Resting Heart Rate
Your heart rate at true rest, ideally measured first thing in the morning before you get out of bed: a slow-moving recovery signal, not a fitness score by itself.
Resting heart rate is simply how many times your heart beats per minute while you're at true rest. The most reliable way to check it is first thing in the morning, before you get out of bed, before coffee, before checking your phone, while your body is still in the calm state it was in overnight. A reading taken later in the day, after you've been moving around, eating, or stressed about a meeting, will usually run higher and isn't a fair comparison point.
Why it tends to run low in endurance athletes
Consistent endurance training changes the heart itself. Training enlarges the left ventricle's cavity and improves how well it fills and empties, so each beat moves more blood. A heart that moves more blood per beat needs fewer beats per minute to do the same job at rest, which is the main reason trained endurance athletes often sit well below the roughly 60 to 100 bpm range considered typical for untrained adults, sometimes into the 40s or lower. Training also raises vagal (parasympathetic nervous system) tone, and researchers have found training-related changes at the level of the heart's own pacemaker cells contribute as well. It's a real physiological adaptation, not a party trick, though the exact split between those mechanisms is still an active research question.
Why the trend matters more than any single number
A low resting heart rate on its own is a sign of fitness, not a daily readiness score. What's useful is tracking it over time and watching for a change against your own baseline. A resting heart rate sitting meaningfully above your normal rolling average for several days in a row, especially alongside heavy legs, poor sleep, or flat performance, is one signal worth factoring into whether you need easier training, not proof on its own. A single morning a few beats higher than usual is common noise and rarely means anything by itself. It's also worth being honest that resting heart rate isn't a perfectly reliable overreaching detector on its own; some research on overloaded athletes found no measurable change in resting heart rate even as performance and mood declined, which is exactly why it's treated as one input rather than a verdict.
How it complements HRV
Resting heart rate and HRV are often tracked together, and it's worth being clear they measure different things. Resting heart rate is simply how fast your heart beats at rest. HRV measures how much the time between those beats varies, a marker more specific to autonomic nervous system balance. A wearable can show a normal resting heart rate alongside a depressed HRV, or the reverse, because they're picking up on related but distinct physiological signals. Neither one replaces a simple gut check on how a session felt, and neither replaces the other. Together, alongside sleep and subjective fatigue, they give a fuller picture than any single number can.