Unlocking Insights: What Your Data Can Reveal

The Large Benelux Trail Running Study revealed that over 83 percent of participants use the running app on their watch to track workouts and review and analyze data. Watches now measure numerous metrics, presenting them in a wide array of tables and graphs. But how can you train smarter with all that data?

In this blog we explains how watches' "direct" measurements work, how reliable they are, and how to use them in training. This includes data like distance, speed, cadence, and heart rate (variability). Brands like Garmin and Polar create "derived" values from these measurements to give users a clear view of their data.

Speed and Distance

Speed and distance are typically measured via GPS or a (calibrated) foot pod. GPS periodically determines the location, calculating the distance between the current and previous location and dividing it by the time between measurements. This relies on signals from multiple GPS satellites, which can be reflected by tall buildings, trees, or mountains, making location and speed readings less accurate. Modern "dual-band" GPS chips in higher-end watches partially address this issue. Footpods use motion sensors to accurately measure distance and time per step based on acceleration and deceleration.

How does this help in training planning?

To improve fitness, we need to challenge our bodies with sufficient training stimuli without overloading to the point of prolonged recovery or injury. A simple approach is the "ten percent rule," ensuring weekly training volume grows by no more than ten percent, either in distance or duration. This is easy to track but has limitations, as it doesn’t account for training intensity, and it's hard to know where to resume after a break.

More advanced methods track the load of each training session, considering both duration and intensity. You can then compare your long-term average training load (chronic load) with the short-term load, like the last week's load (acute load).

Studies suggest that a ratio of acute to chronic load between 1.0 and 1.5 leads to fitness improvement without increased injury risk. You can monitor these values on your watch and use them to guide your training and training volume.

Garmin and Polar offer features to measure load but use different approaches and terminology. Garmin terms "Acute Load" (total training load of the past seven days) and "Chronic Load" (a weighted average of acute load over a longer period, usually four weeks). Polar uses "Cardio Load" (the load from a single session or total daily load) for acute load and "Cardio Load Status" for chronic load.

Heart rate and Heart Rate Variability (HRV)

Most watches now measure heart rate via a mini-sensor on the wrist, detecting blood vessel expansion and contraction, which gives beats per minute. This works well at rest but may be less accurate during vigorous movement if the watch is too loose, or on darker skin tones. A more accurate alternative is a chest strap that reads heart rate based on electrical signals and is less affected by movement or light. However, even chest straps can show incorrect readings if the skin is dry, static electricity is present, or the electrodes aren’t clean.

Heart rate variability (HRV) measures the precise time intervals between heartbeats. If the heart beats 60 times a minute, it doesn’t mean the intervals are exactly one second; for example, intervals might vary from 900 to 1,200 milliseconds. The degree of variation in these intervals is called HRV, and it correlates with our autonomic nervous system. A relatively high HRV reflects a relaxed, recovery state, while low HRV indicates stress. What is considered "high" or "low" HRV is personal and depends on past observations.

How does this prevent injury or overtraining?

When we’re stressed or fighting off an illness, our resting heart rate rises, and HRV tends to drop, often before we notice symptoms. Monitoring these values can prevent heavy training on days we’re vulnerable.

Garmin watches combine HRV, stress, and activity data to calculate "Body Battery," which reflects how much energy you have for an activity. During stressful or high-training periods, your battery might not fully recharge overnight. The emptier your battery, the more cautious you should be about intense activity that day. Polar offers an ANS Recovery value, which includes HRV along with heart rate and breathing rate. A higher ANS recovery level indicates better cardiovascular recovery.

Cadence and Ground Contact Time

A footpod, chest strap transmitter, or the watch itself can measure cadence by counting steps. Higher-end watches and footpods also measure ground contact time, the duration each foot spends on the ground during a step.

Shorter ground contact time is often associated with a higher cadence, a faster push-off, and a mid- or forefoot landing. Runners with longer strides might have longer ground contact times, especially if not trained to efficiently take longer steps. Heel strikers also tend to have longer ground contact times, as rolling from heel to toe takes more time.

How does this help improve running efficiency and speed?

Analyzing ground contact time can help enhance running efficiency and prevent injuries. Cadence and ground contact time indicate mechanical stress on joints; a higher cadence or lower ground contact time can reduce impact forces. If you notice increased ground contact time during a workout or race, it could signal fatigue. As fatigue sets in, technique can deteriorate, increasing ground contact time per step. A difference in ground contact balance might suggest strength imbalances or different foot positioning.

You can do specific running or strength exercises to lower ground contact time or correct imbalances. Try short, fast intervals weekly, or finish your long run with short sprints at a higher cadence. You can also use a metronome app to set a rhythm slightly faster than your current cadence. Strengthen your core and legs with specific exercises.

Measurements Equal Knowledge, or Not?

Given all the data, it might seem we can rely entirely on algorithms for health and sports metrics. However, we should be cautious. As mentioned, measurement errors are always possible, and most apps aren’t great at identifying these errors, which can lead to inaccurate calculations. Moreover, many aspects of our health, like muscle pain, nutrition absorption, or mental stress, simply can’t be measured by sensors.

As long as we continue to listen to our bodies, sports watches and training apps can be highly beneficial, especially for the calculations and analysis involved in structured training plans.

This article was previously published in Trailrunning Magazine (Summer '24).  Text: Jonatan Samoocha in collaboration with Patricia Scheurs.