Lactate and the Feeling of a Hard Sustainable Pace — A Research-Grade Synthesis

Abstract

Lactate threshold is one of the most-studied — and most over-simplified — concepts in endurance physiology. The peer-reviewed literature has converged on a two-threshold model: a first threshold (LT1, often identified by ventilatory threshold VT1) at which lactate first begins to rise above resting values, and a second threshold (LT2, often identified by respiratory compensation point VT2) at which lactate accumulates faster than the body can clear it and sustained exercise becomes a matter of minutes rather than hours ([1] Wasserman & McIlroy 1964, Level 5; [2] Kindermann, Simon & Keul 1979, Level 2b; [3] Stegmann & Kindermann 1982, Level 2b; [25] Poole, Rossiter, Brooks & Gladden 2021, Level 5; [26] Keir et al. 2022, Level 5). The two-threshold framing matters on the indoor rower because the 2K aerobic-anaerobic synthesis sits between LT1 and LT2: the practical "hard sustainable pace" the Concept2 community calls "AT" is roughly LT2 — the point just below which breathing is hard but speech is still possible, and just above which the effort is fundamentally time-limited ([27] Jamnick et al. 2020, Level 5; [28] Seiler & Kjerland 2006, Level 4; [29] British Rowing — Polarised training, Level 5; [30] British Rowing — Homer 2020, Level 5). The rowing-specific literature is sharper: the 1995 [11] Beneke paper showed that in rowers, IAT and AT4 (the fixed 4 mmol/L threshold) are larger than MLSS, and that the fixed 4 mmol/L load produces much higher blood lactate than MLSS does ([11] Beneke 1995, Level 2b; [4] Urhausen et al. 1993, Level 2b). The 1982 [3] Stegmann & Kindermann paper has the most direct rowing-specific anchor: at IAT, 50-minute constant work produced 4.0 ± 1.6 mmol/L lactate and HR 182 ± 13 bpm with no exhaustion, whereas fixed 4 mmol/L loads drove lactate to 9.6 ± 1.2 mmol/L and exhaustion at 14.4 ± 6.3 min in 15 of 19 rowers ([3] Stegmann & Kindermann 1982, Level 2b). The peer-reviewed Borg RPE at the lactate threshold is about 11–13 in trained adults, and the talk test (last positive stage) tracks ventilatory threshold closely across walking, jogging, cycling, and elliptical modalities ([21] Scherr et al. 2013, Level 2b; [22] Gaskill, Skinner & Quindry 2023, Level 2b; [40] Reed & Pipe 2014, Level 5; [41] Kwon, Kang & Chang 2023, Level 2b; [42] Block & Kravitz, Level 5). The modern consensus is that the polarised training distribution — roughly 80% of session time below LT1, 20% above LT2 — is the empirically dominant distribution in elite rowers and elite endurance athletes ([28] Seiler & Kjerland 2006, Level 4; [31] Fiskerstrand & Seiler 2004, Level 2b; [32] Guellich, Seiler & Emrich 2009, Level 2b; [33] Foster et al. 2022, Level 5; [29] British Rowing — Moseley 2019, Level 5; [30] British Rowing — Homer 2020, Level 5). The AI coach that anchors a rower's reference pace on the most recent 2-to-3 familiarised 2K results, treats single-test variabilities as noise, and reads the talk test alongside the monitor is reading lactate threshold correctly; the coach that treats a single 2K as a calibration of physiology is not ([13] Ingham et al. 2002, Level 2b; [51] Kilbey et al. 2025, Level 2b; [14] Ingham et al. 2008, Level 2b; [5] Coyle et al. 1988, Level 2b; [45] Concept2 — 2K race pace, Level 5; [46] British Rowing — Testing Protocols, Level 5).

