Garmin Running Economy Explained for Trail Runners¶
Running Economy: 218 ml/kg/km. The number appeared in the Garmin app after months of qualifying runs, and now it sits there with no reference point. Looking back at which runs fed into it, the singletrack miles are missing. The long runs with a vest. The ridge traversals and the out-and-back on technical ground. None of them contributed.
That is not an accident. This post explains what the number actually measures, what feeds it, and why the runs that make up most of a trail runner's week were never going to count.
What just appeared on your watch¶
Garmin launched Running Economy in May 2025, alongside the Forerunner 970. Getting a first estimate required an HRM-600 chest strap and at least four or five qualifying runs, which means runners who picked up the hardware in mid-to-late 2025 are hitting their baselines right now, in mid-2026.
The delayed arrival is worth naming because it shapes what this post is for. The runners searching for answers today are not the early adopters who wrote about it at launch. They are runners who finally have a number and are asking, for the first time, whether it means anything.
For a runner whose training is mostly trails with a pack, there is a specific follow-on question: why did Garmin only count the road recovery runs? That question is the center of this post.
What ml/kg/km actually measures¶
Running Economy, in exercise physiology terms, is the volume of oxygen a runner consumes to sustain a given pace. The unit, milliliters of oxygen per kilogram of body weight per kilometer, captures what the body is spending to cover ground. A lower number means the body is doing less metabolic work to run the same speed.
Garmin's implementation does not measure oxygen directly. That requires a metabolic cart in a lab. Instead, the HRM-600 captures movement data, and Garmin's model uses that data to estimate efficiency on flat outdoor pavement.
The core sub-metric driving the estimate is called Step Speed Loss, or SSL. It measures the ratio of speed lost during ground contact versus the airborne phase of each stride. A runner whose forward momentum drops more during each foot strike is spending energy fighting deceleration rather than maintaining it. SSL is Garmin's most direct proxy for flat-road mechanical efficiency.
Running Economy is not the same metric as VO2max. VO2max is aerobic ceiling: the maximum rate at which the body can use oxygen. RE is aerobic cost: what the body actually spends at a given pace. Two runners can share a VO2max and have substantially different running economies, which is part of why RE is interesting as a separate measurement. Beyond that distinction, they are different signals and comparing them here would pull us off topic.
The four running dynamics behind the number¶
The HRM-600 captures four running dynamics during qualifying runs. Garmin's RE model draws from all four, though research suggests they do not contribute equally.
| Dynamic | What it measures | Its role in RE |
|---|---|---|
| Step Speed Loss (SSL) | Speed lost during ground contact compared to the airborne phase | Primary driver of the RE estimate |
| Vertical Ratio | Vertical oscillation divided by stride length | Strong research correlation; captures how much energy goes upward rather than forward |
| Ground Contact Time (GCT) | Milliseconds of foot contact per stride | Contributing factor; weaker predictor on its own than Vertical Ratio |
| Vertical Oscillation | Total vertical movement of the torso per stride | Contributing factor; most meaningful when read alongside stride length |
Studies on running biomechanics have found that Vertical Ratio correlates with economy better than Ground Contact Time or oscillation measured in isolation. The reason makes mechanical sense: Vertical Ratio captures the proportion of energy going upward instead of forward, which is a direct measure of wasted propulsive effort. A runner who bobs more vertically per unit of stride length is spending energy on movement that does not translate into forward progress.
SSL is where the carbon plate effect shows up in the data. Carbon plate shoes are designed to reduce braking force during ground contact, which is precisely the mechanical event SSL is measuring. That connection is worth naming, though it goes no further here: this post is about understanding a metric, not about shoe selection.
Why your trail runs didn't count¶
Garmin's Running Economy calculation only accepts flat outdoor runs with no additional load. Trail runs are excluded. Treadmill runs are excluded. Indoor runs are excluded. Any run where a vest, pack, or hydration system adds external weight is excluded.
This is a deliberate methodological choice, not a gap in the firmware.
The model works by making three assumptions: constant grade, consistent surface underfoot, and no external weight altering the runner's natural movement patterns. When a run breaks any of those assumptions, the inputs the model relies on stop reflecting flat-road mechanical efficiency and start reflecting something else.
Trail grade changes the Vertical Ratio reading in ways that have nothing to do with economy on flat ground. A runner climbing a 20% grade has a different oscillation pattern than the same runner on a flat road, and that difference is the grade, not their efficiency. A pack shifts the body's center of mass and alters oscillation and ground contact in ways that contaminate the measurement. Including those runs would make the model's assumptions false: the estimate would be noise rather than signal.
