Training Load Monitoring
Training Load monitoring tracks how your body's fitness and fatigue accumulate over time, helping you find the right balance between building fitness and avoiding overtraining.
The science of training load
Every training session creates two simultaneous adaptations: a short-term fatigue response (which temporarily degrades performance) and a long-term fitness adaptation (which permanently improves capacity). The balance between these two processes determines your readiness on any given day.
This concept was formalised by scientists as the Fitness–Fatigue model (also called the Performance Manager model). It uses exponential moving averages to model how training stress decays over different time horizons.
The most widely used implementation in endurance sports uses the TRIMP (Training Impulse) method to quantify the internal load of each workout — translating heart rate data into a comparable training stress unit.
TRIMP — quantifying workout stress
TRIMP (Training Impulse), developed by Morton and colleagues, calculates an internal training load score for each workout based on:
- Workout duration.
- Average heart rate during the workout.
- Your personal resting and maximum heart rate.
- A physiological weighting factor that gives higher credit to training at higher intensities.
TRIMP accounts for the non-linear relationship between exercise intensity and physiological cost — a workout at 90% of max HR creates a disproportionately larger stress than a workout at 70%, even if the durations are identical. Flux calculates TRIMP automatically from every workout's heart rate data.
ATL and CTL
Flux calculates two key training load metrics using exponential moving averages of daily TRIMP scores:
ATL — Acute Training Load
(~7-day window)A short-term exponential moving average of your recent training stress. ATL rises quickly with hard training and drops quickly with rest. It represents your current fatigue level — how much recent training stress your body is currently carrying.
CTL — Chronic Training Load
(~42-day window)A long-term exponential moving average representing your accumulated training over several months. CTL changes slowly and reflects your current fitness level — the aerobic base you have built over consistent training.
Acute:Chronic Workload Ratio (ACWR)
Flux also calculates the ACWR — the ratio of ATL to CTL. Research has shown that rapid spikes in training load (a high ACWR) significantly increase the risk of both injury and illness:
Under-training zone
Very low recent load relative to fitness. Consider increasing training volume gradually.
Sweet spot
Optimal range. Recent load is proportional to your chronic fitness level.
Danger zone
Sudden spike in load. Elevated injury and illness risk. Reduce intensity and monitor Health Monitor closely.
How to use Training Load data
- Build CTL gradually — avoid increasing weekly training load by more than 10% per week.
- Use ATL as an early warning: if it is rising fast, your next few days should be lighter.
- Plan your race peaks by tapering to reduce ATL while preserving CTL.
- A chronically high ATL over 4+ weeks suggests accumulated fatigue; consider a deload week.
- Cross-reference Training Load with your Recovery Score — low recovery during high ATL is expected and normal.