I commute by bicycle every day, and sometimes I use my commute for training and to improve my fitness (I wear cycling clothes and shower when I arrive, so I can push hard if needed). About a month ago I obtained a heart rate monitor to aid my training. Based on my physical characteristics, male age 31 5'9" 200 lbs, I should have a max heart rate of around 185. However, in the last month my heart rate rarely exceeded 160 BPM, even on some tough climbs, and I've only ever exceeded 170 BPM twice, and only for a few seconds, once reaching 171 and once 174. This is despite numerous "all-out" efforts over the course of the commute. What number should I use for my max heart rate for the purposes of interval training and endurance training?
You can use max heart rate to determine training zones assuming you can get an accurate max heart rate. There are several calculations you can use to guess a max heart rate zone. However, max heart rates can very drastically and in many cases is NOT an indicator of overall health. Some people have really high max heart rates because they have a very high resting heart rate. Some people are the exact opposite. And others have a smaller range between max heart rate and resting heart rate. A good field test can determine your max heart rate better than any calculation. The problem with a field test for a max heart rate is that it is difficult to perform on a bike, outside and do so safely. So, it is commonly done on a bike trainer which may or may not be as accurate as riding outside. Also, if done correctly, your body will be in such a state of stress and fatigue that you will need several days to recover. May I offer an alternative?
Instead of focusing on a max heart rate, let's use your aerobic threshold and build your zones based off that number. Your aerobic threshold is the effort and/or heart rate where you body changes from a mostly aerobic state to a mostly anaerobic state. A common set of training zones, often used in endurance sports, like triathlon is Z1-Z5 where Z1 is easier than Z2 is easier than Z3, etc. Z3 is usually right around your aerobic threshold and Z5 is just about full on effort that can only be handled for very short periods of time. Many people find benefit from training in a Z2 zone (aerobic) for long periods of time and then throwing in some short Z4 and Z5 intervals to build anaerobic performance. But, many more people that do not do zone training suffer from a lot of Z3 training - they are going too hard to build aerobic endurance but aren't working hard enough to build anaerobic performance, speed and power.
I should also note that the Z1-Z5 zones is only one example of zone training. I've also seen and used other programs that have many more zones and are named A1, A2, A3, AT1, AT2, etc. There are several resources available to determine your zones once you know your anaerobic threshold and/or lactic threshold, which are similar, but may not be the same heart rate.
Phil Maffetone has a fairly simple way of figuring out similar to a Z2 heart rate zone for you without any field tests. Some will argue or agree that this is where you should spend the majority of your training.
Joe Friel also describes setting your zones (Z1-Z5) based on your lactic threshold.
And one last point. An example field test to find your aerobic threshold or lactic threshold is to do a time trial. For instance warm up slowly over 15 minutes to a comfortable but low race pace effort. At 15 minutes in the TT will begin. You want to go as hard as you can at an effort (not speed) that you can maintain for 30 minutes. 10 minutes into the TT, hit the lap button on your HRM. And after the full 30 minute TT, hit the lap button again. Cool down appropriately. Your average HR for the 20 minute second part of the TT can be used as the base for the Zones. If you go out too hard and cannot maintain a stead effort for the full 30 minutes, you will probably want to try the field test again in a week or two.
If you want to get the most out of your workouts, then training in your target heart rate zone is one step toward doing that. Hence the word zone, its actually a range of bpm that you want to stay in during your workout. From what I've seen, %50-80 of your max is sufficient, I don't think anyone can train very close to their max for very long.
They also don't say much about if a certain exercise will put you there. If you've been biking long, then biking the same route every day may not put you in your t.h.z. like, say, sprinting for example. It's all about adaptation there.
The calculators are also an estimate, don't be afraid to dial it up or down as needed. If the monitor says your operating at 160 but you're barely breaking a sweat, try wearing a weight vest for added resistance, for example. After listening to your body in addition to where the formula puts you, you'll know your true target heart rate zone.
This is one THR calculator.