I just read this and it seems pretty accurate.
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Maximum HR
Many believe that you can calculate your maximum HR by using the formula of 220 minus your age. For some people this may be accurate, but for many it will be wildly out. I’m 54 years old so, using the formula, my max HR should be 166 (220-54). It’s actually 178, which is a big difference when training in very tight zones.
A much more accurate formula is 210 minus half your age, then subtract 5% of your body weight in pounds. Add four for a male and 0 for a female. The only way to get a truly accurate max HR ï¬gure is to get a physiological test at a sport science centre, such as Fletcher Sport Science, but you can get a reasonable estimate by doing your own max HR test. Only undertake this test if you are ï¬t and exercise regularly, though.
Warm up thoroughly for at least 15 minutes. On a long, steady hill start off fairly briskly and increase your effort every minute. Do this seated for at least ï¬ve minutes until you can’t go any faster. At this point get out of the saddle and sprint as hard as you can for 15 seconds. Stop and get off the bike and immediately check your HR reading. This is your max HR.
“Don’t forget that your max HR ï¬gure is sport speciï¬c,†says Fletcher. “This means that your maximum on a bike will invariably be much lower than it is when you're running because the bike is taking some of your weight.â€
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HR zones
Having established the key numbers (max HR and resting HR) you're now ready to work out your training zones. There are
lots of calculators on the web and, while many people use ï¬ve training zones, I prefer the six-zone system prescribed by the Association of British Cycling Coaches. Fletcher is also a big fan of the six zones, although he points out that there is actually a recovery zone as well which is important. “If athletes are to perform well they need to recover well,†he says. “I monitor every session my athletes do and I can tell very easily when they need to recover and how long that recovery needs to be.â€
Zone 1 (60-65% of maximum heart rate): For long, easy rides, to improve the combustion and storage of fats.
Zone 2 (65-75% of MHR): The basic base training zone. Longish rides of medium stress.
Zone 3 (75-82% of MHR): For development of aerobic capacity and endurance with moderate volume at very controlled intensity.
Zone 4 (82-89% of MHR): For simulating pace when tapering for a race.
Zone 5 (89-94% of MHR): For raising anaerobic threshold. Good sessions for 10- and 25-mile time-trials.
Zone 6 (94-100% of MHR): For high-intensity interval training to increase maximum power and speed
http://www.bikeradar.com/fitness/article/heart-rate-monitor-training-for-cyclists-28838/