Garmin Training Effect Explained: Aerobic vs Anaerobic

Two Scores, Two Energy Systems

Every time you finish a workout on your Garmin, two numbers appear on the summary screen: aerobic training effect and anaerobic training effect. Both sit on a 0.0 to 5.0 scale. Most people glance at them, nod, and move on.

That is a missed opportunity. These two scores tell you exactly what your workout did to your body at a physiological level. One measures how much you challenged your oxygen-based endurance system. The other measures how much you taxed your high-intensity power system. Together, they reveal whether your training is balanced or lopsided.

Understanding training effect takes about five minutes. Using it well can change how you plan every week.

The 0.0 to 5.0 Scale

Both aerobic and anaerobic training effect use the same five-point scale, but the meaning of each level is identical across both:

0.0 to 0.9: No benefit. The effort was too easy or too short to trigger any meaningful adaptation. A casual walk or a very brief warm-up lands here.
1.0 to 1.9: Minor benefit. Recovery-level effort. Good for active recovery days, but not building fitness. Think of a slow 20-minute jog the day after a race.
2.0 to 2.9: Maintaining. This effort maintains your current fitness level without meaningfully improving it. A moderate easy run for a trained runner typically falls in this range.
3.0 to 3.9: Improving. This is the productive zone. The workout was hard enough and long enough to stimulate real adaptation. Most of your key sessions should land here.
4.0 to 4.9: Highly improving. A significant training stimulus. Hard interval sessions, long tempo runs, and race efforts land here. These workouts require proper recovery afterward.
5.0: Overreaching. Maximum overload. Your body received more stress than it can comfortably absorb. Occasional 5.0 scores are fine in a peak training block, but seeing them regularly means you are digging a recovery hole.

The scale is not linear. Moving from 3.0 to 4.0 represents a much bigger jump in physiological stress than moving from 1.0 to 2.0.

Aerobic Training Effect: Your Endurance Engine

Aerobic training effect measures the impact on your oxidative energy system. This is the system that burns fat and oxygen to produce energy, the engine that powers everything from easy runs to marathon pace.

Your aerobic system improves through sustained effort at moderate intensities. Long runs, steady-state rides, and extended zone 2 sessions are the primary drivers. Duration matters a lot here. A 30-minute easy run might score 2.0 aerobic, while the same pace held for 90 minutes could push to 3.5.

What drives aerobic TE higher:

Longer duration at any intensity
Sustained time in zones 2 and 3
Threshold-level efforts (tempo runs, sweet spot cycling)
Higher cumulative EPOC from the session

What keeps aerobic TE low:

Short workouts, even if intense
Long rest intervals that let heart rate fully recover
Very low intensity with minimal cardiovascular demand

For most endurance athletes, aerobic training effect is the more important number. Your aerobic base determines your ability to sustain pace over distance, recover between hard efforts, and handle increasing training volume.

Anaerobic Training Effect: Your Power System

Anaerobic training effect measures the impact on your glycolytic and phosphocreatine energy systems. These are the systems that produce energy without oxygen, powering sprints, hard surges, and high-intensity intervals.

Unlike aerobic TE, anaerobic training effect responds primarily to intensity, not duration. Short, maximal efforts spike it. Long, moderate efforts barely touch it.

What drives anaerobic TE higher:

Repeated high-intensity intervals (above lactate threshold)
Sprint efforts and hard surges
Sustained time above 90% of max heart rate
High variability in heart rate (repeated spikes and drops)

What keeps anaerobic TE low:

Steady-state efforts regardless of duration
Staying below threshold intensity
Long recovery intervals between efforts

A set of 8x400m repeats at 5K pace might score 1.5 aerobic and 3.8 anaerobic. A 2-hour easy long run might score 3.5 aerobic and 0.5 anaerobic. Same athlete, same watch, completely different physiological stress profiles.

How Firstbeat Calculates It

Behind these scores sits Firstbeat Analytics (now owned by Garmin), the engine that processes your heart rate data in real time. The calculation is based on EPOC, or excess post-exercise oxygen consumption. EPOC represents the total oxygen debt your body accumulates during exercise. The bigger the debt, the harder your body worked, and the more adaptation it will trigger.

Firstbeat's algorithm breaks EPOC into two components. The portion of EPOC driven by sustained, moderate-intensity effort feeds the aerobic score. The portion driven by high-intensity surges and anaerobic energy production feeds the anaerobic score.

The algorithm considers several factors:

Heart rate relative to your max and resting HR. This is why accurate heart rate zone setup matters so much. If your max HR is set too high, your training effect scores will read lower than they should.
Duration of effort at various intensities. More time in higher zones accumulates more EPOC.
Heart rate variability during the workout. Rapid HR spikes indicate anaerobic bursts.
Your current fitness level. Firstbeat adjusts the scores based on your VO2 max estimate. The same workout produces a lower score for a fitter athlete because their body handles the stress more easily.

