July 10, 2026 · 6 min read

Intervals 4×8 min vs. 5×5 min — What the Research and the Numbers Say About Building VO2max

4×8 min and 5×5 min are two different tools, not two versions of the same thing. 5×5 min @ 108–115% FTP delivers higher peak power and a stronger cardiovascular hit; 4×8 min @ 100–106% FTP delivers more time in the stimulus (32 min vs 25 min) at lower peak power. For an amateur on 6–8 hours a week building VO2max, 4×8 more often wins — because it's easier to finish every rep at the planned power.

VO2max develops through time spent near maximal oxygen uptake — not through high instantaneous power alone. The key metric is "time at VO2max": the minutes in which your heart and respiratory system work at 90–100% of your ceiling. Both protocols aim for the same thing, but get there differently.

Hard numbers: what each protocol delivers

Take a rider with a 280 W FTP (4.0 W/kg at 70 kg). Compare both workouts:

• 5×5 min @ 112% FTP = 314 W, 5 min recovery. Time in stimulus: 25 min. High power, but the last 2 reps hover on the edge — by the end heart rate hits 178–185 BPM.
• 4×8 min @ 103% FTP = 288 W, 4 min recovery. Time in stimulus: 32 min. Lower power, but you bank 7 more minutes of work at high oxygen uptake.

26 W of extra peak power vs 7 extra minutes in the zone — that's the whole trade-off. One school (including work from Seiler's group) shows that longer, submaximal intervals accumulate more real time at VO2max, because you don't "blow up" in the first reps.

Which protocol delivers more time at VO2max?

More real time at VO2max usually comes from 4×8 min, because the lower power lets you finish the full set without collapsing in the last reps. 5×5 min can be "faster to the ceiling" in a single rep, but if reps 4 and 5 fall below target, the total stimulus is smaller than it looks on paper.

TSS, fatigue and where it sits in the week

Both are high-load sessions. The whole workout (warm-up + reps + cool-down) is usually 75–95 TSS. The practical difference: 5×5 min hits the nervous system harder and needs longer recovery, so you'll rarely run it twice a week. 4×8 min is "more aerobic than neural" and tolerates repetition better. If you plan load through your weekly TSS and PMC chart, 4×8 gives a more predictable cost.

How do you set target power without overcooking it?

Set power from your current FTP from a Ramp Test, not from "feel". For 5×5 aim for 108–115% FTP; for 4×8 aim for 100–106% FTP. If you don't have a fresh number, run a Ramp Test first — VO2max intervals set off an inflated FTP end in a blow-up after the third rep and a wasted session.

When to pick which

• Pick 5×5 min when you're fresh, building the "top" of your power curve and want a strong neuromuscular stimulus. Good 6–4 weeks out from a race, when you tolerate high power.
• Pick 4×8 min when you want to maximize time at VO2max, you're coming off a harder week, or you're adding a second interval session and don't want to break recovery.
• Rotate both in a block — e.g. 4×8 on Monday, 5×5 on Friday. That covers both time in the stimulus and peak power. More ready-made schemes in the piece on interval workouts on the trainer.

Can this raise the power curve?

Yes — after 4–6 weeks of rotating both protocols, power in the 3–8 minute window typically rises, visible directly on your power curve. That's the most sensitive signal that VO2max work is paying off — faster than FTP itself.

Summary

Don't ask "which protocol is better," ask "which one will I finish at the planned power this week." 5×5 min is higher power and a bigger neural cost; 4×8 min is more minutes at the ceiling and easier recovery. Your freshness chart makes the call: with a positive TSB reach for 5×5, with fatigue and a still-needed stimulus reach for 4×8. Verify the effect not by "power feel" but by a shift in the 3–8 minute window of your power curve after the block.

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