Lactate Threshold for Ultrarunners: What It Actually Measures and How to Train Around It

Lactate threshold matters to ultrarunners because it helps define the boundary between effort you can keep under control and effort that starts becoming expensive fast.

Lactate threshold is often treated as a single pace, heart rate, or lab number. That is useful up to a point, but it can also hide what the term is really trying to describe. In practice, lactate threshold is about the highest intensity you can sustain while keeping the byproducts and demands of hard running from accelerating out of proportion to the work you are doing.

For an ultrarunner, that boundary matters even if race pace is usually well below it. Threshold shapes your ability to climb without detonating, to handle surges, to process rolling terrain, and to spend long hours moving at a strong aerobic effort. It also influences how much speed you can access without crossing into a level of strain that drains you disproportionately.

The confusion starts with lactate itself. Lactate is not just a waste product that appears when you are in trouble. Exercise physiologists have spent years correcting that old story. Lactate is produced all the time, even at relatively easy intensities. It is a normal product of carbohydrate metabolism, and it can be reused as fuel by other tissues, including working muscle and the heart. The real issue is not whether lactate is present. The issue is whether production and clearance stay in rough balance as intensity rises.

When you run faster, your muscles need energy at a higher rate. One way they meet that demand is by speeding up glycolysis, which is the pathway that breaks down glucose for energy. Faster glycolysis means more pyruvate is produced. Some of that pyruvate enters the mitochondria, where it can be used aerobically. Some is converted to lactate. As long as the body can clear and reuse lactate at about the same rate it is being produced, blood lactate stays fairly stable. When intensity rises enough that lactate begins to accumulate more quickly than it can be cleared, you are approaching an important physiological boundary.

That boundary is what threshold concepts are trying to capture. Different labs and coaches define it differently. Some use the first clear rise above baseline blood lactate. Some use a fixed value such as 4 millimoles per liter, though that simplification has obvious limits because athletes vary. Some use the highest blood lactate concentration that can remain stable during a sustained effort, often called maximal lactate steady state. In field settings, coaches often estimate threshold from race performances, sustained time trials, breathing changes, heart rate patterns, or pace you can hold for roughly 30 to 60 minutes.

Those are not all the same thing. That is why threshold talk gets messy. The useful common thread is that threshold marks a transition zone. Below it, metabolic strain rises in a more controlled, near-steady way. Above it, the cost of running rises faster, breathing becomes harder to control, fuel use shifts more heavily toward carbohydrate, and fatigue arrives sooner.

This is one reason threshold is so closely tied to endurance performance. Andrew Jones and Michael Joyner have both emphasized that distance performance depends heavily on three broad factors: maximal oxygen uptake, economy, and the fraction of that aerobic capacity you can sustain. Threshold sits inside that third factor. Two athletes can have similar aerobic ceilings, but the one who can operate closer to that ceiling without losing metabolic control will usually perform better over sustained efforts.

For ultrarunners, the point is not that you should race an ultra at threshold. Usually you should not. The point is that raising threshold expands the range of intensities that feel manageable. If your threshold pace improves from, say, 7:30 per mile to 7:00 per mile, then 8:30 pace, 9:00 pace, or a given uphill power output becomes relatively cheaper. The same may be true if threshold heart rate or threshold power shifts, though pace is easier to picture. You are moving the line where running starts to become costly.

That matters especially on trails, where effort is rarely steady for very long. Flat-road models can make threshold seem like a neat laboratory dividing line. Ultras are not neat. You hike steep grades, run moderate climbs, surge over short rises, then descend on damaged legs. On terrain like that, threshold is less a race target than a reference point for how much headroom you have. More headroom means fewer moments where terrain forces you into intensities that burn too much carbohydrate, spike ventilation, and compound fatigue.

Threshold also connects directly to fuel use. As intensity rises toward and above threshold, the body relies more on carbohydrate and less on fat as a percentage of total energy supply. That does not mean fat use suddenly stops. It means carbohydrate becomes increasingly important because it can support ATP production at a faster rate. Since stored carbohydrate is limited, time spent above threshold carries a bigger energetic price. That is one reason ultras punish repeated overpacing, especially early. If you want the background on how this fuel shift works, see glycogen depletion and fat oxidation for ultrarunners.

What changes threshold with training? Mostly, you improve the machinery that lets you produce energy aerobically and manage metabolites at higher workloads. Endurance training can increase mitochondrial content, capillary density, and the enzymes involved in aerobic metabolism. It can improve the transport of lactate between cells and its use as fuel. It can also strengthen the heart's ability to deliver oxygenated blood and the muscles' ability to extract and use that oxygen. The result is not that lactate disappears. The result is that you can run faster before lactate accumulation and associated strain begin to run away from you.