What lactate threshold actually is

Lactate threshold is the exercise intensity above which lactate — produced by glycolysis — accumulates faster than the body's clearance mechanisms can remove it. The original Wasserman & McIlroy 1964 [1] paper framed it as an "anaerobic threshold" detectable by gas exchange, and the 1979 [2] Kindermann, Simon & Keul paper operationalised it as a work-rate at which lactate first rises above resting values during a graded exercise test ([1] Wasserman & McIlroy 1964, Level 5; [2] Kindermann, Simon & Keul 1979, Level 2b). The 1985 [9] Davis review cemented the field's vocabulary, and the 1985 [10] Brooks review challenged the original O2-limitation hypothesis — arguing that lactate production in healthy muscle is not oxygen-limited, but instead reflects a balance between production and clearance ([9] Davis 1985, Level 5; [10] Brooks 1985, Level 5). The 2021 [25] Poole, Rossiter, Brooks & Gladden review — 50 years after the original Wasserman paper — concluded that the original O2-limitation hypothesis is no longer tenable, and that lactate is best understood as a fuel shuttled between producer and consumer cells ([25] Poole et al. 2021, Level 5; [55] Brooks 1986, Level 5; [54] Gladden 2004, Level 5).

The modern view is that lactate threshold is a transition zone, not a single line. The 2008 [23] Binder paper formalised the two-threshold model by defining the first lactate threshold (LT1) as the work rate at which lactate first rises above resting values, and the second lactate threshold (LT2) as the work rate at which lactate accumulates rapidly and clearance fails to keep up ([23] Binder et al. 2008, Level 5; [26] Keir et al. 2022, Level 5). The 2020 [27] Jamnick et al. review mapped LT1/GET/VT1 to the moderate-to-heavy domain and the critical power / LT2 to the heavy-to-severe domain, providing the operational convention that endurance training uses today ([27] Jamnick et al. 2020, Level 5; [24] Faude, Kindermann & Meyer 2009, Level 5). The practical upshot: there is no single "lactate threshold number" — there is a transition zone, and the labeled "AT" or "threshold" on a training plan is one operational choice within that zone.

The two-threshold model: LT1, LT2, VT1, VT2

The two-threshold model is the operational convention for modern endurance training. The 2008 [23] Binder review formalised the definitions: LT1 is the work rate at which blood lactate first rises above resting values, and LT2 is the work rate at which lactate accumulates rapidly and steady-state exercise becomes impossible ([23] Binder et al. 2008, Level 5). The 2022 [26] Keir et al. review added the modern operational definitions: LT1 = onset of lactate accumulation (first lactate turn point); RCP/LT2 = second lactate threshold / onset of metabolic acidosis ([26] Keir et al. 2022, Level 5). The 2020 [27] Jamnick et al. review mapped LT1/GET/VT1 to the moderate-heavy domain and CP/LT2 to the heavy-severe domain ([27] Jamnick et al. 2020, Level 5). The 2021 [25] Poole et al. review placed the field on the modern mechanistic footing: lactate is a fuel, not a waste product, and the thresholds are not rigid lines but transition zones ([25] Poole et al. 2021, Level 5).

The same boundaries appear in the gas-exchange literature. The ventilatory threshold (VT1) is the work rate at which ventilation rises faster than VO2, and the respiratory compensation point (VT2) is the work rate at which ventilation rises faster than VCO2 — the two thresholds of pulmonary gas exchange that correspond loosely to LT1 and LT2 ([23] Binder et al. 2008, Level 5; [26] Keir et al. 2022, Level 5). In healthy trained adults, VT1 and LT1 typically correspond within a few percent of VO2max, and VT2 and LT2 similarly, but the correspondence is not exact — VT1 sometimes precedes LT1, and the two diverge in clinical populations, in high-intensity intervals, and when exercise is not incrementally stacked ([25] Poole et al. 2021, Level 5; [10] Brooks 1985, Level 5). The practical convention in indoor rowing is to label the harder sustainable pace as "AT" or "Threshold" and treat it as an estimate of LT2 — the work rate at which the rowing effort is sustainable for roughly 30 to 60 minutes ([45] Concept2 — 2K race pace, Level 5; [46] British Rowing — Testing Protocols, Level 5; [43] Concept2 — Anaerobic threshold, Level 5).

