Why Your Fitness Tracker Underestimates Your Fitness During Rucking Workouts

Many fitness enthusiasts have experienced this frustrating scenario: after completing a challenging rucking session—perhaps a 5-mile hike with a 30-pound ruck—their fitness tracker reports a decline in their cardio fitness score. Despite feeling accomplished after an intense workout, the device suggests their aerobic capacity has somehow decreased.

This counterintuitive result highlights a significant limitation in how consumer fitness devices estimate VO2 max during weighted activities. Understanding why this happens and how to address it can help you get more accurate fitness data while continuing to benefit from rucking's considerable advantages.

VO2 Max: The Gold Standard of Aerobic Fitness Assessment

To understand why rucking affects fitness tracker readings, it's essential to examine VO2 max—the metric these devices attempt to measure and track over time.

VO2 max represents the maximum volume of oxygen your body can consume and utilize per minute during maximal exercise, typically expressed in milliliters of oxygen per kilogram of body weight per minute (mL/kg/min). This measurement reflects the integrated efficiency of your cardiovascular and respiratory systems in delivering oxygen to working muscles during high-intensity exercise.

VO2 max serves as a fundamental indicator of aerobic fitness capacity. Elite endurance athletes typically demonstrate VO2 max values in the 60-80+ mL/kg/min range, while sedentary individuals may measure in the 20-30 range. Research consistently shows that higher VO2 max values correlate with enhanced athletic performance, reduced cardiovascular disease risk, and increased longevity.

The laboratory standard for VO2 max measurement involves progressive exercise testing to exhaustion while monitoring oxygen consumption and carbon dioxide production through respiratory gas analysis. This direct measurement provides the most accurate assessment but requires specialized equipment and trained personnel, making it impractical for most people.

How Your Smartwatch Guesses Your Fitness Level

Modern fitness trackers have made VO2 max tracking accessible to everyone, but they're essentially playing an educated guessing game. Instead of directly measuring your breath, they use heart rate sensors and movement data to reverse-engineer an estimate.

Apple Watch calls it "Cardio Fitness" and analyzes your heart rate response during outdoor walks, runs, or hikes. It looks at how hard your heart is working relative to your pace, factors in your age, weight, and other personal data, then spits out an estimate. Over time, it refines this number as it gathers more workout data.

Garmin devices use algorithms to estimate VO2 max primarily from running data. The basic logic is simple: the faster you can run at a relatively lower heart rate, the higher your VO2 max. The watch analyzes steady segments of your runs, applies known formulas relating running speed to oxygen consumption, and back-calculates your maximum based on what percentage of your max effort you were working at.

Fitbit takes a slightly different approach, using your resting heart rate as a baseline (lower resting heart rate generally indicates better fitness) and then refining the estimate with GPS exercise data when available.

All these methods share a common thread: they're estimation models that correlate your pace, heart rate, and other variables to VO2 max values observed in actual lab tests. They're reasonably accurate for most people – often within about 5 mL/kg/min of lab values – and more importantly, they're consistent for tracking your personal progress over time.

Where Rucking Breaks the Algorithm

Here's where things get interesting (and frustrating). All these VO2 max estimation algorithms operate under a key assumption: you are only moving your own body weight. When you strap on a 20 or 30-pound ruck, you've effectively changed your weight, but your fitness tracker has no idea.

Remember, VO2 max is measured relative to body weight. If you normally weigh 150 pounds but you're rucking with a 30-pound pack, you're effectively moving 180 pounds. But your watch still thinks you weigh 150.

What happens next is predictable: your pace is slower than a normal run (because you're carrying extra weight), but your heart rate is higher than it would be for that pace without the weight. The algorithm interprets this as "this person is working really hard but moving slowly – they must be in terrible shape." The result? An artificially depressed VO2 max estimate that makes you look far less fit than you actually are.

The irony is that rucking is actually excellent cardiovascular exercise. Carrying extra weight increases oxygen consumption at a given speed – that's exactly why it's such a good workout! But your fitness tracker interprets this increased oxygen demand as a sign that you're out of shape rather than recognizing that you're working harder because you're carrying additional load.

How RuckWell addresses this issue

Understanding the technical challenge, RuckWell has developed an innovative workaround for Apple's fitness tracking limitations. Apple's HealthKit automatically calculates VO2 max estimates for specific activity types—namely outdoor walks, runs, and hikes. Since rucking most closely resembles hiking in terms of movement patterns and GPS tracking requirements, RuckWell defaults to processing workouts as hike activities to ensure optimal sensor utilization and data accuracy.

However, this approach created the same VO2 max distortion issue that affects other fitness trackers when additional weight is involved. To solve this problem, RuckWell introduced a "Disable VO2 Max Updates" feature that prevents rucking sessions from contributing to Apple's system-wide VO2 max calculations.

The technical implementation involves reclassifying the workout type from "hike" to "other" within Apple's workout framework. This designation excludes the activity from VO2 max algorithms while maintaining comprehensive data collection for route tracking, heart rate monitoring, and metabolic calculations specific to weighted activities.

This reclassification does present certain technical challenges. When Apple's system processes a workout as "other" rather than a recognized outdoor activity, it applies different sensor calibration parameters that can affect location accuracy and motion detection algorithms. RuckWell addresses these limitations by implementing custom algorithms for GPS tracking and motion analysis.

The result is a comprehensive solution that captures all relevant workout data—including distance, elevation, heart rate, and weight-adjusted calorie expenditure—while preserving the accuracy of VO2 max estimates derived from other activities. This approach ensures that users can track their rucking progress without artificially deflating their overall fitness metrics.

For serious ruckers who use Apple devices, this represents the most sophisticated currently available solution for maintaining accurate fitness tracking across both rucking and other training sessions.

The Bottom Line

If you love rucking, don't let your fitness tracker's confusion discourage you. When your device tells you your cardio fitness dropped after a great rucking session, it's not because you got less fit – it's because you challenged yourself with added weight, and the algorithm couldn't keep up.

The fitness industry is starting to catch on to this issue. Garmin's recent rucking mode update shows that companies are working on better solutions. We'll likely see more brands follow suit as rucking continues to grow in popularity.

Until then, RuckWell is here to help, but remember that the most important metric is how you feel. If you're getting stronger, building endurance, and enjoying your workouts, you're on the right track – regardless of what your smartwatch thinks about it.

Keep rucking, keep tracking what matters to you, and carry on.