Subtle physical activity poses a challenge to the study of heart rate

In nonhuman primates, our understanding of the effect of physical activity upon heart rate, particularly activity involving only slight, nonlocomotor movements, is poor. However, before inferring that other factors affect an animal's heart rate, the effect of physical activity upon heart rate must be considered. This study describes this effect in five adult female and seven infant rhesus macaques (Macaca mulatta). Interbeat interval (IBI), the reciprocal function of heart rate, was monitored using radiotelemetry during six categories of physical activity. IBI varied between different activities. The largest observed mean decrease in IBI was by 45% of that when sitting still, during locomotion by adult females. However, even subtle physical activity, involving no movement from a sitting position, produced a large, significant reduction in IBI. The effect of activity upon IBI was different between adults and infants. The findings indicate that measures of physical activity that consider only locomotion are insufficiently sensitive to describe the relationship between physical activity and heart rate.