The rower's anchor: IAT, AT4, and MLSS in rowing

The rowing-specific literature is sharper than the general endurance literature, and the differences matter. The 1982 [3] Stegmann & Kindermann paper is the most direct rowing-specific anchor: 19 rowers did prolonged exercise at both IAT and the fixed 4 mmol/L lactate threshold. At IAT, 50 minutes of constant rowing produced a blood lactate of 4.0 ± 1.6 mmol/L and HR of 182 ± 13 bpm with no exhaustion. At the fixed 4 mmol/L load, the same rowers accumulated lactate to 9.6 ± 1.2 mmol/L and exhausted at 14.4 ± 6.3 min in 15 of 19 cases ([3] Stegmann & Kindermann 1982, Level 2b). The 1995 [11] Beneke paper sharpened the picture: in 9 rowers, AT4 (287 W) and IAT (287 W) were larger than MLSS (255 W), and the fixed 4 mmol/L load produced higher blood lactate (4.2 vs 3.0 mmol/L) than MLSS did ([11] Beneke 1995, Level 2b).

The 1993 [4] Urhausen et al. paper quantified the relationship: at 100% IAT, 26 of 30 endurance-trained men reached lactate steady state; at 105% IAT, only 15 of 30 did. AT100% (the work rate at which steady-state lactate is 4 mmol/L) approximated 104% of IAT — in other words, the fixed 4 mmol/L work rate is slightly above the individual's anaerobic threshold, and the gold-standard MLSS is slightly below it ([4] Urhausen et al. 1993, Level 2b). The 2018 [58] Iannetta et al. paper confirmed the slope on the other side: in 11 trained men, exercising 10 W above MLSS dropped time-to-exhaustion to 64.6% of the time-to-exhaustion at MLSS (37.3% at MLSS), showing how steeply performance falls once the rower crosses the LT2 boundary ([58] Iannetta et al. 2018, Level 2b). The read for the indoor rower: the "AT" on a training plan is not the same as the lab-measured MLSS. The training-plan AT is roughly 4–5% above MLSS, and the gap is the difference between "hard sustainable" and "this is over within 30 minutes."

What a hard sustainable pace actually feels like

The hard sustainable pace — the work rate at roughly LT2 — has a specific sensation profile. The 2013 [21] Scherr et al. paper is the most direct anchor: in 2,560 men and women, Borg RPE correlated with blood lactate at r = 0.83 and with HR at r = 0.74. RPE at the lactate threshold was about 10.8; RPE at IAT was about 13.6; RPE at the fixed 4 mmol/L threshold was about 14.1 ([21] Scherr et al. 2013, Level 2b). The 2023 [22] Gaskill, Skinner & Quindry paper confirmed it in a larger and more varied sample: in 863 adults, mean RPE at the ventilatory threshold was 12.5 ± 0.93 ([22] Gaskill, Skinner & Quindry 2023, Level 2b). The practical translation: a hard sustainable pace is roughly 12–13 on the Borg CR-10 scale — "somewhat hard" to "hard." You can say a few words, but you cannot hold a conversation. Your breathing is rhythmic but heavy enough that you would not voluntarily sustain it beyond 30–60 minutes.

The talk test operationalises the same boundary. The 2014 [40] Reed & Pipe review established that above VT/LT, comfortable speech is not likely possible ("negative" stage); below it, the "equivocal/last positive" stage is possible. The 2023 [41] Kwon, Kang & Chang paper validated the talk test in 17 healthy adults on a treadmill: three talk-test stages showed significant linear correlations with HR, VO2, RER, ventilation, tidal volume, and respiratory rate, and the talk test strongly correlated with the ergospirometric variables ([41] Kwon, Kang & Chang 2023, Level 2b). The 2004 [42] Block & Kravitz review — referencing the 2004 Persinger et al. paper — found that the ventilatory threshold was highly correlated with the last positive and equivocal stages of the talk test, and recommended re-assessment every 15 minutes during aerobic exercise ([42] Block & Kravitz, Level 5). The British Rowing beginner training plan operationalises the same idea in stroke-rate terms: 18–22 spm/low (comfortable speech), 22–26 spm/medium (shorter sentences), 26+ spm/high (short bursts) — the talk test by rate zone ([47] British Rowing — Beginner Training Plan, Level 5). The honest read: the hard sustainable pace is the pace at which you can say a sentence but not hold a conversation. Below it, conversation is comfortable. Above it, speech becomes impossible and the effort is fundamentally time-limited.

