week with easy riding because they can’t do more hard work without breaking down. This naturally yields roughly an 80/20 split, but it’s purely a consequence of volume. He and researchers believe it’s not at all clear that this should be the optimal approach for someone who trains a couple of hours per week. Bodybuilding has the same trickle-down problem—split routines work for gym addicts spending 2+ hours a day, not for the average person. The takeaway: don’t copy a pro’s training split unless you also have pro‑level time and recovery resources.

Olrich describes how researchers honed in on the minimal stimulus: two 20‑second maximal sprints, for 40 seconds total of all‑out work. Even though the sprints feel long, the workout is so brief (5–8.5 minutes including warm‑up, recovery, and cool‑down) that it removes the time barrier. In a randomized controlled trial comparing REHIT (3x/week, ~8 minutes per session) to 150 minutes/week of moderate continuous training (zone 2), the REHIT group improved VO₂max by over 12% versus 6.9% in the moderate group, with a lower rate of non‑responders. He adds that the concept of non‑responders is shifting to “slow responders”—most people will eventually respond, but REHIT accelerates the gains. Ben notes he uses this approach with clients who can tolerate it and does it himself on the Carol Bike.

The new feature uses detrended fractal analysis of heart rate variability—a higher‑order metric than simple HRV—to pinpoint the lactate threshold. Olrich explains that respiratory gas analysis (like PNOE or Cosmed) isn’t practical for daily use, and blood pricking is invasive. By analyzing HRV complexity, the bike can lead a user to their exact VT1, automatically adjusting resistance to keep them at the true zone 2 (maximum fat oxidation). He expects this to become available in the next few months as a software update, making the Carol Bike an exceptionally precise tool for those who still want to do zone 2 training.