Heart Rate Training Zones (5 Karvonen Zones)

Karvonen (HRR) is more individualized because it factors in your resting heart rate, which reflects your aerobic fitness. The %MHR straight method gives every athlete with the same MHR the same bpm target at a given intensity, which underestimates the working range of a well-trained athlete with low HRrest. For everyday training prescription, Karvonen is the default in ACSM guidelines and most coaching platforms. %MHR is fine as a rough first pass when you don't have a reliable HRrest measurement.

Yes — Zone 2 (60-70% HRR) should let you speak full sentences without gasping. If you're snatching breaths between every 3-4 words you've drifted into Zone 3. The talk test is one of the most robust low-tech validations of Z2 effort and matches lactate values around 1.5-2.0 mmol/L in most athletes. A common mistake is treating Z2 as 'sort of easy' when it should feel actively boring.

The standard protocol is a 30-minute all-out time trial (treadmill, track, or flat road). Take the average heart rate from minutes 10-30 — that approximates your lactate threshold heart rate (LTHR). For runners, the 30-min TT pace is also a useful threshold pace anchor. Joe Friel popularized this method and it correlates reasonably well with lab LT for most amateur athletes. Repeat every 6-8 weeks during build phases.

The 80/20 ratio refers to time, not sessions. Most weeks for serious endurance athletes shake out to roughly 4-5 easy sessions and 1-2 hard interval sessions, with the easy work massively outweighing the hard work in total minutes. Seiler's research found elites cluster around this distribution naturally. Recreational athletes tend to spend too much time at moderately hard Zone 3 — the gray zone — and not enough at truly easy Z2 or truly hard Z4-Z5.

Wrist optical sensors are acceptable for Zone 1-2 work — typically within ±3-5 bpm of a chest strap at low intensity. They degrade noticeably in Zone 4-5 intervals where wrist motion, cooler skin, and signal lag introduce ±5-10 bpm error and several seconds of delay. For VO2max intervals and threshold sessions, pair a Polar H10 or Garmin HRM-Pro chest strap. For easy aerobic and recovery days, wrist optical is fine.

Once or twice per week, never back-to-back days. VO2max intervals (3-8 minute efforts at 90-100% HRR) carry high neuromuscular and central nervous system load. Most coaches program one Z5 session weekly during build phases, sometimes paired with a Z4 threshold session on a separate day. Recovery between hard sessions is 48-72 hours for most athletes. More is not better.

The Maffetone aerobic threshold heuristic broadly aligns with the top of Karvonen Zone 2 for most athletes. For a 35-year-old that's ~145 bpm, comparable to ~70% HRR. MAF is popular in ultrarunning and base-phase triathlon because it's simple and strict — you never exceed the ceiling. It tends to be a bit conservative for already-fit athletes and a bit liberal for true beginners. Use it as a sanity check against your Karvonen Z2 cap rather than a replacement.

Cardiovascular drift — typically 5-10% upward HR creep over 60+ minutes at fixed pace — is normal and reflects rising core temperature, plasma volume loss, and accumulated fatigue. In Zone 2 training, let HR be the master and back off pace to stay under the cap. Drift greater than ~10% in a single session suggests dehydration, heat stress, or under-recovery. Monitoring drift is itself a useful aerobic fitness marker.

Not very. The legacy Fox formula (220 − age) has a standard deviation of ±10-12 bpm across individuals — meaning two-thirds of people fall within 10 bpm of the estimate and a third are further off. Tanaka's 208 − (0.7 × age) is a meaningful improvement, especially for masters athletes. The only way to know your true MHR is a supervised max test or carefully observed end of a hard 5K race / hill repeat session. If you have a measured MHR, override the calculator's estimate.