There are many training zone systems and sometimes inconsistent terminology describing them. Workouts may be labelled as easy, moderate, tempo, up tempo, threshold or zone 2 but without clear definitions of these terms it’s impossible to know what a coach or plan intended. We’ve found that being precise about what these zones actually mean and grounding them in physiology makes a real difference to how consistently athletes are able to execute their training. Without clear definitions the zones become vague suggestions and vague suggestions are easy to drift away from, usually in the wrong direction.
This figure shows how we think about training intensity at Speedfarm and provides a clear, physiologically grounded reference. We use seven zones: rather than defining them by pace or arbitrary effort descriptions, they are anchored to specific events in exercise physiology: the first lactate threshold (LT1), also referred to as the aerobic threshold (AeT) defines the upper boundary of Zone 2, with the point of maximum fat oxidation (FatMax) just below AeT. The second lactate threshold (LT2, also called the anaerobic threshold or AT) forms the upper boundary of Zone 4. The figure also shows the underlying fuel mix across those zones, which is itself useful information when targeting metabolic adaptations specific to each event.
Zone 1 is recovery territory: warm-ups, cool-downs, the easier portions of long runs and dedicated recovery days. The upper boundary of the zone is a ceiling, not a target, and slower is generally better. As an athlete’s fitness develops, their recovery heart rate tends to settle progressively lower within Zone 1, which is a useful marker of improving aerobic condition in its own right.
Zone 2 is where the bulk of aerobic training happens. Most long runs and general aerobic runs should sit in the low to middle of this zone, which corresponds to the range of highest absolute fat oxidation and is most associated with building genuine aerobic base. The aerobic threshold sits near the top of Zone 2 and is a very useful training intensity as fat burning is very near maximal and the pace is relatively quick. Most athletes find that a well-executed run at the right Zone 2 intensity is harder to get right than it looks. The effort is genuinely easy and the temptation to push just a little harder is always present.
Zone 3 is what we consider the grey zone and we generally avoid targeting it deliberately. It occupies an awkward middle ground where the intensity is too high to maximally train fat-burning aerobic metabolism and too low to develop aerobic capacity or speed in any meaningful way. It generates fatigue without providing the specific adaptations of either the work above it or the easier work below it. The one reasonable exception is marathon pace, which falls right in this zone, and the reason marathon training requires so much care and experience to execute well is at least partly explained by that fact.
Zone 4 and Zone 5a together cover the tempo and threshold range with LT2 separating them. Zone 4 corresponds to longer reps and tempo runs at an intensity just below half marathon pace, while Zone 5a covers shorter harder reps somewhere in the half marathon to 10k range. These are the zones most closely associated with improving the lactate threshold itself. Zone 4 training requires considerable self discipline especially when running with a strong group or feeling good on a given day.
Zone 5b is VO2 max territory, used sparingly, around 5k pace and Zone 5c covers the short speed work and strides. Both zones are important for developing and maintaining high-end speed but neither should represent a large proportion of total training volume and the recovery demands of both are significant enough that slotting them into a training week requires some care.
The reason we want to be specific about these zones when programming training is that endurance adaptation responds to how training intensity is distributed, not just to how much total training is done. There is reasonably strong evidence that athletes who spend most of their training time genuinely below LT1 and combine that with targeted work around LT2 tend to develop better over the long run than athletes who accumulate most of their volume in the middle or attempt too much high intensity work. Spending unplanned time in Zone 3 when the intention was Zone 2, or drifting into Zone 5b when a session was designed for Zone 4, doesn’t just miss the target for that workout. It also distorts the overall intensity distribution across a training block.
We use heart rate (HR) as the primary guide for getting athletes into the right zones with perceived effort and pace as secondary considerations. Pace is context-dependent in ways that heart rate mostly isn’t. A Zone 2 HR effort in the heat, at altitude, at the end of a heavy week or in the middle of a long run will produce a different pace than the same effort on a fresh Tuesday morning, but the physiological state and the adaptation stimulus are essentially the same regardless. Using pace as the primary target for easy and moderate training tends to encourage athletes to push harder than they should when conditions are unfavorable to keep a “good” pace while HR gives a view into the actual physiological strain and helps keep things under control.
The objective measures of training intensity become particularly important earlier in an athlete’s development. Very experienced runners can generally find their target intensities by feel and regulate effort quite accurately across a range of training types, because they’ve had years of feedback from HR monitors, lactate testing and race performances to calibrate their internal sense of where they are physiologically. For newer runners, before that calibration has had time to develop, the tendency is almost always to run a little too hard particularly on the easy days. Recovery runs drift into the grey zone. Long runs get executed at dream marathon pace rather than actual current marathon pace. Tempo sessions in group workouts drift into VO2 max territory when there’s someone faster nearby to chase. None of this happens because athletes are undisciplined. It happens because Zone 2 and Zone 3 can genuinely feel similar while you’re running them and having a concrete heart rate range or ceiling to work within gives athletes something specific to act on rather than a vague instruction to “run easy” or “fast”.
Vague zone definitions also make it very hard to borrow intelligently from what elite athletes are doing. You might want to run double threshold workouts because you heard that Jakob Ingebrigtsen does them, but that requires knowing what “threshold” means to him in this context, the specific physiological targets those sessions are built around and how these relate to your personal zones and training status.
We’ve found that adopting a clearly defined seven zone framework and using specific HR targets to help guide athletes into the desired zones for each workout to be a powerful way to approach training. It’s a consistently effective way to regulate intensity according to the underlying physiology and to get athletes executing their workouts the way they were actually designed to be run.