Measuring lactate threshold without a lab

The gold-standard MLSS test requires multiple 30-minute constant-work-rate trials at progressively higher intensities to find the highest work rate at which blood lactate stays in steady state ([56] Goodwin et al. 2007, Level 5; [59] Iannetta et al. 2020, Level 2b). The IAT test approximates the same boundary in a single incremental test by identifying the work rate at which lactate first rises above resting values, or the inflection point of the lactate curve ([2] Kindermann, Simon & Keul 1979, Level 2b; [3] Stegmann & Kindermann 1982, Level 2b). The 30-minute all-out test on a rowing ergometer — Concept2's recommended surrogate — uses a 30-minute maximal effort at low stroke rate to estimate Functional Threshold Power (FTP), which approximates the LT2 boundary ([43] Concept2 — Anaerobic threshold, Level 5; [56] Goodwin et al. 2007, Level 5). The 2009 [17] Bourdon et al. paper showed that a single combined incremental + 2K test reproduces both LT and 2K performance parameters from separate tests in elite rowers — the practical anchor for pairing a 2K with an LT read in a single session ([17] Bourdon et al. 2009, Level 2b).

The 2002 [13] Ingham et al. paper compared step-wise and ramp-wise incremental rowing exercise tests and showed that ramp and step VO2max were indistinguishable (r = 0.97), power at LT vs VT correlated at r = 0.73, and maximum minute power correlated at r = 0.98 with 2K performance ([13] Ingham et al. 2002, Level 2b). The 2025 [51] Kilbey et al. systematic review — 24 studies, 797 athletes — found that power at 4 mmol/L lactate correlated with 2K time at r = 0.53–0.96, with the strongest correlations in trained populations ([51] Kilbey et al. 2025, Level 2b). The practical read for the indoor rower: the 30-minute all-out test is the most practical surrogate for LT2, and Concept2's pace calculator can be used to cross-check the 2K and 5K against the LT2 estimate ([43] Concept2 — Anaerobic threshold, Level 5; [17] Bourdon et al. 2009, Level 2b; [45] Concept2 — 2K race pace, Level 5).

Lactate threshold and the 2K: what the rower can read

The 2K is a six-to-eight minute maximal effort, and the peer-reviewed literature places it on the aerobic-anaerobic spectrum. The 2002 [13] Ingham et al. paper is the most direct anchor: in 41 elite rowers, a regression model with power at VO2max, VO2 at lactate threshold, power at 4 mmol/L lactate, and peak power explained 98% of 2K speed variance ([13] Ingham et al. 2002, Level 2b). The 2004 [15] Bourdin et al. paper showed that peak power output alone correlated with 2K time at r = 0.92 in 54 male rowers ([15] Bourdin et al. 2004, Level 2b). The 2017 [16] Bourdin et al. paper showed the same pattern in 70 female rowers: Ppeak r = 0.89, PLa4 r = 0.87, VO2max r = 0.83 ([16] Bourdin et al. 2017, Level 2b). The 2025 [51] Kilbey et al. systematic review synthesised the literature: across 24 studies and 797 athletes, power at 4 mmol/L lactate correlated with 2K time at r = 0.53–0.96, with the strongest correlations in trained populations ([51] Kilbey et al. 2025, Level 2b). The 2022 [39] Boillet et al. paper added the pacing-strategy caveat: in 13 French national-level rowers, the degressive pacing strategy was the least-preferred strategy and produced more time at high blood lactate and high RPE than the stable or progressive strategies, even though final lactate and final RPE were similar ([39] Boillet et al. 2022, Level 2b).

