One effort, at least five names
Tempo run, threshold run, LT session, cruise intervals, "comfortably hard", MLSS, LT2. Runners and coaches use these words for overlapping efforts, and sometimes for quite different ones. A club "tempo" can mean a steady 20 minutes at roughly one-hour race effort, a 10 km progression, or a long run that finishes briskly. Lab scientists add their own vocabulary, and a 2009 review counted 25 separate lactate threshold concepts in the literature.
Before untangling the terms, some perspective. For nearly all runners, the biggest improvements come from running more, consistently, for months at a time. Whether Tuesday's session is called tempo or threshold, and exactly where it sits, is a second-order detail. The vocabulary is still worth getting right, because it helps you and anyone you train with mean the same thing.
Key takeaways
- In Jack Daniels' system, threshold (T) pace is the effort most runners could race for 50 to 60 minutes, and his "ideal" tempo run is a steady 20 minutes at that pace.
- Daniels uses two formats for threshold work: continuous tempo runs and cruise intervals, which are repeats with short recoveries.
- Many runners and coaches run longer "tempo" efforts a little slower than true threshold. Daniels says this can work, and his tables slow the pace as the run gets longer.
- Pete Pfitzinger's lactate-threshold runs sit at roughly 15K to half-marathon race pace, which for many runners is a touch slower than Daniels' one-hour effort.
- Physiologists describe two boundaries: a first lactate or ventilatory threshold (LT1 or VT1) and a second (LT2, often tied to the maximal lactate steady state). Coaching "threshold" usually means the second.
- Labels help communication. The training effect comes mostly from how much running you do, week after week.
What coaches mean by threshold
Daniels' T pace
Daniels defines threshold running as an effort that produces an elevated but steady blood lactate level. In a Runner's World excerpt from Daniels' Running Formula, he places it a little faster than a pace you could hold for two hours or more (marathon pace for most people) and slower than a pace you could hold for 30 minutes. In practical terms, most runners can treat T pace as the speed they could race for 50 to 60 minutes. For slower runners that can be close to 10K race pace, because their 10K takes nearly an hour.
He gives the intensity as about 83 to 88% of VO2max, or 88 to 92% of maximum heart rate, and he describes the feel as "comfortably hard". The purpose, in his words, is to stress the body's lactate-clearance capability without overstressing it. If it feels hard, you have drifted into interval territory.
Daniels' paces come from a table keyed to a recent race, and the table assumes the rest of your training can absorb them. kaizen works from the other direction. It is hyper-personalised, using the overall intensity of the running you actually do, measured from each run's pace and elevation, to set your training target for the week, and it recalculates as you run.
Tempo runs and cruise intervals
Daniels splits threshold work into two formats. A tempo run is, ideally, a steady 20-minute run at T pace. Cruise intervals are repeated runs at the same pace with brief recoveries, which in his system follow roughly a 5:1 work-to-rest ratio. He caps threshold running at about 10% of weekly mileage in a session.
That cap is a percentage of whatever you happen to run. kaizen sets the week's target from your current training load and your goal, and any threshold work you do counts towards it at the intensity it really was.
When tempo drifts below threshold
This is where most of the confusion starts. Daniels himself notes that many coaches and runners do longer tempo runs at slower than true threshold pace, and that this can yield positive results, including a strong sense of holding a solid pace for a long time. His tempo tables reflect it: the longer the run, the slower the pace. At VDOT 50, a 20-minute tempo is 6:51 per mile, and a 60-minute tempo is 7:13 per mile.
So a "tempo" in one training group might be a 20-minute T-pace effort, while in another it is a 50-minute steady run 15 to 20 seconds per mile slower. Both are legitimate sessions, and they put somewhat different demands on the body.
Pfitzinger's lactate-threshold runs
Pfitzinger and Scott Douglas's Advanced Marathoning uses "tempo" and "lactate-threshold run" almost interchangeably. Their LT runs last at least 20 minutes, often 4 to 7 miles of tempo after a 2 to 3 mile warm-up, at about 15K to half-marathon race pace, or 82 to 91% of maximum heart rate. On his coaching site, Pfitzinger describes lactate-threshold pace as roughly what experienced runners could race for 45 to 60 minutes.