Threshold training is one way to target those adaptations, but it is easy to misuse because the sensation of threshold is subtler than many runners expect. It is hard, but not desperate. Breathing is controlled but clearly elevated. You can usually speak in short phrases, not full relaxed sentences. You feel pressure, not panic. The workout should leave you worked over, not wrecked. If you want a second set of eyes on whether you actually hit the intended intensity, Train Ultra gives you an evaluation of every workout based on data imported from Garmin, COROS, Apple Health, or Health Connect.

Classic threshold sessions include continuous efforts of 20 to 40 minutes near threshold, or cruise intervals such as 4 to 6 by 5 to 10 minutes with short recoveries that keep lactate from fully dropping. A runner preparing for trail ultras might do 3 by 12 minutes at threshold effort on smooth uphill grades, or 5 by 8 minutes on rolling terrain where the goal is to hold even physiological strain rather than even pace. The exact pace matters less than keeping the effort in the right place.

That last point matters because threshold can be hard to pin down in the field. Pace on a hot day, technical trail, climb, or fatigued week may not match pace on a cool flat road. Heart rate can help, but it lags and drifts, especially in long intervals or hot conditions. If you have ever watched your heart rate keep climbing during a steady run, that is a separate but related phenomenon explained in this article on cardiac drift in running. Lactate testing is more direct, but most athletes do not have regular access to lab or field testing with blood samples. So in practice, threshold training often works best when anchored by a mix of recent race data, terrain-adjusted pace or power, heart rate trends, and honest perceived effort. It also helps to ask the coach about your own training when the numbers from one session do not match how the work actually felt.

This is also where ultrarunners get into trouble. Many threshold sessions become moderate-hard gray-zone running that drifts too easy to produce a clear stimulus or too hard to repeat well. On trails, the usual failure mode is chasing pace uphill, letting effort spike above threshold, then jogging the flatter sections too softly to restore rhythm. The session becomes a set of mini races instead of controlled work.

A better way to think about threshold in ultra training is as a durable aerobic pressure. You are trying to spend meaningful time near the top of sustainable aerobic work, where breathing is heavy but coordinated, mechanics are still under control, and the effort is repeatable week to week. That is enough stress to force adaptation without turning every quality day into a recovery problem.

Threshold is useful, but it does not explain everything. It is one lens, not a complete model of ultramarathon performance. Ultras depend heavily on fatigue resistance, fueling, downhill durability, terrain skill, heat management, and pacing discipline. An athlete with a strong threshold can still unravel late if their legs cannot handle eccentric damage, if they underfuel, or if they have not built the ability to preserve form and force output after several hours. That is part of why threshold is more predictive for shorter endurance events than for mountainous 50 milers or 100 milers.

It also has limitations as a training obsession. Many runners like threshold because it is measurable. You can test it, assign a pace, and watch it improve. But a clean number can create false precision. Threshold on a treadmill in cool conditions is not automatically your threshold on a hot exposed climb after three heavy weeks. It is better to treat threshold as a moving marker of current metabolic control than as a fixed identity.

There is also an interaction with critical speed, another concept that tries to locate a boundary between heavy and severe exercise intensity. The two ideas overlap, but they are not interchangeable. Threshold concepts usually center lactate stability and sustainable metabolic balance. Critical speed comes from the relationship between work and time to exhaustion, and often describes the highest intensity at which a physiological steady state is still possible. For runners, both can be useful, but each sees performance through a slightly different frame. If you want that distinction unpacked, read critical speed for trail and ultra runners.

So how should an ultrarunner actually use threshold? First, use it to calibrate intensity, not ego. If threshold pace or power is improving, your sub-ultra efforts are likely becoming cheaper. That is good news even if your easy pace has not jumped dramatically. Second, include threshold work in a way that matches your event demands. For flatter ultras and faster trail races, longer continuous threshold segments may fit well. For mountain races, broken uphill threshold intervals may be more specific and more mechanically manageable. Third, protect the quality of the work. Threshold training is only useful if you can stay near the intended intensity and recover well enough to absorb it. Fourth, count your climbing. On steep climbs you reach threshold heart rates just by continuing to run, with less impact than fast flat running, so a hilly long run can quietly add a lot of time near threshold. If you train on hills, one structured threshold session a week is usually enough, and your easy days need to stay truly easy.

Finally, remember what threshold is really telling you. It is not a badge. It is a description of how much hard running your body can manage before the cost starts escalating too quickly. When that line moves up, more of racing and training falls below it. For an ultrarunner, that usually means better control on climbs, less needless carbohydrate burn, and more room to make good decisions late in the day.

Written by Wade Wegner. Train Ultra is a private AI coach that reads every workout from Garmin, COROS, Apple Health, or Health Connect. Try it free.