The read is direct: the 2K is a synthesis test, and the rower's lactate threshold is one of the strongest single predictors of 2K performance. The 1984 [12] Hagerman review anchored the 2K as a synthesis of aerobic and anaerobic energy systems, and the 1997 [18] Messonnier et al. paper showed that the lactate removal rate constants γ1 + γ2 explained 67% of 2K performance variance in 12 male rowers ([12] Hagerman 1984, Level 5; [18] Messonnier et al. 1997, Level 2b). The 2016 [19] Maciejewski et al. paper showed that MCT4 transporter content correlated with the lactate removal rate at r = 0.63 in 18 lightweight rowers ([19] Maciejewski et al. 2016, Level 2b). The 2007 [20] Lacour et al. paper showed that 38 of 94 elite rowers had a VO2 plateau, and those rowers had right-shifted lactate curves, higher peak power, and better 2K performance ([20] Lacour et al. 2007, Level 2b). The 1999 [52] Cosgrove et al. paper confirmed the pattern in 13 club-standard rowers: VO2max correlated with 2K velocity at r = 0.85 ([52] Cosgrove et al. 1999, Level 2b). The honest read for the indoor rower: lactate threshold is one of several independent predictors of 2K performance, and the rower who trains it sees a real but bounded improvement in 2K performance.

The lactate shuttle: what lactate actually is

The original Wasserman & McIlroy 1964 framing was that lactate is a waste product — produced when oxygen is insufficient and accumulating in the blood. The 2021 [25] Poole et al. review rejects that framing. The 1986 [55] Brooks paper introduced the lactate shuttle: 75%+ of lactate formed during steady-rate exercise is removed by oxidation, and 20% is converted to glucose ([55] Brooks 1986, Level 5). The 2004 [54] Gladden review consolidated the modern view: lactate is a fuel, not a waste product, and the thresholds are not abrupt failures but transitions where the production-clearance balance shifts ([54] Gladden 2004, Level 5). The 2018 [49] Volianitis, Secher & Quistorff paper added the rowing-specific mechanistic anchor: after 5 minutes of rowing, arterial lactate reached 17.5 ± 1.6 mM and intramuscular pH recovery was 3.5-fold slower than after handgrip, because the elevated arterial lactate reduces the intra-to-extracellular gradient driving lactate removal ([49] Volianitis et al. 2018, Level 2b). The 2013 [57] Messonnier et al. paper showed that trained men have a 34% higher metabolic clearance rate at LT than untrained men — the lactate clearance itself is trainable ([57] Messonnier et al. 2013, Level 2b).

The practical read: lactate threshold is not a metabolic failure point. It is a transition zone where the body's clearance mechanisms begin to fall behind production, and the practical marker of that transition is the change in sensation — the shift from "hard sustainable" to "this is over within minutes." The lactate shuttle means that training at LT2 increases the body's clearance capacity, which is why the threshold itself is trainable ([5] Coyle et al. 1988, Level 2b; [8] Coyle et al. 1985, Level 2b; [14] Ingham et al. 2008, Level 2b). The 2012 [60] Smith & Hopkins review of rowing performance metrics consolidated the field's understanding: peak power, VO2max, and lactate threshold are the three core physiological determinants of 2K performance, and training shifts all three ([60] Smith & Hopkins 2012, Level 5; [13] Ingham et al. 2002, Level 2b).

The 30-minute test on the ergometer

The 30-minute all-out test is the most practical surrogate for LT2 on a rowing ergometer. The 2009 [17] Bourdon et al. paper showed that a single combined incremental + 2K test reproduces both LT and 2K performance parameters — the practical anchor for combining a 2K with an LT read in a single session ([17] Bourdon et al. 2009, Level 2b). The 2002 [13] Ingham et al. paper showed that maximum minute power correlated with 2K performance at r = 0.98, and power at LT vs VT correlated at r = 0.73 — the methodological anchor for using a 30-minute test as a 2K proxy ([13] Ingham et al. 2002, Level 2b). The 2015 [46] British Rowing Performance Talent testing protocol uses the 30-minute test at rate 20 as the aerobic-capacity anchor, alongside the 2K (power at VO2max), 250 m (maximum power), and seven-stroke tests ([46] British Rowing — Testing Protocols, Level 5). The 2023 [37] Mujika et al. paper showed that in 12 highly trained rowers, HIIT increased power at IAT and OBLA by approximately 3% — the training-adaptation anchor for the 30-minute test ([37] Mujika et al. 2023, Level 2b).

