Originally Posted by Trek Rider
A max heart rate doesn't decrease as you get more physically fit. As you excercise and increase your level of fitness, your cardio-vascular system also becomes more efficient. What this means is that it takes your heart less work to get oxygen to where it's needed in your body, which means you have a lower heart rate for the same level of effort.
After reading Ric Stearn's comment I did some searching specifically for any references to decreasing HRmax with fitness. I did find several claims to that effect but only one claim with any specifics and that claimed up to 6-10 bpm reduction with increased fitness. I also see as many claims stating that HRmax will decrease at a slower rate with age for higly fit individuals. However, it seems certain to me that the HRmax is primarily determined by genetics and age.