“Low aerobic shortage” is a message from your Garmin watch telling you that your recent training has been too heavily weighted toward moderate and high-intensity exercise, without enough easy, low-intensity activity to balance it out. Garmin’s recommendation is straightforward: add more low aerobic activities to provide recovery and balance for your higher intensity workouts.
This message comes from a feature called Training Load Focus, which sorts your recent workouts into three categories based on heart rate data: low aerobic, high aerobic, and anaerobic. When one category falls short relative to the others, your watch flags it as a “shortage.” In this case, you’re not doing enough easy effort.
How Your Watch Categorizes Training Load
Garmin devices use analytics developed by Firstbeat to measure the physiological impact of each workout. The system tracks your heart rate, heart rate variability, and breathing rate to estimate how hard your body is working at any given moment. Each session gets scored on a Training Effect scale from 0.0 to 5.0, where anything below about 2.0 falls into the low aerobic bucket.
Low aerobic activity is exercise where you could comfortably hold a conversation the entire time. Think of a relaxed bike ride with a friend, a brisk walk, or an easy jog. These sessions might feel almost too easy, and your watch may even show a surprisingly low Training Effect score for a workout that lasted an hour or more. That’s normal. The intensity is too low to directly improve your peak fitness (VO2max), but it serves a different and equally important purpose.
Why Low Aerobic Training Matters
Easy exercise builds the aerobic foundation that all your harder training depends on. When you spend time at low intensities, typically in heart rate zones 1 and 2 (roughly 50% to 70% of your maximum heart rate), your body makes several key adaptations. Your muscles grow more capillaries, improving blood flow and oxygen delivery. The number and efficiency of mitochondria in your muscle cells increase, which determines how well your body can produce energy using oxygen. And your metabolism shifts toward burning a higher percentage of fat for fuel, sparing your limited carbohydrate stores for when you really need them.
These adaptations also improve insulin sensitivity, blood sugar control, and your body’s ability to store glycogen. None of this happens during a hard interval session. It’s the slow, steady work that creates the infrastructure your body needs to perform during intense efforts and recover from them afterward.
There’s a reason elite endurance athletes across nearly every sport spend 75% to 80% of their training time at easy intensities. Without that base, high-intensity work becomes less effective and harder to recover from. A “low aerobic shortage” is your watch’s way of saying you’re skipping this foundational layer.
What Causes the Shortage
Most recreational athletes see this message for one of two reasons. The first is that nearly all of their workouts are moderate to hard. They run at a “comfortable but challenging” pace, push through group fitness classes, or ride at a steady effort that keeps their heart rate in the upper zones. There’s very little truly easy activity in their week.
The second reason is simply not exercising enough overall. Garmin can also flag a message saying your training load is lower than optimal in all intensity categories, recommending that you increase the duration or frequency of your workouts. If you’re only working out a couple times a week and those sessions are intense, you’ll naturally end up short on low aerobic volume.
How to Fix It
The fix is adding easy sessions to your week. These don’t need to be long or structured. A 30 to 60 minute walk, an easy bike ride, a light swim, or a slow recovery jog all count. The key is keeping your heart rate low enough that the effort feels genuinely easy. For most people, that means staying below about 70% of your maximum heart rate. If you’re breathing hard or can’t speak in full sentences, you’re going too fast.
On rest days, a walk or easy bike ride can contribute low aerobic load without interfering with recovery. Firstbeat’s own guidance suggests keeping the Aerobic Training Effect below 2.0 on these days. You don’t need to turn recovery days into training days, just add light movement.
If you’re already training four or five days a week at moderate to high intensity, you don’t necessarily need to add more days. Instead, consider converting one or two of those sessions into genuinely easy efforts. Many runners, for example, find that slowing their “easy” runs by 30 to 60 seconds per mile is enough to shift those sessions into the low aerobic category. The pace should feel almost embarrassingly slow. That’s the point.
What to Expect After Adjusting
Once you start including more low aerobic work, the shortage message on your watch will resolve within a few days to a week as the Training Load Focus recalculates based on your recent activity. More importantly, you may notice that your harder sessions start to feel more productive. Better aerobic base fitness means faster recovery between intervals, a lower resting heart rate over time, and more energy for the workouts that really count.
Long, easy efforts can feel anticlimactic in the moment, especially when your watch gives them a low Training Effect score. But those sessions are building the metabolic machinery, more capillaries, more mitochondria, better fat burning, that makes everything else work. The shortage message is a useful nudge to stop neglecting them.