Seiler 4x8 — Free Zwift Workout
(.zwo Download)
Four 8-minute reps at 106-110% FTP with 6 minutes easy between. In the trial that compared the three formats it out-gained both the 4x4 and the 4x16, and at a lower perceived effort than the 4x4.
Built by Jim Camut. JB Cycling Academy 2004–09 · PCS-verified · Cyclingnews diarist alumnus.
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Trains consistently — doesn't race
What do you want from this workout?
110–120% FTP — engine-builder intervals
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Seiler 4x8: prescription + science
Four 8-minute reps at 106–110% FTP with 6 minutes easy between — the protocol that beat both the 4×4 and the 4×16 in the trial that compared them.
The trial that made the 4×8
Trained cyclists ran one of three interval formats twice a week for seven weeks, all told to ride at their maximal tolerable intensity. The 4×8-minute group gained 11.4% across VO2 peak, power at VO2 peak and power at 4 mmol lactate, against 5.5% for the 4×4-minute group and 5.6% for the 4×16-minute group [1]. Accumulating 32 minutes of work near 90% of peak heart rate beat accumulating 16 minutes near 95%, and it did so at a lower rating of effort.
That is one trial in recreational cyclists, not a law. But it is the trial this session is named for, and the result points the opposite way to the intuition that harder reps must train you more.
Why the watts look low
Handed the same instruction to ride at maximal session effort, well-trained cyclists settled 4×8-minute reps at about 106% of their 40-minute time-trial power and 4×4-minute reps at about 117% [2]. The lower number is not a softer session. It is what holding the effort for twice as long actually costs.
Perceived exertion in that study tracked the shorter format: 61% of 4×4 sessions were rated at the very top of the exertion scale, against 8% of 4×16 sessions. The 4×8 sits between them, which is most of its appeal.
One place we depart from the study
Seiler ran all three formats on 2-minute recoveries. This page gives you 6. That is the ratio the rest of our coaching holds for a rep this long, roughly 0.75 to 1.25 times the work duration: a 2-minute break after 8 minutes is a quarter of the work, and rep four then becomes a different session from rep one. Take the longer rest and the wattage on the last rep is the thing that stays honest.
Hold 106–110% FTP on every rep rather than opening at 4×4 wattage and fading. A 2025 network meta-analysis pooled 51 trials and found high-intensity intervals, sprint intervals and repeated sprints all raised VO2 max, with no significant difference between those three families [3]. It compared families rather than rep durations, so treat the 4×8 as a strong option to rotate through rather than a proven replacement for the 4×4.
Peer-reviewed sources
- 01Seiler et al. 2013. Adaptations to aerobic interval training: interactive effects of exercise intensity and total work duration. Scandinavian Journal of Medicine & Science in Sports, 2013.
- 02Seiler 2017. How Does Interval-Training Prescription Affect Physiological and Perceptual Responses?. International Journal of Sports Physiology and Performance, 2017.
- 03Yang et al. 2025. Comparison of different interval training methods on athletes' oxygen uptake: a systematic review with pairwise and network meta-analysis. BMC Sports Science, Medicine and Rehabilitation, 2025.
Seiler 4x8: common questions
What power is a Seiler 4x8?
Around 106–110% of FTP — lower than a 4×4 on purpose. In the source study cyclists self-selected about 106% of their 40-minute time-trial power when told to go as hard as they could sustain.
4x8 or 4x4 — which is better?
In the seven-week trial that compared them, the 4×8 group improved roughly twice as much. That is one trial with nine riders in the 4×8 group, so the honest answer is that the 4×8 is underused and worth a block, not that the 4×4 is wrong.
The study used 2-minute recoveries. Why does this give 6?
Deliberately. A 2-minute break after an 8-minute rep is a quarter of the work, and it costs you the wattage on rep four. We hold roughly 0.75–1.25× the work duration for reps this long, which is what the rest of our coaching prescribes too.
Is this too hard for a first VO2 session?
It is a full high-intensity day, so once a week with 48 hours before the next hard session. But the wattage is lower than a 4×4, which makes it a reasonable way into a VO2 block.
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