SYNTH-51 · Guide
The aerobic base ("Zone 2")
Keep easy work comfortably below the first threshold rather than pinned to it, buy the adaptation with hours, frequency and one long session, and stop adding easy time when the week is too short for it to be the lever
The question it answers
How hard is easy, how is that ceiling found without a lab, how much easy work and how long a long session, what does it build — and when is more easy volume not the answer?
Default (what to prescribe first)
Intensity: comfortably below the first threshold, conversational — not "as close to LT1 as you can hold". In 20 trained amateur runners (6 men and 4 women per group), six volume-equated weeks of easy running AT the first threshold or 10-15% below it gave the same neuromuscular and performance gains; the at-threshold group paid a higher psychophysiological load at some time points, within acceptable limits (REF-705, B+; running only — transfer to cycling is inference).The expert definition of Zone 2 is the intensity immediately below LT1/VT1, and the panel noted slightly higher or lower intensities may induce the same adaptations (REF-025, B, consensus of 14 experts).
Keep the middle out of the easy days. With hard-work time held similar (8.3% vs 8.5%), sub-elite runners who spent 80.5% of time below VT1 and 11.8% between thresholds improved a 10.4-km race by 157 s versus 121.5 s for 66.8% easy and 24.7% between thresholds, over ~5 months (REF-094, B, n=12, running).
Volume: as many easy hours as the athlete's real week holds after the quality sessions, distributed per SYNTH-26 (house bands: under ~5 h count sessions; 5-8 h = 2 quality + one long-ish easy session + the rest easy).
One long session a week that grows. Not from a study; the base's own practice: start from the athlete's current longest session and extend it gradually toward the event's duration (SYNTH-50 for cyclists). For events up to about two hours the long session is optional (practice), and the same hours can be spread across the week (SYNTH-84).
Decision (who → what)
| Situation | Do this | Source and how sure |
|---|---|---|
| LT1/VT1 measured (lab, lactate) | Easy = below that power/pace/HR; do not chase its top | REF-025 B (anchor), REF-705 B+ (top buys nothing, runners) |
| No test, runner with a known critical speed | T1 ≈ 82% of CS (80.6% in slower, 84.2% in faster groups) | REF-178 A, meta-analysis of 26 studies, n=527, running only; the ~80-85% CP transfer to cycling is inference |
| No lab, accurate chest strap | Ramp run read at DFA a1 = 0.75 → HR at VT1; an estimate, keep easy below it | REF-873 B: 15 people, one treadmill ramp, HR 154 vs 152 bpm at gas-exchange VT1 (ICC 0.96), no individual error reported; running and chest strap only |
| No test, no power meter | Talk test and RPE: full sentences; roughly RPE ≤13 on the 6-20 scale | Talk test: in 16 healthy volunteers the point where speech first became difficult matched the ventilatory threshold on both treadmill and cycle (REF-929, B; an incremental test, so its use late in a long session is inference). Using it as the primary marker is the base's own practice (SYNTH-40); the RPE ≤13 band is how REF-079 (C+, narrative) defines "light", not a validated LT1 test |
| Zones set as % of FTP | Check the "endurance" zone against LT1; it can overshoot | REF-025 B for the LT1 anchor; that a %FTP zone can overshoot it is that entry's own note, not the panel's finding |
| Long ride past ~2 h | Cap by breathing and talk (the talk test as a proxy for ventilation is inference), not by fresh watts or fresh HR | REF-179 B: after 2 h at 90% VT1, VT1 power −10% (217 → 196 W), HR at VT1 +9 bpm; REF-180 B: ventilation at VT1 unchanged; n=14 trained cyclists/triathletes each |
| Hot day | Expect HR to drift and compare only against similar-temperature rides; inference: cap by talk and RPE rather than slowing to hold a cool-day HR | REF-3496 B+: HR +11% at 35 °C vs +2% at 22 °C over minutes 15-45, fluids replaced, n=10 trained men |
| Time-crunched (≤ ~8 h) | House practice: two quality sessions carry the adaptation; easy time is what is left, not the plan | REF-465 A: per hour, SIT ~3.9× and HIT ~1.7× ET for mitochondria; hour bands = SYNTH-26 practice |
| Masters | Keep total load; house practice (SYNTH-35): protect the hard sessions rather than swap them for easy volume | REF-595 B (former elite runners: ~6%/decade VO2max loss if hard training continued vs ~15% if stopped; amount, not intensity, was graded); REF-598 B (age effect on recovery smaller than assumed) |
How to progress / periodise
Grow duration and frequency, not intensity. Mitochondrial gains were larger with more sessions a week (6 > 4 > 2) and did not differ by modality once frequency, weeks and initial fitness were adjusted: ET +23%, HIT +27%, SIT +27% (REF-465, A, 5,973 participants, untrained to trained).