The practical read for the indoor rower: the 30-minute all-out test — Concept2's "30-minute test" — provides a practical surrogate for the LT2 boundary. The 2007 [56] Goodwin et al. paper anchored the clinical reference: MLSS averages 3.7 mM (range 1.5–7.0 mM), and peak [La⁻] 15–25 mM 3–8 min after 30–120 s all-out ([56] Goodwin et al. 2007, Level 5). The 2018 [58] Iannetta et al. paper showed that exceeding MLSS by even 10 W dropped time-to-exhaustion to 64.6% of the time-to-exhaustion at MLSS — the steepness of the threshold boundary ([58] Iannetta et al. 2018, Level 2b). The honest read: the 30-minute test is a useful estimate, but it is not the gold-standard MLSS test, and the rower who uses the 30-minute test as a calibration of LT2 should treat the result as a corridor, not a line.

Polarised training and the lactate threshold

The modern training consensus is that the polarised distribution — roughly 80% of session time below LT1, 20% above LT2 — is the empirically dominant distribution in elite rowers and elite endurance athletes. The 2006 [28] Seiler & Kjerland paper established the foundational review: elite endurance athletes train ~75% below VT1, 7–8% between VT1 and VT2, and 17–22% above VT2 ([28] Seiler & Kjerland 2006, Level 4). The 2004 [31] Fiskerstrand & Seiler paper showed that Norwegian international rowers moved toward a polarised distribution over three decades: low-intensity training (below 2 mmol/L) rose from 30 to 50 h/month, and race-pace/supra-maximal training dropped from 23 to ~7 h/month ([31] Fiskerstrand & Seiler 2004, Level 2b). The 2009 [32] Guellich, Seiler & Emrich paper showed that 36 young German male rowers over 37 weeks spent 95% of total rowing at below 2 mmol/L, 2% at 2–4 mmol/L, and 3% at >4 mmol/L ([32] Guellich et al. 2009, Level 2b). The 2022 [33] Foster et al. paper is the modern consensus position: polarised training is optimal for endurance athletes ([33] Foster et al. 2022, Level 5).

The 2019 [29] British Rowing article by Sarah Moseley synthesised the federation interpretation: polarised distribution in elite rowers is ~80% of sessions below 2 mmol/L and ~20% above 4 mmol/L ([29] British Rowing — Moseley 2019, Level 5). The 2020 [30] British Rowing article by Dr Mark Homer added the elite-junior data: in elite German junior rowers, distribution is 71% below 2 mmol/L, 21% between 2–4 mmol/L, and 8% above 4 mmol/L, with more successful rowers showing more distinct polarisation ([30] British Rowing — Homer 2020, Level 5). The 2008 [14] Ingham et al. paper showed the experimental anchor: in 18 trained rowers, the LOW group (polarised distribution) gained 23.5 ± 12.2 W at LT vs 5.1 ± 5.0 W in the MIX group over 12 weeks ([14] Ingham et al. 2008, Level 2b). The 2023 [37] Mujika et al. paper showed that HIIT in 12 highly trained rowers increased power at IAT and OBLA by approximately 3% — the modern training-adaptation anchor ([37] Mujika et al. 2023, Level 2b). The practical read for the indoor rower: the training distribution is the controllable variable on top of physiology, and the polarised distribution is the empirically dominant shape in elite rowers.

Age, sex, and what changes the threshold

Lactate threshold is trainable and age-aware. The 2003 [36] Tanaka & Seals paper showed that the age-related decline in LT is slower than the age-related decline in VO2max — LT is preserved in older athletes up to a certain age, and the age-LT relationship is not a simple linear decline ([36] Tanaka & Seals 2003, Level 5). The 1993 [35] Steinacker paper showed that elite rowers have 70–85% slow-twitch fibres and the aerobic-anaerobic threshold reaches 80–85% of maximal performance — the muscle-fibre anchor for LT ([35] Steinacker 1993, Level 5). The 2007 [53] Ebert et al. paper showed that 2% body-mass loss reduced time-to-exhaustion by 28% in trained cyclists — the dehydration-performance anchor that applies to the 2K back half ([53] Ebert et al. 2007, Level 2b).