That last point is important. Training effect is personalized. A 3.0 for a beginner represents a very different absolute workload than a 3.0 for an elite runner. But both represent roughly the same relative training stimulus for that individual.

What Good Numbers Look Like

Here is a quick reference for typical training effect values across common workout types. These assume reasonably accurate heart rate data and properly configured zones.

Easy Runs and Recovery

Aerobic TE: 1.5 to 2.5
Anaerobic TE: 0.0 to 0.5
If your easy runs consistently score above 3.0 aerobic, you are running too hard for recovery purposes.

Long Runs (Endurance Pace)

Aerobic TE: 3.0 to 4.0
Anaerobic TE: 0.0 to 1.0
Duration is the main driver. A 60-minute long run and a 2-hour long run at the same pace will produce very different aerobic TE scores.

Tempo Runs and Threshold Work

Aerobic TE: 3.0 to 4.0
Anaerobic TE: 1.0 to 2.5
These sessions stress both systems. The aerobic score is high because of sustained effort. The anaerobic score creeps up because you are flirting with threshold intensity.

Hard Intervals (VO2 Max, 5K Pace)

Aerobic TE: 2.0 to 3.5
Anaerobic TE: 3.0 to 4.5
The anaerobic score dominates because of repeated high-intensity efforts. The aerobic score is moderate because total sustained time is relatively short.

Sprints and Short Repeats

Aerobic TE: 1.0 to 2.0
Anaerobic TE: 3.0 to 4.5
Very short, very intense. Not much total volume, so the aerobic system barely notices. But the anaerobic system gets hammered.

Common Confusions

"My training effect is always low"

Usually one of three issues. First, your max heart rate might be set too high in your Garmin profile. If the watch thinks your max is 200 but it is actually 185, every workout looks easier than it really is. Second, you might be relying on wrist-based heart rate, which can underread during intense intervals. A chest strap fixes this. Third, you might genuinely be training too easy for your fitness level. If you have been running the same pace for months, your body has adapted and the stimulus has diminished.

"I got a 5.0, that must be great"

Not necessarily. A 5.0 means your body received an extreme training stimulus. If it happens during a race or a planned peak workout, fine. If it happens during a regular Tuesday interval session, it means the session was harder than your body was ready for. Repeated 5.0 scores lead to overtraining, not faster race times.

Think of it like sunlight exposure. Some sun builds a healthy tan. Too much causes a burn. A 5.0 is a sunburn. Productive, but only if you give your body time to heal before the next exposure.

"Both scores are high, is that bad?"

Not inherently, but it means the session taxed both energy systems hard. A workout with 4.0 aerobic and 4.0 anaerobic is a massive training stimulus. You probably need 48 to 72 hours before another hard session. If this happens often, you are not recovering between key workouts.

Using Training Effect for Balance

The real power of tracking both scores over time is seeing your training balance. Most endurance athletes should accumulate far more aerobic training effect than anaerobic across a typical week. The classic 80/20 rule applies here: roughly 80% of your training stimulus should come from aerobic development, with 20% from high-intensity anaerobic work.

If you look back at your last month and your total anaerobic TE outweighs your aerobic TE, you are likely doing too many hard sessions and not enough easy volume. This is one of the most common mistakes in self-coached athletes. Hard workouts feel productive. Easy runs feel like wasted time. But the aerobic base is what supports everything else.

Conversely, if your anaerobic TE has been near zero for weeks, you might be neglecting the intensity work that sharpens fitness and improves speed. Even in a base-building phase, one session per week with some intensity keeps the anaerobic system engaged.

Look at your training load ratio alongside training effect to get the full picture. Load ratio tells you if the total volume is appropriate. Training effect tells you what type of adaptation that volume is producing.

Putting It All Together

Training effect is most useful as a pattern indicator, not a single-workout metric. Any individual score can be thrown off by a bad heart rate reading, an unusually hot day, or a watch that lost GPS for three minutes. But trends over weeks and months tell you something real.

Check your training effect scores after each workout and ask two questions. First: did the score match my intention? If you planned an easy run but got a 3.5 aerobic, you went too hard. If you planned hard intervals but only hit 2.0 anaerobic, you did not push enough or rested too long between reps. Second: over the last two weeks, what is the balance between aerobic and anaerobic stimulus? If it matches your goals, stay the course. If not, adjust.

Gneta tracks your training effect alongside every workout, so you can see your aerobic and anaerobic balance over weeks and months. The AI coach uses this data to suggest what type of session to do next. Start free →

Training effect will not make you faster by itself. No metric can do that. But it removes the guesswork from one of the most fundamental questions in training: did that workout actually do what I needed it to do? When you can answer that question honestly after every session, you make better decisions. And better decisions, compounded over months, are what produce results.

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