Put the two side by side and the overlap is large. The difference sits at the slower end: half-marathon pace takes most club runners well over an hour to sustain, so a Pfitzinger LT run at that pace will often land a little below Daniels' T pace. That reading is our interpretation of their published definitions, and the gap is small.
What the lab means by threshold
Two boundaries
Physiology textbooks describe intensity in domains. During a graded test, blood lactate first rises above resting levels at the first lactate threshold (LT1), which roughly coincides with the first ventilatory threshold (VT1). Higher up, lactate starts to climb steeply. That second turn point goes by several names, including LT2, the respiratory compensation point and, approximately, the maximal lactate steady state (MLSS).
Faude, Kindermann and Meyer's 2009 review in Sports Medicine framed this as an aerobic-anaerobic transition with two breakpoints: the intensity where lactate begins to rise above baseline, and the highest intensity where lactate production and removal stay in balance, which is the MLSS. They found 25 different threshold concepts, and grouped them into those using fixed lactate values, those detecting the first rise, and those aiming at the MLSS or a sharp bend in the curve. Across 32 studies, thresholds correlated strongly with endurance performance, particularly in running.
MLSS and the heavy-severe boundary
MLSS is measured with constant-pace efforts rather than a ramp test. It is the fastest speed at which blood lactate stays roughly stable over about half an hour. Daniels' description of T pace, an elevated but steady lactate level, lines up with this second boundary, which is why coaching "threshold" usually means LT2 or MLSS and seldom LT1.
Jamnick and colleagues' 2020 review in Sports Medicine adds a caution. They found some support for LT1, VT1 and the gas exchange threshold marking the line between moderate and heavy exercise. For the line between heavy and severe exercise, they found little evidence for most common methods, with critical power and critical speed as the exceptions. They also found that a fixed percentage of VO2max or maximum heart rate does a poor job of placing different people in the same intensity domain.
That last finding applies directly to training plans. A prescription such as "88 to 92% of max heart rate" or "half-marathon pace" will put different runners at different points relative to their own thresholds. kaizen sidesteps the table by working from your own runs: the pace and elevation of each one set its intensity, and that intensity feeds your weekly training target.
A working vocabulary
If you want terms your training partners will recognise, this set holds together reasonably well:
- Threshold or T pace: roughly one-hour race effort, comfortably hard, close to MLSS or LT2. Usually done as a steady 20 minutes or as cruise intervals.
- Tempo: a continuous run at or a little below threshold. Say how long and how hard, since the word alone covers a range.
- Steady or LT1-ish running: a notch above easy, still well below threshold. Some groups call this tempo too, which is worth clarifying before the session starts.
The most useful habit is to describe a session by duration and effort ("25 minutes at about one-hour race effort") instead of trusting the label to carry the meaning.
Sources and how sure we can be
Daniels' definitions come from his own book excerpt in Runner's World, and the VDOT 50 tempo paces from his published tables. Pfitzinger's figures come from Advanced Marathoning summaries and his coaching site, and published summaries differ slightly on session lengths. The physiology is drawn from two reviews, Faude et al. (2009) and Jamnick et al. (2020), and we read both through their abstracts and published summaries. The percentages attached to threshold differ between sources and editions by several points, so treat every number here as approximate. The observation that Pfitzinger's LT pace often sits slightly below Daniels' T pace is our interpretation. No study we found compares the training effect of a 20-minute T-pace run with a longer, slightly slower tempo in recreational runners.
Getting the label right, then getting the weeks in
Clear terms make a session easier to plan and easier to share. They do not change where most of the improvement comes from. For almost every runner, threshold, tempo and steady running refine a training picture that is built mostly from running often, at a volume you can absorb, across many consecutive weeks. A well-named workout is one small part of that.
See what your threshold work adds up to in kaizen
You've probably spent more time than most debating what your Tuesday session should be called. kaizen calculates your fitness from your recent running and past races, then sets the weekly training target your goal requires. Every run counts by its pace and elevation, whether it was a 20-minute tempo, a set of cruise intervals or an easy hour, and the target recalculates as you run. You choose the sessions; kaizen shows you how they add up. Open kaizen on iOS or Android and see your fitness.
