Aerobic Threshold (AeT)
Your aerobic threshold (AeT) corresponds to a “conversational” nose-breathing effort. You should be able to breathe through your nose and hold a back-and-forth conversation with someone running next to you when you’re at or below your AeT; breathing is moderate and not labored.
In most athletes, the AeT corresponds to the top of Zone 2 if you’re using the Friel heart rate zones or Zones 1-2 (“easy to moderate”) if you’re using the Stryd power zones as a guide. On a Borg 1-10 rating of perceived exertion (RPE) scale, this would be an RPE of 5-6 (“moderate”).
Training at or below your aerobic threshold is foundational to building your aerobic endurance during base training, allowing you to better metabolize fat and spare glycogen (stored carbohydrate) as a long duration energy source. Work at or below your AeT during other phases of training still comprises the bulk of your training time.