What lactate curves really tell coaches and athletes
You’ve felt it—the late-interval burn when legs go heavy and breathing goes choppy. Old story: “lactic acid” makes you slow. Better story: lactate is useful. It’s a meter (it shows where your system is shifting gears) and a molecule (it’s high-octane fuel many tissues love). Read the curve right and you stop guessing. You start training with intent.
The big idea (plain and simple)
Lactate is both a marker of rising strain and a fuel moved by monocarboxylate transporters (MCTs). If you pair lactate thresholds (LT1/LT2) with ventilatory thresholds (VT1/VT2) from a DexaFit VO₂ test, you get clean boundaries for your Zones—top of Zone 2 (around LT1/VT1), working Zone 3 (between VT1 and VT2), and into Zone 4 (around LT2/VT2). That lets you pull the lever that actually matters. [1–6]
Myth vs. reality
Myth: Lactate = lactic acid = “the burn.”
Reality: The burn is mostly acid (H⁺) buildup. Lactate itself is a shuttled fuel and a signal. Trained muscle and the heart burn it eagerly. [1,9,10]
Myth: One “threshold,” cross it and you’re cooked.
Reality: You have two useful inflection points—an early one (LT1/VT1) and a later one (LT2/VT2). They mark gears, not catastrophe. [2–6,11]
Myth: “Tempo” always equals “threshold.”
Reality: Tempo is a training flavor (“comfortably hard,” long-hold effort). Threshold is a physiology point near LT2/VT2—often a bit harder than most people’s “tempo.” [3–6,11]
Marker and molecule: the 30-second shuttle tour
As intensity rises, fast fibers make lactate. MCT-4 exports it. MCT-1 imports it into oxidative fibers and the heart to be burned. The liver can recycle some to glucose (Cori cycle). With training, you get more MCT-1, more capillaries, more mitochondria—so the same blood-lactate reading can mean more power with less distress. Context matters. [1,7–10]
LT1/LT2 vs. VT1/VT2 (and where Zone 2 lives)
LT1 (aerobic threshold): first steady rise in blood lactate from baseline (often ~2 mmol·L⁻¹, but individual).
LT2 (lactate turnpoint): accumulation outpaces clearance; curve steepens.
VT1: on a gas-exchange test, breathing gets noticeably deeper; the ratio of breathing to oxygen use (VE/VO₂) rises while VE/VCO₂ stays steady.
VT2: breathing ramps more sharply relative to CO₂ (respiratory compensation)—clear redline approach. [2–6,11]
How they line up (practically):
Zone 2 sits just below or near VT1/LT1—steady, efficient, “I can talk in short sentences.”
Zone 3 lives between VT1 and VT2—comfortably hard; strong but sustainable.
Zone 4 starts around VT2/LT2—hard work near your ceiling.
In well-run tests, LT1 ≈ VT1 and LT2 ≈ VT2. They’re not identical, but they point to the same neighborhoods. VT1/VT2 from your DexaFit VO₂ test are excellent proxies—especially where finger-stick lactate isn’t available (some locations do offer it; not all). [2–6,11–12]
Two plain-English terms while we’re here:
Tempo = “comfortably hard” you can hold 20–60+ min;
VE is simply “how much you breathe” relative to the gases your body uses/produces, which helps spot VT1/VT2 in the lab. [11–12]
Where Redline Ratio fits (your “usable fraction”)
Redline Ratio = VO₂ at VT2 ÷ VO₂ Max × 100.
It tells you how much of your ceiling you can actually use before things unravel.
Same VO₂ Max, different outcomes:
Athlete A: VO₂ Max 50; Redline Ratio 90% → can sit near the ceiling longer.
Athlete B: VO₂ Max 50; Redline Ratio 75% → fades earlier at the same pace.
That’s fractional utilization in plain clothes. If your Redline Ratio is high but VO₂ Max is low, you still need to raise the ceiling. If VO₂ Max is high but Redline Ratio is low, you need more clearance/tolerance (the shuttle, the sinks). Use both metrics together. [3–6]
Why “tempo” ≠ “threshold” (and why that matters)
Threshold (near LT2/VT2) is the ragged edge many fit athletes can hold ~30–60 minutes. “Tempo” often sits just under that—by design—to build durability without frying you. If your “tempo” feels chatty, it’s not threshold; anchor it with VT2/LT2 instead of vibes. [3–6,11]
Choose the right lever: ceiling, floor, or cost
If you redline early (LT2/VT2 shows up too soon)
Signals: lactate spikes fast; breathing “breaks” early; long efforts crumble.
Do more:
Zone 2 (45–90 min): mitochondria + capillaries.
Zone 3 blocks: 2–4×(8–20 min) right at/just under VT2, easy between—train clearance/tolerance.
Occasional Zone 4 (3–5 min) to gently raise the ceiling. [3–6,8–10]
If your ceiling is low (VO₂ Max needs work)
Signals: solid durability below redline, missing a top gear.
Do more:
Zone 4 intervals 1–2×/week (5×3 or 4×4 min at ~90–95% HRmax, generous recovery).
Keep Zone 2 volume to support the engine. Don’t “polish” a threshold you haven’t powered. [3,5,11]
If economy is the bottleneck (oxygen is expensive)
Signals: VO₂ and thresholds look fine, but speed/watts per VO₂ disappoint.
Do more:
Technique tuning (stride/cadence), short hills or high-cadence spins.
Smart strength/plyo (quality > quantity).
Economy is the oxygen price of speed. Lower the price. [3]
DexaFit: test, don’t guess
VO₂ Max → your ceiling.
VT1/VT2 → your zone boundaries (strong proxies for LT1/LT2).
Optional lactate testing (at some locations) → confirms exactly where your curve bends.
Together, these define where Zone 2 ends, Zone 3 lives, and Zone 4 begins—for you.
Simple 8–12 week sketches
Raise the floor (LT2/VT2)
2–3×/week Zone 2 (45–75 min).
1×/week Zone 3: 3×12–15 min at ~VT2 minus ~10 bpm (RPE ~7/10), easy between.
Every 10–14 days: 4×4 min Zone 4 (90–95% HRmax), 3–4 min easy.
Raise the ceiling (VO₂ Max)
2×/week Zone 4: 5×3 or 4×4 min hard, full recovery.
2–3×/week Zone 2 for support.
Improve economy (cost)
1–2×/week technique (strides/cadence), short hills or high-cadence spins.
Strength/plyo 1–2×/week.
Keep most volume Zone 1–2 so you can absorb the work.
Retest every 6–9 months. If VT2 shifts right and the same pace/watts costs less oxygen (or a lower HR), you’re winning—even if VO₂ Max holds steady for a block.
References
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