Heart rate tells you how hard you're working, and your body's numbers between runs give clues about how well you're recovering. Trainer uses heart rate to check that easy runs are easy, and shows recovery trends so you can spot when to back off.
Max heart rate
Your maximum heart rate sets the scale for heart-rate training. Trainer takes the middle of the three highest heart rates recorded in your runs over the last year. Using the middle one means a single sensor glitch can't set it.
If your runs haven't reached a higher value, Trainer estimates it from your age instead, using Tanaka's formula: 208 − 0.7 × age. Age formulas are only averages, and an individual's real maximum is often 10 or more beats away from them, so a value from your own runs is better when you have one.
Easy heart rate ceiling
Easy running should stay under about 79% of your max heart rate, the top of Daniels' Easy range. The dashboard shows this ceiling, and your plan's easy runs refer to it. It's normal for this to feel surprisingly slow at first. Many runners find they have to slow down a lot, and sometimes walk briefly on hills, to stay under it.
Heart rate drifts up in heat, with dehydration, and late in long runs, so use it alongside feel ("could I hold a conversation?").
Resting heart rate
Your resting heart rate tends to fall as your aerobic fitness improves. A rise of a few beats above your usual for several days can be a sign of fatigue, illness, poor sleep, stress, heat or alcohol. The dashboard compares your last 7 days with your last 28. A noticeably higher 7-day number is a reason to take an extra easy day.
Heart rate variability
Heart rate variability (HRV) is how much the time between heartbeats varies. Apple Watch measures it as SDNN, in milliseconds, at moments through the day and night. Higher than your usual generally means you're well recovered; a sustained drop can mean accumulated fatigue or stress.
HRV varies a lot from one reading to the next and between people, so compare your own trend over days, never single readings or other people's numbers. In research, adjusting training to daily HRV has worked about as well as fixed plans, or better (Kiviniemi et al., 2007), and HRV trends have been useful for monitoring elite endurance athletes (Plews et al., 2013).
Sleep
Sleep is when most adaptation to training happens. In one study, college basketball players who extended their sleep to about 10 hours a night for several weeks got faster and more accurate (Mah et al., 2011). Most adults need 7–9 hours. The dashboard shows your average over the last 7 nights. If it's falling short during a hard block of training, prioritising sleep is one of the most effective things you can do.
Sources
- Tanaka H, Monahan KD, Seals DR. Age-predicted maximal heart rate revisited. J Am Coll Cardiol. 2001;37(1):153–156.
- Daniels J. Daniels' Running Formula. 3rd ed. Human Kinetics; 2014.
- Kiviniemi AM, et al. Endurance training guided individually by daily heart rate variability measurements. Eur J Appl Physiol. 2007;101(6):743–751.
- Plews DJ, et al. Training adaptation and heart rate variability in elite endurance athletes: opening the door to effective monitoring. Sports Med. 2013;43(9):773–781.
- Mah CD, et al. The effects of sleep extension on the athletic performance of collegiate basketball players. Sleep. 2011;34(7):943–950.