Longevity Biomarkers Made Clear: VO₂max, Heart Rate Variability and Grip Strength

What the three markers actually measure

The three markers capture very different bodily systems – and that is precisely what makes them interesting as a group. VO₂max (maximal oxygen uptake) describes how much oxygen the body can take in, transport and utilize in the muscles per unit of time under maximal exertion. It is the most common measure of cardiorespiratory fitness and condenses the performance of the heart, lungs, circulation and muscles into a single number. It is precisely determined via spiroergometry (analysis of respiratory gases under exertion); fitness watches merely estimate it from heart rate and pace data and are correspondingly less accurate.

Heart rate variability (HRV) measures the small fluctuations in the time intervals between two heartbeats. A healthy heart does not beat to a rigid rhythm but continuously adapts to breathing, exertion and recovery. HRV is therefore regarded as an indirect window onto the autonomic nervous system and the interplay between tension (sympathetic) and recovery (parasympathetic). Higher variability is generally interpreted as a sign of good adaptability.

Grip strength, finally, is a simple indicator of overall muscular strength. It is measured with a hand dynamometer and correlates surprisingly well with whole-body strength. Because muscle mass and strength decline with age (sarcopenia), grip strength serves as an inexpensive proxy for general physical condition.

• VO₂max: a measure of cardiorespiratory fitness (heart, lungs, circulation, muscle); precise only via spiroergometry
• HRV: fluctuation in the intervals between heartbeats – an indirect measure of the autonomic nervous system
• Grip strength: a simple proxy for overall muscular strength, measured with a hand dynamometer
• Three different systems – endurance, nervous system, musculature

What the research actually shows

For all three markers there are large human studies showing an association with mortality risk – that is the robust core finding. On cardiorespiratory fitness, an analysis of 122,007 people who had completed an exercise stress test (treadmill) examined the relationship between fitness and long-term mortality. The result: the higher the fitness, the lower the mortality – with no discernible upper limit to the benefit. Even very high fitness levels were associated with a continued lower risk. This is an observational study; it shows a strong association but cannot, on its own, prove that more fitness causally extends life.

For grip strength, the international PURE study with nearly 140,000 participants from 17 countries provided robust figures: each decrease in grip strength of 5 kilograms was associated, on average, with roughly a 16 percent higher risk of all-cause mortality and about a 17 percent higher risk of cardiovascular mortality. Remarkably, grip strength predicted mortality even better than systolic blood pressure.

On HRV, a meta-analysis of 32 studies with around 38,000 participants showed that lower HRV values were a predictor of higher mortality across various age groups and populations. An earlier meta-analysis found a 32 to 45 percent increased risk of a first cardiovascular event with low HRV in populations without known cardiovascular disease. Here too the same applies: these are associations in observational data.

• Fitness (122,007 people): higher cardiorespiratory fitness, lower mortality – with no discernible upper limit to the benefit
• Grip strength (PURE, ~140,000 people): −5 kg ≈ +16 % all-cause mortality; a better predictor than systolic blood pressure
• HRV: low values associated with higher mortality and more cardiovascular events
• All the evidence comes from observational studies – they show correlation, not proof of causation

Correlation is not causation – the limits of the markers

The most important caveat applies equally to all three markers: a statistical association with mortality risk does not automatically mean that the marker is the cause. Low values may simply indicate an already existing, possibly still undetected illness (reverse causation). Someone with incipient heart disease, for instance, will often show lower fitness, lower HRV and less strength – without those values having caused the illness. The markers may therefore be a symptom rather than a cause.

There are also measurement problems. The VO₂max estimate from fitness watches sometimes deviates considerably from the lab value. HRV fluctuates strongly from day to day and depends on breathing, sleep, caffeine, alcohol, time of day and measurement method – a single value is therefore of little significance, and absolute figures can hardly be compared between people. Grip strength depends on height, sex, age and daily form and must be interpreted accordingly. None of the three values is a diagnostic test; they are risk indicators at the group level whose individual predictive power is limited.

• Low values can be the consequence of an existing illness, not its cause (reverse causation)
• VO₂max from wearables is an estimate and can deviate from the lab value
• HRV fluctuates strongly depending on breathing, sleep, caffeine, time of day and measurement method – single values and person-to-person comparisons are problematic
• No marker is a diagnostic tool; all are risk indicators at the population level

Trend rather than snapshot – how to use the values sensibly

Despite these limits, the three markers have practical value when understood correctly. Their worth lies less in the single measurement than in the trend over time. An HRV baseline falling over weeks or months, declining fitness or a noticeable loss of strength can be signs of overload, insufficient recovery or a health change – and a reason to look more closely or seek medical advice.

It is also well established that fitness and strength in particular can be improved through exercise. Endurance training tends to raise VO₂max, strength training builds muscular strength. This makes these markers among the few health indicators that not only measure but can also be influenced. That makes them attractive as a tool for motivation and tracking. This article deliberately describes these relationships only in a contextualizing way and gives no specific training, measurement-interval or target-value recommendations – such individual decisions belong in knowledgeable hands, in case of doubt medical or sports-medicine professionals, especially where pre-existing conditions are present.

• The trend over weeks/months is more meaningful than the single daily value
• Persistently falling values can be a reason to look more closely or seek medical advice
• Fitness and strength can be influenced through exercise – this sets them apart from many other markers
• Specific target values or training prescriptions belong in knowledgeable hands, especially with pre-existing conditions

Putting the hype into sober perspective

In the longevity and biohacking scene, VO₂max, HRV and grip strength are sometimes elevated into almost magical "longevity numbers" that one must optimize. This portrayal distorts the evidence. What is true: the three markers are robust, well-studied statistical risk indicators that are linked to mortality in large studies. What is false is the claim that a particular target value guarantees a long life or that a single wearable display reflects one's own biological reality exactly.

Anyone tracking these values should keep three levels apart: the documented association with mortality risk (well established), the plausible assumption that improving the values also lowers individual risk (partly supported, but not causally proven for every marker), and marketing promises around devices, apps or substances (often claim rather than evidence). None of the three markers replaces a medical examination, and no value should tempt anyone into self-diagnosis or the unsupervised use of substances. As points of orientation over time they are useful – as a final verdict on one's own health they are unsuitable.

• The markers are well-studied risk indicators – but not a guarantee and not a diagnosis
• "Target value X = a long life" is an oversimplification the evidence does not support
• Keep three levels apart: documented association, plausible assumption, marketing claim
• No marker replaces a medical examination