Modern running watches collect an extraordinary amount of data. Heart rate, heart rate variability (HRV), sleep metrics, recovery time, estimated VO₂max, training status and race predictions are all available at the flick of a wrist. The problem isn’t the data itself. It’s the assumption that those numbers tell the whole story.
Imagine waking up feeling refreshed, well-rested and ready to train. Then you check your watch. It reports a low HRV, suggests you’re not fully recovered and recommends an easy session instead. Do you trust the watch, or your own body? And if you go ahead with a hard workout anyway, have you really compromised your progress?
When it comes to distance and pace, modern running watches perform remarkably well. GPS technology has improved dramatically over the past decade, allowing most devices to deliver highly accurate measurements in everyday training. That said, no GPS is perfect. Tall buildings, tunnels and dense tree cover can all interfere with satellite signals and reduce accuracy.
Many runners finish a marathon only to find that their watch has recorded more than 42.195 kilometres. This often leads them to question the official course distance. In reality, certified race courses are exceptionally accurate. They are measured along the shortest possible route, 30 centimetres from the kerb, with an additional 0.1% safety margin to ensure the course can never be too short.
Runners, however, rarely follow that exact line. Overtaking other competitors, running wide through corners and weaving around aid stations all add extra distance. Small GPS errors and the way elevation changes are recorded can also contribute to discrepancies between your watch and the official course measurement.
Heart rate is one of the most widely used training metrics, but not all sensors measure it in the same way. Chest straps and upper-arm monitors remain the gold standard because they detect the heart’s electrical activity directly. Using electrodes, they identify the QRS complex on an electrocardiogram (ECG), allowing heart rate to be calculated from each heartbeat as it occurs.
Running watches take a different approach when measuring heart rate in the wrist. Instead of measuring electrical signals, they use photoplethysmography (PPG), an optical technique that estimates heart rate by shining LED light into the skin and detecting changes in blood volume beneath the surface.
Optical sensors have improved substantially in recent years, yet they still cannot consistently match the accuracy of ECG-based devices. Sweat, skin temperature, sensor placement, strap tightness and arm movement can all affect the quality of the signal. During high-intensity exercise, wrist-based devices are particularly prone to overestimating heart rate (Chow & Yang, 2020).
Most running watches estimate sleep timing and total sleep duration reasonably well. Determining exactly when you fell asleep and when you woke up is relatively straightforward. Distinguishing between light sleep, deep sleep and REM sleep is considerably more difficult. These sleep-stage estimates are based on movement patterns and optical heart rate measurements rather than polysomnography, the laboratory gold standard for sleep assessment. As a result, they should be viewed as informed estimates rather than precise measurements.
Heart rate variability (HRV) has become one of the most popular recovery metrics among endurance athletes. It reflects the activity of the autonomic nervous system and can provide valuable insight into how the body is responding to training. However, HRV is influenced by far more than exercise alone. Age, genetics, biological sex, physical fitness, illness, medication, alcohol consumption, smoking, body composition and psychological stress can all influence HRV values. Even seemingly minor factors, such as poor sleep, dehydration or emotional stress, may alter a single night’s reading.
Most running watches measure HRV only during sleep, using the same optical sensors that estimate heart rate. Because PPG measurements are sensitive to movement and other external influences, a single HRV value tells you very little in isolation. The real value lies in monitoring long-term trends and comparing each night’s result with your own established baseline rather than with population averages (Li et al., 2023).
Imagine you’ve been training consistently, recovering well and eating appropriately, yet your Garmin reports that your training status is Unproductive. Training status is primarily derived from three variables: estimated VO₂max, acute training load and HRV. While each of these metrics provides useful information, they also have important limitations.
A running watch does not measure VO₂max directly. The most accurate measurement of VO₂max requires laboratory-based respiratory gas analysis during an incremental exercise test. Instead, your watch estimates VO₂max by modelling the relationship between your running pace and heart rate. It is, by definition, an estimate rather than a direct physiological measurement.
Training load is also an estimate. It reflects the cardiovascular stress imposed by recent training sessions and provides a useful overview of changes in workload over time. What it cannot capture are many of the factors that influence adaptation, including psychological stress, nutrition, muscle soreness, illness, travel or accumulated life stress. For this reason, recovery recommendations also should be interpreted as guidance, not objective truth. Your watch has no understanding of your training programme, your performance goals or the purpose of a particular session. During a planned overload block, for example, it may repeatedly indicate inadequate recovery, even though the accumulated fatigue is an intentional part of the training process.
Successful training is determined by far more than the metrics displayed on your watch. Sleep quality, nutrition, muscle glycogen availability, psychological stress, illness, the menstrual cycle, environmental conditions, travel and the phase of the training season all influence how the body adapts to exercise. No wearable device can measure all of these variables directly. Instead, running watches estimate readiness using proxy measures such as HRV, resting heart rate, sleep metrics and recent training load. These variables provide valuable information, but they represent only one piece of a much larger physiological puzzle.
Yes, but use it wisely. The most useful insights come from long-term trends rather than individual data points. Monitor your training load, pace, distance and heart rate over weeks and months, particularly if your heart rate zones are based on physiological testing.
HRV can also be a valuable tool, provided it is interpreted alongside subjective measures such as perceived fatigue, muscle soreness, and overall wellbeing. A poor HRV score does not necessarily mean you should cancel a hard workout. Likewise, an excellent recovery score does not guarantee that your body is ready to perform. Your watch cannot detect food poisoning, emotional stress, disrupted travel or countless other factors that influence performance.
The most effective monitoring systems combine objective data with subjective feedback. A running watch is one of the most valuable training tools available today, but it is still only a tool. Use the data to inform your decisions, not to make them for you.
Chow H, Yang C. (2020) Accuracy of Optical Heart Rate Sensing Technology in Wearable Fitness Trackers for Young and Older Adults: Validation and Comparison Study, JMIR Mhealth Uhealth, DOI: 10.2196/14707
Li K, Cardoso C, Moctezuma-Ramirez A, Elgalad A, Perin E. (2023) Heart Rate Variability Measurement through a Smart Wearable Device: Another Breakthrough for Personal Health Monitoring? Int J Environ Res Public Health. 6;20(24):7146. doi: 10.3390/ijerph20247146. PMID: 38131698;
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