We congratulate Zulfiqar et al for their valuable study demonstrating the relation between high heart rate (HR) variability and healthy longevity. In their report, they discussed HR variability due to autonomic control and nothing else. HR variability obviously is caused by autonomic control, as demonstrated by autonomic blockade (see, e.g., Figure 1 in Tan et al ). Fluctuation of a variable is caused by control mechanisms in steady-state conditions. Control mechanisms, including baroreflex, detect any blood pressure drift and change HR, with a delay to compensate for the drift. Indeed, not any regulatory mechanism can instantaneously compensate for drift of the regulated variable. The delay of autonomic control results in fluctuations of blood pressure and of HR chasing after it. In return, fluctuations inform about autonomic control.

Holter monitors include HR fluctuations caused by the delay of autonomic control. However, they also include HR adaptations of a permanently disturbed cardiovascular system coping with daily life activities. Indeed, energy expenditure could be assessed by means of HR monitoring.

Aoyagi et al demonstrated that body movements participate in HR variability. They logically participate at long time scales, while the fast regulation by autonomic control influences HR at a short time scale. However, we demonstrated that body movements, even slight ones such as fidgeting, influence HR variability, including at the time scale of autonomic control. The study by Zulfiqar et al raises an important question: what are we really measuring when we assess HR variability by means of Holter monitors, autonomic control or cardiovascular adaptation to physical activity, or both?

Zulfiqar et al showed an 8-decade reversal of the decrease in Holter HR variability with aging. Because high daily physical activity comes with better longevity, we suggest that this reversal is caused by the selection of a healthy active population by aging. In contrast, younger populations include active and inactive subjects. Interestingly, the results of Zulfiqar et al suggest that comparison between Holter and short–time scale motionless HR variability might provide a physical activity index.