Generally, you should aim for raising your heart rate during exercise to about 70-85% of its maximum capacity. Subtract your age by 220 and multiply the result by .70%, then also multiply the result by .85%. This is your target heart rate zone, which will give you the greatest benefits available while not straining your heart or depleting the life energy necessary for maintaining your system. For example, if you are 40 years old, your maximum rate is 220 minus 40, which equals 180. Your target heart rate zone, then, is 70-85% of 180, or 126-150.

Age                  Beats Per Minute
20                    140-170
25                    137-166
30                    133-166
35                    130-157
40                    126-153
45                    126-153
50                    119-145
55                    116-140
60                    112-136
65                    109-132
70                    105-128

You should aim for raising your heart to its target rate and keeping it there for 15 minutes at a time, for a minimum of about 2 hours a week. In other words your looking for four half-hour sessions of vigorous exercise per week.

However, if you haven’t exercised in a while, plan to working up this ideal rate gradually, over a period of three or four months. To determine your period of vigor, try talking while exercising. If you’re so out of breath that you can’t talk, you’re working too hard. Slow down. And don’t start with half-hour workout sessions—try ten or even five minutes at first—until your stamina increases.

If possible, though, try to exercise at least four times a week, even if for only five minutes at a time. Repeated attempts of exercise will build up your muscles as well as strengthen your heart and lungs. The more often you exercise, the more easily exercise will come to you.