The Efficiency Factor (EF) is an advanced metric that measures how efficiently your cardiovascular system converts effort into power during a workout. It helps you understand your aerobic fitness by essentially showing how much power you generate for each heartbeat.
Efficiency Factor (EF) is a powerful way to gauge your aerobic conditioning, often described as "power per heartbeat." Since this metric is highly personal, its true value comes from tracking your own trends over time in similar workouts, rather than comparing your numbers to others.
The Outsiders categorizes your EF based on your personal 30-day history:
To calculate your Efficiency Factor, The Outsiders divides your Weighted Average Power by your average heart rate for the entire session.
Context is crucial when interpreting this metric. A single low EF reading after a particularly hot or stressful day is likely just noise. However, a declining trend over several comparable workouts can be a meaningful signal that your body is accumulating fatigue or that your fitness may be dipping.
Tracking your EF provides one of the clearest windows into whether your cardiovascular system is getting stronger and your endurance training is paying off.
A consistently rising EF trend means your aerobic engine is improving—you're able to generate more power with less cardiovascular strain. Conversely, a declining trend can serve as an early warning of overtraining, allowing you to adjust your recovery and training load. When analyzed alongside Decoupling, a high EF with minimal heart rate drift is a strong indicator of a well-developed endurance base.
Efficiency Factor (EF) measures your aerobic efficiency by calculating the power you produce per heartbeat. Tracking EF trends over time provides a clear window into your cardiovascular conditioning, helping you gauge your fitness progress and identify potential signs of fatigue or overtraining.