The long session: in 26 well-trained male runners matched on 10-km time and VO2max, those with regular ≥90-min long runs lost less running economy over a 90-min run at lactate threshold (+3.1% vs +6.0%), and economy durability correlated with the length of the weekly longest run (r = −0.67) (REF-160, B, cross-sectional — association, not a trial).Keep it one continuous session when durability is the point: split into two, the drift in HR, lactate and RPE reset (REF-159, B, n=14 runners, acute only). That session was 6 × 10 min at moderate intensity, and the entry says it does not apply to splitting long low-intensity work; the transfer to the long easy session is inference.
Runners: grow weekly distance under 20% per week — recreational runners who jumped 20-60% had ~23 percentage points more injuries at 21 days (95% CI 0.9-44.3), with no difference left at 56 and 98 days (REF-623, B+, n=261). Not from a study; the base's own practice: cyclists, without the impact cost, may tolerate somewhat larger jumps in easy hours, still inside the "well under 20-30%" week-to-week convention of SYNTH-29 that SYNTH-50 uses.
Phase: in 60 well-trained male runners over 16 weeks all four distributions improved, and pyramidal-then-polarized improved most (~1.5% 5-km TT) (REF-509, A-; transfer to cyclists). In elite road cyclists a winter period with 77% of training below VT1 was followed by VT1 rising from ~52% to ~60% of VO2max and MLSS power ~225 → ~250 W (REF-095, B, observational, one season, no DOI in the base).
Keep a weekly hard touchpoint in base phases. Not from a study; the base's own practice (SYNTH-26; SYNTH-19 is superseded).
When NOT to use it / red flags
"More Zone 2" as the answer at 4-6 h a week. 80% of a small week is too few easy hours to compound (inference, SYNTH-26); the quality sessions are the efficient lever (REF-465). Low-volume sprint intervals matched continuous endurance training for aerobic capacity (~3.6 mL/kg/min, ~8%) in healthy young people (REF-696, A-, 16 RCTs, n=318, non-athletes) — efficient, but not a replacement where the event demands duration (inference).
Easy days drifting into the middle. Threshold-dominant training gave no further gains in any endurance-performance variable over 9 weeks, apart from a slight economy gain (REF-303, A-, n=48 well-trained).
Low-cadence "strength" easy rides. In 22 well-trained veteran cyclists (47 y), 12 weeks of 40-rpm intervals at 73-82% HRmax changed nothing, while the group that added the same 90 min/week at free cadence raised VO2max 58.9 → 62.2 mL/kg/min and 30-min power 284 → 297 W (REF-118, B+). The positive low-cadence result is 50-70 rpm inside SIT and HIIT, in 24 well-trained women (REF-084, B+) — inference: low cadence is not an easy-day variant.
A fasted long ride when durability is the point. Carbohydrate during 150 min of moderate riding limited the VT1 power drop to −3% vs −6% without (REF-181, B, n=12 trained cyclists/triathletes); applying that to fasted training rides is inference.
Fasted and low-carbohydrate easy sessions (only what the base supports)
Acute effect: a fasted or protein-only start raised fat oxidation during submaximal work with no difference in interval power (367 ± 51 W), RPE or hunger vs a carbohydrate breakfast (REF-157, B+, n=17 trained male cyclists, one session).
