What factors can affect maximum heart rate?

Your genes account for about 50% of your maximum heart rate (max HR). Smaller hearts beat faster than larger ones. And there are other factors. Your max HR is affected by altitude, drugs, and antihistamines. It cannot be increased by training, and a high max HR does not predict better performance. If you go to a gym and see the max HR charts, be cautious -- they aren't very accurate. Maximum heart rate is genetically determined; it doesn't necessarily decrease over time like those charts suggest. In fact, if you stay fit as you get older, your max HR may not change much at all.