Training load and readiness tell you how much to train and when, while advanced workout metrics tell you what's happening inside a session. They break down the quality of your effort, the efficiency of your engine, and how your body holds up as the work adds up.
Each metric offers a different angle on the same workout. Together, they give you a clearer, more honest picture of your performance than any single number can.
All four metrics use Weighted Average Power in their calculations. Weighted Average Power is a smarter version of average power. It gives more weight to harder efforts, so it better represents the true physiological cost of a session, especially when your output wasn't perfectly steady.
Intensity Factor (IF) shows how hard a session was relative to your personal limit. It compares your Weighted Average Power to your Functional Threshold Power (FTP), the maximum power you can sustain for about one hour.
The calculation is straightforward: Weighted Average Power divided by FTP.
An IF close to 1.0 means you were working near your threshold, the kind of effort you could sustain for roughly an hour. Below 1.0, the session was less demanding. Above 1.0, you were pushing beyond your threshold, which is only sustainable in shorter bursts.
Not all sessions are created equal. A one-hour criterium, a four-hour endurance ride, and a tempo run will all feel completely different, but IF reveals how close each came to your physiological ceiling, on the same scale.
This makes IF a powerful tool for comparing efforts across different durations, terrains, and workout types. It cuts through the noise of raw numbers and shows you, in a single value, how hard a session really was relative to your capacity.
Track IF over time to see whether your training load is balanced. It helps answer the question most athletes eventually face: am I pushing hard enough to grow fitness, or so hard that I'm flirting with burnout? If IF is consistently high across multiple sessions in a week, your body may be accumulating more stress than it can absorb. If it's consistently low, you might be leaving gains on the table.
Variability Index (VI) shows how steady your power output was during a session. It's calculated by dividing your Weighted Average Power by your Average Power.
A VI close to 1.0 means your effort was smooth and consistent, you held a steady output from start to finish. The further it climbs above 1.0, the more your power fluctuated throughout the session.
A low VI reflects steady, even output, what you want for tempo runs, endurance rides, and long aerobic efforts where holding a consistent effort is the goal. A high VI reflects a more variable effort with surges and recoveries throughout, expected and appropriate in workouts like intervals, hill repeats, or races with frequent pace changes.
Neither is inherently better. It depends on what you planned to do. A criterium is supposed to be punchy. A half marathon is supposed to be smooth. VI tells you which one you actually achieved.
Track VI over time to see whether your pacing is improving. If you're working on even effort distribution, especially for endurance events, a declining VI trend is a sign that your execution is getting sharper. If you're training for variable-effort racing, a higher VI may be exactly what you're after.
VI also adds context to other metrics. For example, Decoupling is most reliable in sessions with a low VI, where any heart rate drift can be attributed to genuine aerobic fatigue rather than power fluctuations.
Efficiency Factor (EF) measures how efficiently your cardiovascular system converts effort into power. It's calculated by dividing your Weighted Average Power by your average heart rate for the session.
Think of it as power per heartbeat.
A higher EF means you're generating more power without your heart working harder. This is a sign of strong cardiovascular conditioning and good aerobic efficiency, exactly what endurance training is designed to build.
A lower than usual EF can mean several things: accumulated fatigue, a dip in fitness, or external factors like heat and dehydration making your heart work harder for less output. Context matters here. A single low reading after a hot day is noise. A declining trend over weeks is a signal.
EF is highly personal. Comparing your number to someone else's tells you very little. What matters is how your EF changes over time for similar workouts. Same route, same conditions, same type of session, improving EF: that's aerobic progress you can trust.
Paired with Decoupling, EF becomes even more useful. A high EF with minimal heart rate drift shows a well-developed aerobic engine, your body doing more with less and sustaining it. A lower EF with significant drift suggests your endurance base could use some work.
Track EF across comparable sessions to spot trends in your aerobic development. It won't tell you everything, but it's one of the clearest windows into whether your cardiovascular system is getting stronger.
Decoupling measures how the relationship between your heart rate and power output shifts during a session. In an ideal scenario, if you hold the same power from start to finish, your heart rate stays nearly the same throughout. In reality, it tends to creep upward, a phenomenon known as heart rate drift.
To calculate decoupling, the app compares your Efficiency Factor from the first half of your session to the second half. If EF stays consistent across both halves, you have low decoupling. If it drops noticeably in the second half, meaning your heart is working harder for the same output, you have higher decoupling.
Decoupling is most reliable in sessions where your effort is steady and sustained, think tempo runs, threshold rides, or long aerobic efforts where your Variability Index stays close to 1.0. When the effort is even, any heart rate drift can be attributed more clearly to aerobic fatigue. In sessions with higher variability, decoupling can still reveal useful trends, but the signal is noisier since some of the drift may come from power or pace fluctuations rather than genuine fatigue.
Low decoupling, generally under 5%, means your cardiovascular system is efficiently supporting your output over the full duration of the session. Higher decoupling points to fatigue, an endurance base that may need more development, or external factors like heat and dehydration. It's also normal to see higher decoupling in highly variable workouts, where repeated surges naturally drive heart rate up over time.
Interpreted in the right context, decoupling is one of the most honest metrics in endurance training. Power and pace can be held through sheer will. Heart rate drift is your body telling you what that effort is actually costing.
Track decoupling over time in comparable workouts. As your fitness improves, you'll see it shrink, meaning you can hold steady output without your heart needing to work progressively harder to keep up. A shrinking trend is one of the clearest signs that your endurance is genuinely improving.
These metrics are most powerful when read together. IF tells you how hard the session was. VI tells you how steady. EF tells you how efficient. And Decoupling tells you how well that efficiency held up over time. Each one answers a different question, but together they show you something no single metric can: how your body is really performing, and where there's room to grow.