Adaptation vs performance: train-low augmented muscle signalling (73% of 11 studies) and oxidative-enzyme adaptation (78% of 9) but improved performance in only 37% of 11 (REF-543, B+, framework tally); pooled periodised carbohydrate restriction in endurance-trained athletes: SMD 0.17 [−0.15, 0.49], not significant (REF-218, A-).One 3-week sleep-low block in 21 trained triathletes improved 10-km run time 2.9% vs 0.1% (REF-003, B+, randomised per the full text (corrected 2026-10-12)).
Decision (the base's own practice, following [REF-543]'s framework): optional, on easy sessions only, never on quality days; promise a metabolic signal, not a faster race. For slow Ironman amateurs (beyond ~12 h) an opinion piece with a modelled energy budget says a low-carbohydrate or ketogenic diet "might" suit some, mainly those with gut trouble on race-day carbohydrate who aim simply to finish, and that its performance effects are not known (REF-142, C+, no trial).
What the evidence does and does not show
Does — mitochondria and capillaries: ET raised mitochondrial content as much as HIT/SIT and capillaries per fibre +15% (REF-465, A); capillaries rise ~10-20% within weeks in the untrained, more slowly in trained athletes, with causal evidence for performance scarce (REF-466, B+, narrative).Lactate is a fuel and signal whose repeated rise drives mitochondrial biogenesis (REF-172, B+, narrative) — inference: that argues against treating easy work as the ONLY mitochondrial stimulus.
Does — cheap on the system: HRV returned to baseline within 5-10 min after up to 120 min below VT1 in 9 highly trained runners, vs ~30 min after threshold or VO2max work (REF-161, B). In 14 professional cyclists ~2000 kJ of low-intensity riding did not lower 3-min power vs fresh, while less work at 105-110% CP did (REF-193, B; not work-matched). Fatigue at exhaustion in the moderate domain was not the metabolite ceiling seen above CP (REF-306, B, n=11 men, acute).
Does — fat oxidation and durability, by association: professionals oxidised more fat and accumulated less lactate than less-fit groups (REF-473, B, cross-sectional, all male); FatMax averaged 63% VO2max with an SD of almost 10 points (REF-451, B, n=55 trained men) — an individual test, not a percentage, locates it.VT1 power fell non-linearly during prolonged riding, and how long it held tracked rested fat oxidation (REF-143, B, n=12, correlation). Durability was not correlated with VO2max or FTP in amateurs (REF-427, B, n=20), so it is trained and tested separately (SYNTH-32).No trial in the base shows that easy work trains fat oxidation better than harder work: in a 3-month two-arm trial in 32 obese adults maximal fat oxidation rose only after HIIT, not after moderate continuous training (intensities not given in the abstract) (REF-1091, B+, not athletes), and a narrative review concluded that current evidence does not support Zone 2 as the optimal intensity for mitochondrial or fat-oxidative capacity (REF-1092, B+, general population).Easy work earns its place through volume and low cost, not a unique fat-burning effect (inference).
Does — volume goes with speed, observationally: across 119,452 marathoners the faster groups did over three times the volume, added almost entirely below 82.3% of CS, with the dominant distribution pyramidal (REF-197, B+); easy-run volume correlated with performance in 85 elite runners (r ≥ 0.68) (REF-090, B).In 8 subelite runners (VO2max 70) over 6 months, time below VT1 correlated with cross-country race time (r = −0.97 over 10 km; −0.79, P = 0.06, over 4 km) (REF-817, B); highly trained and elite distance runners train pyramidal, marathoners most so (REF-811, B+, systematic review of 10 descriptive studies).All are associations: the athletes who tolerate most volume may simply be faster.
Does not — raise the ceiling on its own in trained athletes: high-volume training brought no further gains in endurance-performance variables over 9 weeks while polarized training raised VO2peak 11.7% (REF-303, A-); gross efficiency rose only after a high-intensity block, not with intensity restricted (REF-488, B+, n=29 competitive cyclists).The opposite error is also real: in the already well trained, more HIT showed no convincing long-term gain (REF-087, B+, narrative).
Does not — settle polarized vs pyramidal: a polarized 10-week program did not beat between-thresholds training significantly in 30 recreational runners (5.0% vs 3.6%), only in a post-hoc subset of 12 who adhered (REF-086, B+); a commentary argues elites train pyramidal when time in zone is counted (REF-467, C+).
