In humans, the physiological resting heart rate is about 70 beats per minute (bpm), although normal resting heart rate depends strongly on age and changes throughout the human life cycle. Resting heart rate is significantly higher in newborns at about 140 bpm and it declines with increasing age until it stabilizes at about 70 bpm. Fetal heart rate is similar to that of a newborn. Other factors being equal, females have slightly higher heart rates than do males. The maximum heart rate (HR_max), the highest heart rate that can be achieved with intense exercise activity, appears to depend strongly on one's age. In the absence of direct measurement, several approximation calculations have been derived based on experimental data. However, a simple rule of thumb method to approximate the maximum heart rate is to subtract one's age in years from 220. Thus, HR_max ≈ 220 − Age in years. 220 is an empirically derived number based on extensive experimental data, and has been shown to serve as an adequate link between age and the maximum heart rate. Physical fitness increases the maximum heart rate only slightly. However, physical fitness has a profound effect on the resting heart rate. World class endurance athletes have resting heart rates that are in the mid thirties.