The read for the indoor rower: LT is preserved with age, varies with menstrual cycle phase in women, drops with dehydration, and is bounded by the muscle-fibre composition. The AI coach that reads a 2K result alongside the rower's age, hydration, and recent training load is reading lactate threshold correctly; the coach that treats a single 2K result as a calibration of physiology is not ([13] Ingham et al. 2002, Level 2b; [53] Ebert et al. 2007, Level 2b; [36] Tanaka & Seals 2003, Level 5).

What the AI coach actually does with lactate threshold

For an AI coach that reads the rower's logbook and writes the rower's session, lactate threshold is one of the controllable variables the coach can shape directly. The coach's rule is:

• A rower who has done three or more 2Ks — the coach uses the moving average of the most recent two-to-three 2Ks as the reference pace, and treats the lactate threshold as the implicit boundary above that pace. The 2K is a synthesis test, and the LT contribution is captured implicitly in the average split ([13] Ingham et al. 2002, Level 2b; [51] Kilbey et al. 2025, Level 2b; [45] Concept2 — 2K race pace, Level 5).
• A rower who has done a 30-minute all-out test — the coach uses the average split as the LT2 surrogate, and the 2K reference pace is set 4–6% faster than the 30-minute split ([43] Concept2 — Anaerobic threshold, Level 5; [56] Goodwin et al. 2007, Level 5; [59] Iannetta et al. 2020, Level 2b).
• A rower with no 2K or 30-minute test — the coach widens the reference-pace band and leans on the rower's stated training distribution, recent RPE, and the talk-test self-report. The reference pace is a corridor, not a line, and the coach that treats a single 2K as a calibration of physiology is over-fitting ([5] Coyle et al. 1988, Level 2b; [21] Scherr et al. 2013, Level 2b; [40] Reed & Pipe 2014, Level 5; [41] Kwon, Kang & Chang 2023, Level 2b).
• A rower whose recent 2K result is faster than the expected LT2 corridor — the coach notes the result, widens the confidence band, and waits for a familiarisation trial before anchoring the reference pace. The first 2K after a layoff is a familiarisation piece, not a calibration of physiology ([39] Boillet et al. 2022, Level 2b).
• A rower whose training distribution is high-intensity-heavy — the coach notes the distribution and prescribes a polarised pattern with roughly 80% below LT1 and 20% above LT2. The 2022 Foster et al. consensus position is that polarised training is optimal for endurance athletes ([33] Foster et al. 2022, Level 5; [28] Seiler & Kjerland 2006, Level 4; [29] British Rowing — Moseley 2019, Level 5; [30] British Rowing — Homer 2020, Level 5; [31] Fiskerstrand & Seiler 2004, Level 2b; [32] Guellich et al. 2009, Level 2b).

The four-numbers dashboard (CTL, ATL, TSB, ACWR) is the how much the coach is asking the body to absorb in a given week. The 2K trajectory is the where the coach expects the body to be in a few months. The reference pace is the today. The lactate threshold is the corridor the today sits inside, and the rower's training distribution is the shape the corridor is built from ([28] Seiler & Kjerland 2006, Level 4; [33] Foster et al. 2022, Level 5; [14] Ingham et al. 2008, Level 2b; [37] Mujika et al. 2023, Level 2b).

Limitations and open questions

The rowing-specific LT literature is small. Most of the foundational papers are in cycling, running, or general exercise physiology, with rowing-specific work concentrated on the 1995 [11] Beneke sample (n=9 rowers), the 1997 [18] Messonnier et al. sample (n=12 male rowers), and the 16-rower Ingham 2002 sample ([11] Beneke 1995, Level 2b; [18] Messonnier et al. 1997, Level 2b; [13] Ingham et al. 2002, Level 2b). The 1995 [11] Beneke paper is the most direct experimental test of MLSS vs IAT vs AT4 in rowers, and the 1982 [3] Stegmann & Kindermann paper is the most direct experimental test of IAT vs AT4 in rowers ([3] Stegmann & Kindermann 1982, Level 2b; [11] Beneke 1995, Level 2b). The 5K and 6K rowing-specific LT literature is sparser still. The reader should weight the rowing-specific evidence more heavily than the cross-sport evidence when the two diverge.