Evidence gaps
No cycling trial of easy riding at LT1 vs below it; REF-705 is 20 runners over 6 weeks.
Field methods for LT1: DFA a1 has one group-level validation (REF-873, 15 people, running, no test-retest or individual error); the talk test has one validation against the ventilatory threshold, in 16 healthy volunteers on an incremental test (REF-929), none late in a long session; no HR-at-LT1 as a fraction of threshold HR.
No dose-response trial of easy volume at matched intensity in amateurs at 4-10 h/week, and no long-session progression trial in cyclists; the long-ride growth rule is practice.
Long-term fasted or low-carbohydrate easy training with performance outcomes in women and masters.
Masters-specific easy-volume trials; most cited samples are male.
How this entry was checked and changed
Why this level
(the placement rule rests on one small B+ running trial, REF-705; the adaptations on an A meta-regression, REF-465, and A-/B+ distribution trials; the volume–performance link is observational; hour bands and long-session progression are house practice)
Content correction 2026-09-27
Revision 2026-09-29: folded in REF-873 (Rogers 2021, B: DFA a1 = 0.75 matched gas-exchange VT1 at group level in 15 people) as a new decision row for finding the easy ceiling without a lab, and narrowed the field-LT1 gap; REF-817 (Esteve-Lanao 2005, B) and REF-811 (Casado 2022, B+) added to the observational volume-speed evidence. Default unchanged; level unchanged (B).
Verification 2026-10-01: all 45 cited REF entries opened in the KB.md snapshot. Every number checked matches its entry (REF-705, 025, 094, 178, 873, 079, 179, 180, 3496, 465, 595, 598, 160, 623, 509, 095, 696, 303, 118, 084, 181, 157, 543, 218, 003, 142, 466, 172, 161, 193, 306, 473, 451, 143, 427, 197, 090, 817, 811, 488, 087, 086, 467); every level/design/n next to a refId matches the head block (no NUMBER, WRONG or LEVEL findings).
Verification 2026-10-01: the core placement rule is stated with its limits (one B+ trial, 20 runners, 6 weeks, 'transfer to cycling is inference'); the Level line B is honest and I raise no LEVEL-OVERALL. For a cyclist the rule is an inference from running, which the card says.
Verification 2026-10-01: no SOURCE flag on REF entries by the spec's rule; REF-095 has no DOI but a verified PMID (the card says 'no DOI in the base'), and its abstract gives no n. REF-873 and REF-817/811 are abstract-only.
Verification 2026-10-01: house bands match SYNTH-26 (under ~5 h count sessions; 5-8 h = 2 quality + one long-ish easy ride); the masters line matches SYNTH-35 (house practice; REF-595 graded amount, not intensity). The superseded SYNTH-19 also appears in Cross-References.
2026-10-01: talk test: the base now holds REF-929 (cited by SYNTH-75), so 'not from a study' and 'no talk-test validation against VT1' were stale; both lines now cite it with its limits. Superseded pointer SYNTH-12 dropped from a source citation (SYNTH-32 replaces it).
Revision 2026-10-06: folded REF-1091 (Vaccari 2020, B+, RCT in 32 obese adults: maximal fat oxidation rose only after HIIT) and REF-1092 (Storoschuk 2025, B+, narrative review: Zone 2 not the optimal intensity for mitochondrial or fat-oxidative capacity) into the fat-oxidation line and the cross-references. Defaults, decision rows and level unchanged.
Verification 2026-10-06: STRETCH: 'no trial shows' is absolute; abstract does not say randomised; MICT intensity unknown.
Revision 2026-10-08: the weekly long session is now conditional on event duration (SYNTH-84). Default, decision rows and level unchanged.
Revision 2026-10-14: REF-142 claims re-checked against the full text (Maunder 2018 is a Current Opinion piece with a modelled energy budget, regraded B+ -> C+; "fat can cover more of the demand" was not shown, since fat oxidation rates are model inputs; the low-carbohydrate option now carries the paper's conditions — gut trouble on race-day carbohydrate, aiming to finish — and its statement that performance effects are not known). Decision unchanged.