Logging Heart Rate (Avg / Max)

Heart rate is an internal-load metric: it reflects the physiological cost of the work, not the external output. The two session summaries to log are:

• Average HR — overall intensity of the session.
• Maximum HR reached — the peak, useful for interval and threshold sessions.

HR's strength is that it accounts for terrain, heat, fatigue, and fitness — a hilly run and a flat run can match in HR even if pace differs. This is why pace/power paired with HR is more informative than either alone (see Pace, Speed, and Distance, Cardio Efficiency (Pace/Power at HR)).

To turn HR into zones you need an anchor — measured or estimated HRmax (see Estimating Maximal Heart Rate) or heart-rate reserve via the Karvonen method using resting HR (see Heart-Rate Reserve (Karvonen Method), Resting Heart Rate (RHR)). Zone minutes are then derived (see Time-in-Zone).

Caveats that the data layer must respect:

• Cardiac drift — HR rises over a long session at constant effort (heat, dehydration), so average HR overstates intensity late in long efforts.
• HR lag — HR responds slowly, so it is a poor intensity guide for short, hard intervals (use power or pace there).
• Device accuracy — optical wrist HR errs more during exercise, especially high-intensity and wrist-flexing activity (see HR Monitor Accuracy (Chest vs Optical)). Caffeine, stress, sleep, and heat also shift HR independent of effort.

• HR is internal load: physiological cost, not external output.
• Log average HR (session intensity) and max HR (peak).
• Needs an HRmax or HRR anchor to compute zones.
• Cardiac drift inflates late-session average HR at constant effort.
• HR lags fast intervals; optical wrist HR errs most during exercise.