{
  "calculator": "zone-2-heart-rate-calculator",
  "title": "Zone 2 Heart Rate Calculator",
  "site": "AI Fit Hub",
  "input": {},
  "defaults_used": {
    "age": 30,
    "resting_hr": 0,
    "max_hr": 0,
    "max_hr_formula": "fox",
    "maffetone_adjustment": 0,
    "methods": [
      "maffetone",
      "percentage",
      "karvonen",
      "lactate"
    ]
  },
  "result": {
    "methods": [
      {
        "key": "maffetone",
        "label": "Maffetone (MAF 180)",
        "zone2Low": 140,
        "zone2High": 150,
        "maxHr": 190,
        "maxHrLabel": "MAF heart rate: 150 bpm (180 − 30)",
        "zones": [
          {
            "zone": "Zone 2 (MAF aerobic)",
            "lowBpm": 140,
            "highBpm": 150
          }
        ],
        "recommended": true,
        "requiresRhr": false,
        "note": "Phil Maffetone's 180 − age ceiling for aerobic training, adjusted for health and training history. No equipment needed; aim to stay within 10 beats below it."
      },
      {
        "key": "percentage",
        "label": "% of Max HR (estimated)",
        "zone2Low": 114,
        "zone2High": 133,
        "maxHr": 190,
        "maxHrLabel": "Max HR 190 bpm (220 − age)",
        "zones": [
          {
            "zone": "Zone 1 (Recovery)",
            "lowBpm": 95,
            "highBpm": 114
          },
          {
            "zone": "Zone 2 (Aerobic Base)",
            "lowBpm": 114,
            "highBpm": 133
          },
          {
            "zone": "Zone 3 (Tempo)",
            "lowBpm": 133,
            "highBpm": 152
          },
          {
            "zone": "Zone 4 (Threshold)",
            "lowBpm": 152,
            "highBpm": 171
          },
          {
            "zone": "Zone 5 (VO2 Max)",
            "lowBpm": 171,
            "highBpm": 190
          }
        ],
        "recommended": false,
        "requiresRhr": false,
        "note": "Zone 2 = 60-70% of max HR. Max HR estimated as 220 − age = 190 bpm (220 − age gives 190, Tanaka 208 − 0.7 × age gives 187)."
      },
      {
        "key": "karvonen",
        "label": "Karvonen (Heart Rate Reserve)",
        "zone2Low": 0,
        "zone2High": 0,
        "maxHr": 190,
        "maxHrLabel": "",
        "zones": [],
        "recommended": false,
        "requiresRhr": true,
        "note": "Requires resting heart rate. Enter your RHR above to enable this method."
      },
      {
        "key": "lactate",
        "label": "Lactate Threshold Estimate",
        "zone2Low": 125,
        "zone2High": 141,
        "maxHr": 190,
        "maxHrLabel": "Est. LTHR: 166 bpm (87.5% of max 190, 220 − age)",
        "zones": [
          {
            "zone": "Zone 1 (Recovery)",
            "lowBpm": 108,
            "highBpm": 125
          },
          {
            "zone": "Zone 2 (Aerobic Base)",
            "lowBpm": 125,
            "highBpm": 141
          },
          {
            "zone": "Zone 3 (Tempo)",
            "lowBpm": 141,
            "highBpm": 158
          },
          {
            "zone": "Zone 4 (Threshold)",
            "lowBpm": 158,
            "highBpm": 166
          },
          {
            "zone": "Zone 5 (VO2 Max)",
            "lowBpm": 167,
            "highBpm": 190
          }
        ],
        "recommended": false,
        "requiresRhr": false,
        "note": "Lactate threshold HR estimated at 87.5% of max HR (166 bpm); Zone 2 taken as 75-85% of it. Published LTHR systems put zone 2 lower or higher (Coggan 69-83%, Friel cycling 81-89%), so treat this as an approximation; a lab or field LTHR test is far better than 87.5% of an estimated max."
      }
    ],
    "estimatedMaxHrTraditional": 190,
    "estimatedMaxHrTanaka": 187,
    "maxHrUsed": 190,
    "maxHrSource": "220 − age",
    "mafHr": 150,
    "hasRhr": false,
    "hasCustomMaxHr": false
  },
  "provenance": {
    "run": "https://aifithub.io/api/run/zone-2-heart-rate-calculator",
    "page": "https://aifithub.io/zone-2-heart-rate-calculator/",
    "methodology": "https://aifithub.io/methodology/zone-2-heart-rate-calculator/",
    "contract": "https://aifithub.io/contracts/zone-2-heart-rate-calculator.json",
    "engine": "https://aifithub.io/engines/zone-2-heart-rate-calculator.js",
    "engine_sha256": "9bc28400df86beb389d56b28e42ded8a38ea18d49c7a6551d466f643abab511f",
    "build": "9e8a56a16bda662ba90601b49561a2b176e9c379"
  },
  "disclaimer": "General fitness estimates — not medical advice.",
  "license": "https://creativecommons.org/licenses/by/4.0/",
  "cite": "AI Fit Hub, Zone 2 Heart Rate Calculator (engine 9bc28400df86), https://aifithub.io/api/run/zone-2-heart-rate-calculator"
}