Making Sense of Your Health Tracker in July

Slow down and look at the others around you the next time you visit the fitness centre. You are likely to see intelligent watches that track sleep, estimate recovery, monitor stress, and then tell us to either push it or slow it down. Devices from Garmin, Apple, Fitbit, Samsung and Aura have become incredibly sophisticated tools that provide more information about our health than ever before, and the technology behind these devices is genuinely impressive.

Physiological Measurements

The premise behind most health trackers is their continuous use of optical sensors to measure your heart rate. They do so by illuminating the skin and analyzing changes in blood flow and surface temperature. They combine this information with movement data from accelerometers and GPS, which is plugged into advanced algorithms to estimate a surprisingly wide range of physiological metrics.

Modern devices can track or estimate:

• Optical Heart Rate (Elevate Sensor): Shines LEDs onto the skin to measure blood flow via photoplethysmography (PPG)

• Heart Rate Variability (HRV): The variation in time between heartbeats. Higher HRV is generally associated with better recovery and adaptability, while lower HRV can indicate fatigue, illness, or stress.

• Stress Levels: Calculated primarily from heart rate and HRV patterns. Most devices reflect physiological stress on the body rather than emotional stress alone.

• Respiration Rate: An estimate of how many breaths you take per minute during rest, sleep, and exercise.

• Sleep Tracking and Sleep Scores: Estimates sleep duration and stages to provide insight into recovery and overall health.

• Battery:  An estimate of your energy reserves, combining sleep, HRV, stress, and activity data to estimate how prepared your body is for exercise or daily life.

• VO₂ Max: An estimate of your maximum aerobic capacity and one of the most widely used measures of cardiovascular fitness. Devices calculate this by analyzing the relationship between your performance and physiological effort.

• Training Load: Measures the cumulative stress from your workouts using heart rate and excess post-exercise oxygen consumption (EPOC) to determine how hard your body is working over a period.

• Training Load Focus: Breaks your training into aerobic, high aerobic, and anaerobic categories so you can see if your workouts are balanced.

• Training Status: Combines fitness trends, recovery, and recent workouts to tell you whether your training is productive, maintaining fitness, overreaching, or detraining.

• Recovery Time:  Estimates how long your body needs before it is ready for another hard workout.

• Calories Burned: An estimate based on heart rate, movement, age, sex, and other factors.

For the most part, these devices are very good at identifying trends. If your resting heart rate is gradually decreasing over several months, that is usually a sign that your cardiovascular fitness is improving. If your HRV is steadily increasing and your sleep scores are improving, your body is probably adapting well to your training. If your training load has been rising while your training status remains productive, your fitness is likely improving.

The important thing to remember is that most of these numbers are estimates, not direct measurements. Your watch is not measuring VO₂ max in the same way a laboratory would, nor is it directly measuring calorie expenditure, stress, or recovery. High level athletes achieve maximum accuracy from their smart device by pairing it with chest straps and arm bands, which pick up the electrical signal produced by each heartbeat. While the standard optical values are not perfectly precise, they are great at tracking general changes and trends over longer periods. The mistake people often make is treating every number as an absolute truth. Your watch may tell you that your VO₂ max dropped by one point overnight or that you had a "poor" night's sleep despite feeling fantastic the next morning. These metrics are best used as guides rather than verdicts as they provide context, not certainty. The real value comes from looking at weeks and months of data and not obsessing over a single day's score. It is also important to remember that your watch doesn't know everything. It cannot measure motivation, enjoyment, or how accomplished you feel after a workout. It cannot tell you that a walk outside helped clear your mind or that hanging out with your friends was exactly what you needed that day. Trust the data, but trust yourself too.