HRR

HRR is an initialism that can refer to several concepts in different disciplines. Among the most widely encountered in professional writing are heat release rate, used in fire safety engineering, and heart rate recovery, a measure used in exercise physiology and clinical assessment of autonomic function.

Heat release rate (HRR) is the rate at which heat is liberated by a burning material or fire, typically expressed in kilowatts (kW) or megawatts (MW). It is a key parameter in fire dynamics, influencing fire growth, flame spread, smoke production, and the effectiveness of suppression and ventilation strategies. HRR curves describe how heat output changes over time, and maximum HRR is used to compare materials and to size protection systems such as sprinklers or certification tests.

Heart rate recovery (HRR) refers to the rate at which heart rate declines after peak exercise or stress. It is usually assessed by measuring the decrease in beats per minute within a specified interval, commonly one or two minutes post-exercise. HRR is used as an indicator of cardiovascular fitness and autonomic nervous system function; slower recovery has been associated with higher risk of adverse cardiovascular outcomes in some populations.

Other, less common uses of HRR exist in specialized domains and depend on context. Because HRR is not tied to a single universally accepted definition, the intended meaning should be inferred from the subject area and accompanying explanation.