First of all, just know that your watch’s estimate of HR zones and VO2 max is probably not super accurate. It might be, but knowing for sure requires fancy machines and testing, and your watch is doing a lot of guessing. So don’t stress about it by any means.

VO2 max, max heart rate, and lactate threshold are different types of upper limits of physical performance, and when you talk about one, you usually also talk about the others. Knowing these metrics gives athletes, coaches, and physiologists tools to approximate the most effective training zone for an athlete.

The prerequisite knowledge to the rest of this explanation is that the body has two ways of producing energy: in the presence of oxygen (aerobic) and without oxygen (anaerobic).

The short version is that VO2 max refers to the maximum amount of oxygen that body can be using to produce energy at one time. Lactate threshold refers to a particular spike in blood lactate at high levels of aerobic exertion. Maximum heart rate is exactly what is sounds like, though endurance athletes spend very little time training at or near max heart rate, where energy production is short term and anaerobic. When your watch sets heart rate zones it will rely on its ability to guess these metrics.

The training effect on the body is different depending on what percentage of your VO2 max and lactate threshold your training heart rate falls at. LT and VO2 are a pain to measure, whereas current HR is easy to measure, and monitors are getting more accurate. So when your watch guesses at things like VO2 max, LT, and max heart rate, it can feed you back training zones with specific heart rate ranges as a guideline for you to use when training.

It doesn’t matter what your watch tells you your VO2 max is. There’s a genetically determined upper limit for VO2 max anyway, and any conditioned athlete is operating pretty close to their VO2 max as it is. The real training benefits come from increasing your LT, with the ideal scenario being that HR @ LT = HR @ VO2 max. Training zones are an important training tool for doing this, but the question is whether your watch’s training zones are accurate.

An athlete who mostly trains at comfortable, conversational pace and occasionally throws in some intervals or races can probably ignore the watch’s zones in favor of focusing on perceived exertion, or use the zones to ensure your easy runs are actually easy enough. An athlete trying to improve performance at race-level effort will need to do specific intervals at certain percentage of VO2max and LT to improve their LT. At that point, the watch’s guess is probably not close enough. There are other tools coaches and athletes use at that point to get more accurate training zones if needed, including ways to hone in on training by perceived exertion (training by feel).

So ignore your watch’s VO2 estimate entirely, optionally use training zones to ensure you’re actually going EASY some of the time, and if you want to check on the accuracy of the watch’s training zone estimates, ask your coach.

I will leave the technical responses to these questions to the other coaches, but I am happy to chime in on the utility of fitness devices in general. The two basic functions of fitness devices are to count your steps and measure your heart rate throughout the day and night. Given these data, fitness device developers create algorithms that chart your fitness over time in various categories. Simple algorithms count your steps throughout the day and the device can offer a challenge for you to meet, such as, walk and run 5,000 steps a day. On top of that your device can notify you during the day to let you know if you are on pace to reach 5,000 steps or, if not, send you an encouraging message – “Time to move!”

When you add heart rate sensors to the mix, algorithms become much more complicated. The companies that develop fitness devices want to make them fun to use so you will buy one. One way to do this is to create unique algorithms that use multiple calculations, averaging, ratios, etc. (I do not claim to understand these algorithms) to come up with measurements like “Suffer Score”, available on Strava and developed through a partnership with Garmin. Garmin’s software, Trainingpeaks, goes one step further and offers integrations with which you can easily build structured workouts you can upload to your device in minutes.

I sheepishly admit that I have spent a lot of money on fitness devices and for years wore two air a time, one on each wrist. For me it has become one runner’s search for the Holy Grail, the device that does everything, including a battery that will hold up for a 24-hour workout. I do not begrudge Garmin and FitBit for marketing their lines of devices that they seem to upgrade every week! My advice is to have fun with your devices and if “Suffer Score” helps to motivate you – Great!

Like many devices, my Garmin Instinct monitors my heart rate throughout the day and night and calculates my resting heart rate. If my resting heart rate goes up 5+/- beats per minute from the day before, I take that as an indication that my body was stressed and is still in recovery mode. So, I will go a little easy for 24 hours or so and see if it comes back down. Another good use of fitness devices is to keep track of your steps per day over time. I find that it takes me about 2,200 walking steps per mile and about 1,800 running steps per mile, so on an average day 2,000 steps per mile is about right. If I average 10,000 steps a day, I am walking and running about 5 miles a day, 35 miles a week. As I prepare for a race, particularly a longer race like a marathon, I want to see my steps per day approach an average of about 20,000 per day or 70 miles a week during a training block where I’m peaking out my weekly mileage. If there is one measurement you can trust it’s steps per day. Want to both save money, track your fitness and get ready for your next important race? Use a pedometer or simple FitBit-type device to measure your daily steps and measure your heart rate by hand each morning.

Probably I will continue to buy new fitness devices, the search for the Holy Grail goes on. But, I will do so with my eyes wide open and not get carried away with setting a new PR for my “Suffer Score”.