So Garmin's RE is measuring one specific thing: flat-road mechanical efficiency with no external load. It is measuring that one thing well, within its own constraints. For a runner whose training is mostly singletrack with a vest, the RE score reflects the road recovery runs and the flat shakeout miles. Those are a meaningful slice of the total picture. They are not the whole picture.
Garmin's documentation notes that the sensors and calculations underlying the RE feature have not been formally validated in published research. That is worth sitting with. The metric is an estimate built on a model, not a laboratory measurement. It is a direction and a trend, not a clinical reading.
What the number tells you, and what it honestly can't¶
A single RE baseline tells you less than watching the number across months of consistent qualifying runs.
In general exercise physiology terms, lower RE values reflect greater flat-road efficiency. Elite distance runners typically operate at the lower end of the range, with well-trained recreational runners sitting higher, and less-experienced runners higher still. Garmin aligns their RE ranges with those population norms. But the specific number matters less than what happens to it over time in your own data.
A runner who sees their RE estimate falling across a training block is seeing flat-road efficiency improving. A runner who sees no movement may be doing most of their adaptation on terrain that Garmin cannot count: trail volume, loaded runs, hill work. The training is happening. The metric just cannot see it.
Two things worth keeping in front of the number:
Trend over baseline. One RE estimate means almost nothing. Six months of estimates, taken from consistent qualifying runs on similar routes, start to tell a story about how flat-road efficiency is moving across a build.
What the metric excludes is not what it gets wrong. RE does not fail on trail runs. It simply does not attempt to measure them. Those are different statements, and conflating them leads to wrong conclusions about the number's reliability on the pavement runs it does include.
The validation caveat from Garmin belongs here again: this is a model, not a lab test. Use the trend as a directional signal and hold the absolute value with some patience. It will sharpen as the qualifying run sample grows.
The runs Garmin didn't see are still in your history¶
Every trail run Garmin filtered out of the RE calculation was still logged. The vert, the pack weight, the miles on technical ground, the back-to-backs that built most of a runner's fatigue tolerance. All of it is in the activity history on Strava.
When a training plan draws from that history, it is drawing from the complete record of what a runner has actually trained, not just the flat road sessions the RE model could use. A runner holding a RE baseline of 218 ml/kg/km and wondering whether it captures their actual fitness has a more complete answer in the aggregate of what they have run across the past year than in the metric alone.
NavRun syncs from Strava, which means the entire record comes in: road runs and trail runs together, loaded days and unloaded days, the long weeks and the recovery weeks. That is what generates the plan.
Your Strava history has all of it. Connect at navrun.app and let the full record build the plan.
FAQ¶
Why do I need the HRM-600 specifically?¶
Running Economy relies on running dynamics that standard optical wrist sensors cannot capture accurately. The HRM-600 measures chest-based movement, giving it access to Vertical Oscillation, GCT, and the sub-beat timing needed to calculate SSL. A wrist optical sensor is sufficient for heart rate and pace. It is not sufficient for the dynamics inputs the RE model requires.
My RE estimate seems to move around between runs. Is that normal?¶
Yes, at least early on. The model stabilizes as more qualifying runs accumulate. Conditions also affect individual estimates: heat, wind, fatigue from recent hard training, and route surface all influence the dynamics inputs. A single RE reading on an unusually fatigued day will come in higher than a reading on a fresh day. This is why trend matters more than any individual data point.
Does a lower RE score mean I should run more road miles?¶
This post is a translation of a metric, not a training prescription. Whether to shift training surfaces is a question about your race goals and your training history, not something that follows from the RE number itself.
Can I compare my RE score to another runner's?¶
With real caution. RE values are speed-dependent: the same runner's economy looks different at 5K pace versus easy pace, because the dynamics of the stride change. Unless two runners are being measured at exactly the same speed under the same conditions, their RE scores are not directly comparable. Your own trend, across consistent qualifying runs at roughly similar efforts, is the meaningful comparison.
Why are my trail runs excluded from RE even if they're flat?¶
Garmin's documentation specifies outdoor flat runs as qualifying, but the exclusion of trail runs appears to apply broadly, not only to hilly segments. The likely reason is surface consistency: trail ground, even flat trail, introduces irregular footing that alters the dynamics inputs in ways that pavement does not. The model was built on the assumption of consistent, predictable ground contact.
Is Running Economy available on the Forerunner 965 or Fenix 7?¶
As of the time of publication, Garmin has not confirmed Running Economy via firmware update to the Forerunner 965, Forerunner 255, or earlier Fenix models. The feature launched with the Forerunner 970. Speculating about future firmware availability here would not be accurate, and Garmin's own release notes are the right place to track that.