Cardio
Running / intervals
ProvisionalTargetAbout 1×/week hard VO2 work — usually Norwegian 4×4 — or at least one all-out sprint session/week. Weekend-warrior style can match high-frequency VO2 gains if weekly volume matches.
Why it matters
Intervals are the hard cardiorespiratory lever next to Zone 2. Structured HIT (4×4) and short sprint work are time-efficient ways people raise VO2; a volume-matched weekend-warrior trial and a SIT meta (both now full-texted) are why frequency alone is less magic than total hard + steady work — and why stacking endless sprint reps isn’t required for the VO2 adaptation in Wingate-style SIT.
Protocol
- Norwegian 4×4: 4 min @ ~85–95% HRmax (study posts often ~95%), 3 min rest/active @ ~60–70%, 4 rounds · about 1×/week (sometimes once every ~10 days when strength leads) — Siim-informed.
- Sprint floor: ≥1 all-out sprint session/week (e.g. 3–5 × 60–100 m after warm-up, or 10 × 100 m walk-back) — Siim-informed.
- Weekend-warrior option (study he posts): high-frequency 4×/week shorter vs 2×/week longer → similar VO2 if volume matched — study-informed (Tripp 2025) + Siim-informed catch.
- High-frequency: 2× 35 min continuous + 2× HIIT 4 min work / 3 min rest × 4 (Siim’s ~80% / ~95% HRmax labels; paper uses GET/RCP power domains).
- Weekend warrior: 1× 70 min continuous + 1× HIIT × 8 rounds (doubled units).
Evidence
Tripp TR et al. Cardiorespiratory Fitness Improvements Following Low-Frequency Training Are Not Inferior to High-Frequency Training Matched for Intensity and Volume. Scand J Med Sci Sports. 2025;35(2):e70024. doi:10.1111/sms.70024. Full-text note: Tripp 2025.
Key finding: 8-week RCT (n=28); WW (2×/week) not inferior to HF (4×/week) for V̇O2max when intensity + weekly volume matched (WW 43.5→47.8; HF 42.3→47.3 mL/kg/min; change difference 95% CI −1.5 to 2.9 vs 3.5 mL/kg/min margin).
Catch: Short trial, young adults, cycle ergometer; intensity prescribed by GET/RCP power, not %HRmax; non-inferiority ≠ identical.
vs Siim: Headline Yes; %HR wording Partial.
Vollaard NBJ et al. Effect of Number of Sprints in an SIT Session on Change in V̇O2max: A Meta-analysis. Med Sci Sports Exerc. 2017;49(6):1147–1156. doi:10.1249/MSS.0000000000001204. Full-text note: Vollaard 2017.
Key finding: 38 trials / 34 studies; typical SIT → +7.8% ± 4.0% V̇O2max; −1.2% ± 0.8% per 2 extra sprint reps/session (more reps not better; possibly slightly worse).
Catch: Wingate-type cycling SIT only; magnitude-based inference framework; “2–3 enough” is discussion/REHIT pointer, not a pooled optimal-dose estimate.
vs Siim: Yes on 34 / ~8% / more reps not better; Partial on “2–3 enough” slogan.
Catch (Siim names it): you have to match the volume.
How Siim frames it
YT OceysodInIo 31:52–32:07 — Norwegian 4×4 at 85–95% / 60–70% × 4. YT F__fxx_E9p4 5:38 — “at least one sprint workout per week.” X weekend-warrior thread: https://x.com/siimland/status/2094442770047893557
Status
Provisional — weekend-warrior VO2 + SIT meta now full-texted with solid (partial) Siim matches. Gap: Norwegian 4×4 still lacks a named full-text note for the specific Helgerud/Wisløff paper he leans on; weekly 4×4 / sprint-floor doses remain Siim-informed. Not Locked until that 4×4 cite is pinned + full-texted (or Product accepts intervals Locked on WW+SIT alone).
Full dump / backlog (Tabata/Gibala ranking video): corpus → Intervals.