The traditional strategy is to use the formula of 220 minus age to 'guestimate' your max. This is often used in health clubs. Here, charts show age-related MHR and training heart rates for cardiovascular fitness development. For example, if you are 40, your estimated MHR would be 180 (ie 220-40). You can then calculate training heart rates from this, using a formula such as 70 per cent MHR (which would be 126).
It's quite simple, but unfortunately it's not accurate for everyone. American sports scientists have modified the basic formula to allow for gender: 214-(0.8 x age) for men, and 209-(0.9 x age) for women. However, this still gives a generalised result.
If you want to find your true MHR, you'll have to do a little work and some measurement with your heart rate monitor. But it's not as straightforward as taking a peak reading from a race or a hard training session, no matter how exhausted you might make yourself. When it comes to your heart, it's how you work up to your max that counts.
Sports science laboratories often use a graded treadmill run to establish MHR. The speed of the track is gradually increased until you can no longer keep up, and your heart rate at this point is assumed to be your MHR.
However, findings from Oslo have suggested that a combination of short runs will give you higher readings still, and this would seem to be your best option. Run as fast as you can evenly for three minutes (ideally on a treadmill), rest with two or three minutes gentle running, and then repeat your three-minute maximal run. During the second run you should get a higher MHR value than with any other method, though use your monitor to take readings throughout it, as your heart rate may peak before the end (see below). Shorter, faster bursts don't appear to work, as the leg muscles then become exhausted before the cardio-respiratory system.
Other factors contribute to MHR values (see below) and should be taken into account before you set off on your rush to exhaustion. Needless to say, you should be in good physical health before you do any intensive exercise, let alone running to your body's upper limits. If you are in any doubt at all, always get a medical check-up.