The fixed 4 mmol/L threshold is not the gold standard. The 1993 [4] Urhausen paper showed that at 100% IAT, 26 of 30 endurance-trained men reached steady-state lactate, while at 105% IAT only 15 of 30 did — the lab-measured MLSS is below AT4, and the training-plan "AT" is roughly 4–5% above MLSS ([4] Urhausen et al. 1993, Level 2b; [11] Beneke 1995, Level 2b). The 2009 [24] Faude et al. paper catalogued 25 LT concepts and concluded that the fixed 4 mmol/L threshold is one operational choice among many — not the gold standard ([24] Faude et al. 2009, Level 5). The rower who treats the AT on a training plan as the same as the lab-measured MLSS is over-fitting the model.

The RPE and talk-test anchors are robust but bounded. The 2013 [21] Scherr et al. paper established that RPE at LT is about 10.8 in trained adults, but the standard deviation is wide — the individual variability is large enough that a single RPE read is not a calibration of physiology ([21] Scherr et al. 2013, Level 2b). The 2023 [22] Gaskill, Skinner & Quindry paper confirmed it in 863 adults: mean RPE at VT was 12.5 ± 0.93 ([22] Gaskill et al. 2023, Level 2b). The talk test is a useful practical tool, but it is one of several — the AI coach that anchors the rower's reference pace on a single talk-test read is over-fitting.

The sex-specific LT response is not as well characterised as the male response. The broader literature on female rowing LT is much smaller than the male literature, and the within-sex variance is large enough that a single 2K does not pin down the rower's optimal LT-based training zone. The 2017 [16] Bourdin et al. paper provided female-specific 2K-determinant data (n=70 female rowers), but the within-sex variance is large enough that a single test should not be read as a calibration of LT ([16] Bourdin et al. 2017, Level 2b). The AI coach that uses a single test's LT as the rower's "true" LT is over-fitting.

The LT estimates are bounded to indoor rowing. The on-water 2K profile is more front-loaded than the ergometer profile, so an LT measurement made on the Concept2 ergometer does not transfer directly to on-water LT. The 2009 [50] Volianitis & Secher paper on the rowing physiology in elite rowers noted that the demands of on-water rowing stress pulmonary diffusion, cerebral blood flow, and neuromuscular activation beyond the ergometer's profile ([50] Volianitis & Secher 2009, Level 5; [34] Volianitis, Yoshiga & Secher 2020, Level 5). The reader who rows both should treat the ergometer and on-water LT estimates as distinct.

What to do with this article

Read the principle: lactate threshold is a transition zone of exercise intensities above which lactate accumulates faster than the body can clear it. The modern convention recognises two thresholds — LT1 (the first sustained rise above resting) and LT2 (the rapid-accumulation point above which steady-state exercise is impossible). Read the evidence: the 1982 [3] Stegmann & Kindermann paper (Level 2b) and the 1995 [11] Beneke paper (Level 2b) anchor the rowing-specific claim that IAT and AT4 are larger than MLSS; the 2013 [21] Scherr et al. paper (Level 2b) and the 2023 [22] Gaskill, Skinner & Quindry paper (Level 2b) anchor the RPE-at-LT claim; the 2006 [28] Seiler & Kjerland paper (Level 4) and the 2022 [33] Foster et al. paper (Level 5) anchor the polarised-distribution claim; the 2002 [13] Ingham et al. paper (Level 2b) and the 2025 [51] Kilbey et al. systematic review (Level 2b) anchor the LT-as-2K-predictor claim. Read the practical read: the hard sustainable pace is roughly 12–13 on Borg CR-10, just below LT2, and roughly 4–5% above the lab-measured MLSS; the 30-minute all-out test is the most practical surrogate for LT2; the polarised training distribution is the modern consensus position for distance training.

When you want to anchor your reference pace, the practical recipe is: row a familiarisation piece first if it has been more than six weeks since your last 2K, then row your calibration piece with the pace plan that fits your level; trust the moving average of your most recent two-to-three 2Ks more than any single test; use the talk test as a real-time check on intensity — long sentences are below LT1, short sentences are around LT1, and broken words are at or above LT2; and treat the lactate threshold as a corridor, not a line. The AI coach that reads the threshold as a corridor, treats single-test variabilities as noise, and uses the polarised distribution as the default training shape is reading lactate threshold correctly.