When your Apple Watch labels your cardio fitness as “Low,” it means your estimated VO2 max falls in the bottom range for your age and sex. VO2 max measures how efficiently your body uses oxygen during exercise, and it’s one of the strongest predictors of long-term health. A low reading isn’t a diagnosis, but it is a signal worth paying attention to.
What Your Apple Watch Is Actually Measuring
Cardio fitness on Apple Watch is an estimate of your VO2 max, reported in milliliters of oxygen per kilogram of body weight per minute (mL/kg/min). Your watch calculates this using data from its heart rate sensor and motion sensors during outdoor walks, runs, or hikes. It tracks how hard your heart works relative to how fast you’re moving, then compares your result against population data for people of your age and sex.
Apple groups results into four categories: Low, Below Average, Above Average, and High. If you’re seeing “Low,” your estimated oxygen uptake capacity is in roughly the bottom tier for your demographic group.
A few important caveats about the measurement itself. Only outdoor walks, runs, and hikes count toward your estimate. Indoor workouts, cycling, swimming, and gym equipment sessions don’t contribute. Your watch also needs at least 24 hours of wear plus several qualifying outdoor workouts before it generates an initial reading. If you’ve only recently started using the feature, or if most of your exercise happens indoors, the estimate may not reflect your true fitness level.
How Accurate Is the Estimate?
Apple Watch provides a reasonable ballpark, not a clinical measurement. A 2025 study comparing the Apple Watch Series 10 against gold-standard lab testing found a mean absolute percentage error of about 13%. The watch tended to underestimate VO2 max by an average of roughly 6 mL/kg/min, with wide variability between individuals. Some people’s readings were close to their lab values; others were off by a significant margin.
This means a “Low” reading could genuinely reflect poor cardio fitness, or it could be an underestimate driven by sensor limitations, how tightly your watch fits, or the specific conditions of your workouts. If you’re consistently getting low readings across many outdoor sessions, the trend is more meaningful than any single data point. But if you’re reasonably active and the number seems surprisingly low, the measurement’s inherent margin of error may be a factor.
Why Low Cardio Fitness Matters for Health
Setting aside Apple Watch specifically, low cardiorespiratory fitness is one of the most well-studied risk factors in preventive medicine. People with low VO2 max have higher rates of heart disease, stroke, type 2 diabetes, and certain cancers. In one study of people already at risk for type 2 diabetes, 96% had VO2 max results below the healthy range for their age and sex, and 85% of those fell into the high-risk category for developing disease linked to low aerobic capacity.
The relationship runs deeper than just heart health. Low cardio fitness is associated with greater arterial stiffness (meaning your blood vessels are less flexible, which raises blood pressure over time) and with clustering of metabolic problems like high blood sugar, excess abdominal fat, and abnormal cholesterol. These connections hold even after accounting for body weight, meaning fitness level itself matters independently of how much you weigh.
The encouraging flip side: improving your VO2 max, even modestly, reduces these risks. Moving from “Low” to “Below Average” carries real health benefits. You don’t need to become an elite athlete for the reading to matter less.
Common Reasons for a Low Reading
A sedentary lifestyle is the most straightforward explanation. If you spend most of the day sitting and don’t regularly exercise at a moderate or vigorous intensity, your cardiovascular system simply hasn’t been challenged enough to adapt. Age also plays a role: VO2 max naturally declines by roughly 10% per decade after your mid-20s if you don’t actively maintain it, though the Apple Watch categories already adjust for age.
Other factors can contribute. Excess body weight lowers your score because VO2 max is calculated per kilogram. Certain medications that affect heart rate, like beta-blockers, can alter the readings. Chronic conditions including anemia, lung disease, or heart conditions reduce oxygen delivery. Poor sleep, high stress, and dehydration can temporarily suppress your numbers too. And again, if your outdoor workout data is limited, the watch may simply not have enough information to generate an accurate estimate.
How to Improve Your Cardio Fitness Score
Any aerobic exercise that elevates your heart rate will help. If you’re currently inactive, even brisk walking can be vigorous enough to trigger improvements in VO2 max. You don’t need to start running or join a gym. Consistency matters more than intensity at first: aim for regular outdoor walks at a pace that makes conversation slightly difficult.
Once you’ve built a base, the fastest way to push your score up is high-intensity interval training (HIIT). This means alternating between short bursts of hard effort and recovery periods. For example, walking briskly for two minutes, then walking at a comfortable pace for one minute, repeating for 20 to 30 minutes. If you’re already a runner, that might mean alternating between a fast run and a jog. Research from Harvard Health suggests doing interval sessions several times per week, mixed with longer, slower-paced activities on other days.
If you typically exercise at a steady, moderate pace, two changes help: occasionally push harder, and occasionally go longer. Adding 10 minutes to a 30-minute routine or picking up the pace for portions of your workout both challenge your cardiovascular system in ways that steady-state exercise alone doesn’t.
To make sure your Apple Watch captures these improvements, do your workouts outdoors using the Outdoor Walk, Outdoor Run, or Hiking options in the Workout app. Indoor treadmill sessions or cycling won’t update your cardio fitness estimate, no matter how hard you work.
What to Watch For Over Time
The trend line matters more than any single reading. Open the Health app, tap Browse, then go to Heart and select Cardio Fitness to see your history. A gradual upward trend over weeks and months confirms that your training is working. Most people who go from sedentary to regularly active see measurable improvement within four to eight weeks.
If your score stays stubbornly low despite consistent outdoor exercise over several months, or if it drops suddenly without a clear reason like illness or reduced activity, that’s worth discussing with a doctor. A persistently low VO2 max in someone who exercises regularly can occasionally point to an underlying cardiovascular or pulmonary issue that warrants evaluation.