What efficiency factor tells you
Output for every beat
Two rides can look alike on the power meter and feel quite different at the heart. Efficiency factor puts the two together: how much output did you record for the heart rate it took? On a bike it is power divided by heart rate. 200 W at an average of 140 bpm gives 200 ÷ 140 = 1.43. Form uses Normalized Power on rides, so a variable ride is not flattered or punished by its average.
Higher means more output for each beat per minute. The interactive example compares a ride with an earlier one at 200 W and 140 bpm. Change the power or heart rate and watch the ratio move. The numbers are illustrative, not measurements.
Efficiency = power ÷ average heart rate. Form uses Normalized Power for rides; this example uses a steady power. Illustrative numbers.
Output for every beat
Two rides can look alike on the power meter and feel quite different at the heart. Efficiency factor puts the two together: how much output did you record for the heart rate it took? On a bike it is power divided by heart rate. 200 W at an average of 140 bpm gives 200 ÷ 140 = 1.43. Form uses Normalized Power on rides, so a variable ride is not flattered or punished by its average.
Higher means more output for each beat per minute. The interactive example compares a ride with an earlier one at 200 W and 140 bpm. Change the power or heart rate and watch the ratio move. The numbers are illustrative, not measurements.
What a change in the ratio can and cannot say
If the ratio rises, you produced more for the same heart rate, or the same output at a lower heart rate. That is a useful observation, and it is the reason endurance athletes track the figure across similar sessions.
It does not tell you why. Heat, hydration, sleep, caffeine, a recent hard day, a different sensor or a different route can each move heart rate without any change in fitness. Equally, a higher ratio on a downhill ride says little about the athlete. Treat efficiency factor as a prompt to look at the session, not as a fitness score.
Why Form shows it for each half
Form leaves out the first 10 minutes, then splits the rest of the ride in two and works out the ratio for each half. A ratio that holds from the first half to the second means heart rate and output stayed in step. A second-half ratio that falls means heart rate rose against the same output, or output fell against the same heart rate, or both.
The percentage change between the two halves is the aerobic decoupling figure. The drift article explains where it comes from, and the power fade figure shows the output side on its own. Together they separate a heart rate that climbed from a power that dropped.
Comparing rides fairly
Compare like with like: the same sport, a similar length and intensity, the same sensor, and as far as you can, similar temperature and terrain. A ratio from a 40-minute tempo ride and one from a four-hour endurance ride answer different questions, because heart rate behaves differently over a long effort.
Platforms also differ. TrainingPeaks describes its cycling efficiency factor with Normalized Power and its running version with normalized graded pace. Another tool may use average power or leave the warm-up in, so two services can show different numbers for the same ride without either being wrong.
Not the same as mechanical efficiency
The word efficiency has another meaning in cycling science: the fraction of metabolic energy that becomes mechanical work, usually called gross efficiency. Efficiency factor is a simple ratio of two recorded numbers. It does not measure how much energy your muscles used, and the two should not be mixed up.
Check the recording first
A heart-rate strap that slips, an optical sensor that loses the signal or a power meter that drops out can all move the ratio on their own. Look at the heart rate and power traces before trusting a surprising number, and compare with another recording if the result matters.
Then use the figure the way you would any trend: across several comparable sessions, with a note of what changed. One ride is an observation. A pattern across similar rides is the useful part.