Chronic Load, also called Endurance Fitness, tracks how much structured training your body is adapted to handling. It is a metric designed for endurance athletes and is calculated from heart rate data. It is expressed as a weighted average of your training load over the past 42 days. The higher the number, the more durable your endurance base.
Recent workouts carry more weight than older ones, but all 42 days contribute. This smooths out short-term fluctuations while remaining responsive to genuine changes in your fitness over time.
Because the metric relies on heart rate, it is most accurate for activities where heart rate reflects true effort, such as running, cycling, swimming, and rowing. Some athletes override the heart rate-based load with a rating of perceived exertion (RPE), often for strength or resistance workouts where the effort feels higher than heart rate alone suggests. This is understandable, but worth knowing: strength work contributes little to endurance fitness by nature. Regularly overriding load with RPE for these sessions can inflate your Endurance Fitness score and give a misleading picture of your aerobic base.
A rising Chronic Load means your body is adapting to progressive work. A falling Chronic Load means your training base is eroding, whether from a rest period, illness, or an unplanned reduction in volume.
Unlike day-to-day metrics, Chronic Load moves slowly by design. That is what makes it reliable. It is not thrown off by a single hard session or a rest day. It reflects the cumulative effect of weeks of work.
Chronic Load is most useful for spotting trends. Use it to plan volume increases without outpacing your base, manage a taper before a goal event, or catch early signs that your fitness is quietly slipping between training blocks. Paired with Training Load Ratio, it gives you a clear picture of where your current training sits relative to what you have actually built.