VO2max, RER, 1RM - Three Tests Every Endurance Athlete Should Get

Most endurance runners who take their health seriously get blood tests done periodically. Iron, ferritin, vitamin D, B12, a lipid panel, maybe a thyroid screen. That's sensible - do it if you don't already. But bloodwork answers one (rather, two) narrow question(s): what's circulating in your blood, and in what concentration. They might produce indicators of endurance and recovery capabilities and potential, but blood tests reveal almost nothing about your aerobic capacity, fat oxidation profile, or strength potential - all of which matter significantly to ultrarunning and endurance sports.

Those things require different tests entirely, and most runners or endurance athletes never get them.

In this post, we explore three assessments that together paint a meaningfully complete picture of where one actually stands as an endurance athlete, and that generate specific, actionable data that no general health panel provides. None of them needs a research institution. Each one answers something your bloodwork, your watch, and your gut can't.

VO2max - The Aerobic Ceiling Test

VO2max is the maximum rate at which your body can consume and utilise oxygen, and it is the single most well-validated predictor of endurance performance across literature. Knowing your actual VO2max - not an estimate, but a measured value - gives a precise anchor from which to set training intensities, track development over time, and understand where your ceiling currently sits.

The problem is that most runners think they already know their VO2max because their watch tells them. They do not, and it does not.

Studies comparing smartwatch-estimated VO2max against laboratory values have found that in highly trained athletes, smartwatches underestimate VO2max by an average of over 6 ml/kg/min, with mean absolute percentage errors approaching 10%. In other words, if your real VO2max is 62, your watch might say 55. That is not a rounding error - that is a different athlete. Wearable algorithms are calibrated against population-average data, and at the extremes - which is where trained athletes tend to sit - the models break down. A highly trained athlete with a low resting heart rate and high stroke volume produces heart rate patterns that the algorithm interprets as a lower VO2max than reality.

Field tests - Cooper test, time trial protocols, Beep test - are closer to the truth than a wearable estimate, but they introduce their own confounders around pacing, motivation, terrain, temperature, and the degree to which performance on a short maximal test reflects true maximal oxygen uptake rather than anaerobic capacity or pain tolerance. Research validating VO2max prediction models against directly measured values found only low-to-moderate associations, with models tending to underestimate VO2max for younger and higher trained athletes and overestimate for older and lower trained individuals.

A genuine lab-based VO2max test, conducted on a treadmill or cycle ergometer with a metabolic cart measuring expired gases, is the most definitive and direct measurement, and not a model's estimate. It also gives you your ventilatory threshold data, which is more actionable than VO2max alone for training intensity prescription. The test takes roughly 15 to 20 minutes of exercise and is available at most sports science facilities, performance labs, and a growing number of physiology clinics. It is worth doing at least once as a baseline, and repeating every 12 to 18 months or after a significant training phase to track genuine progress.

One cautionary consideration for trail runners specifically is that lab-based protocols using treadmills or stationary ergometers may not fully capture the physiological demands of trail running, where gradient, technical terrain and effort differ meaningfully from lab conditions. If a portable metabolic testing option is available in a real-terrain context, that is a more specific measurement. If not, a lab treadmill test with incline protocol is still substantially more accurate than any wearable or field estimate.

RER - The Fuel Utilisation Spectrum

Blood tests tell you what nutrients are present in your system in what quantities. They do not tell you how many calories your body burns at rest, at what intensity you switch from fat to carbohydrate as a primary fuel source, or how much of your current training load your current eating actually supports. A metabolic nutrition assessment answers all three.

The core of this assessment is indirect calorimetry - measuring the ratio of oxygen consumed to carbon dioxide produced during rest and during graded exercise. From this ratio, called the respiratory exchange ratio (RER), a laboratory can calculate your resting metabolic rate, your fat oxidation rate at various exercise intensities, and the intensity at which you cross from fat-dominant to carbohydrate-dominant metabolism - sometimes called Fatmax or the fat-burning peak.

These numbers matter in ways that most runners underestimate.

Your resting metabolic rate (RMR) is the baseline caloric requirement your body has before any training is accounted for. RMR accounts for as much as 60 to 75% of total daily energy expenditure in most individuals, and accurate measurement of it allows for precise tailoring of nutrition intake to actual metabolic needs. Most runners are operating from estimated RMR values calculated by formulas that assume average body composition and average hormonal function. The reality is that endurance athletes frequently have suppressed RMR relative to formula predictions - a consequence of chronic training load, underfuelling, or both - and the only way to know if yours is suppressed is to measure it directly.

Your fat oxidation profile is the second critical output. Fat oxidation rates during exercise vary by roughly threefold between individuals at similar fitness levels. This variation determines how much carbohydrate you burn at a given intensity, which directly sets your fuelling requirement per hour of running. Two runners of similar fitness completing a mountain 50K at similar effort levels may have dramatically different glycogen depletion rates and dramatically different gel requirements as a result, and neither of them can determine which category they fall into from blood tests or wearable data.

Knowing your personal Fatmax - the intensity at which fat oxidation peaks - also directly informs training intensity decisions in a way that heart rate zones and RPE bands alone cannot. If your Fatmax sits at an effort significantly below where your easy runs actually land, you are training in a more carbohydrate-dependent state than you might assume, which has implications for both training adaptation and race nutrition strategy.

This test is available at sports science labs and a smaller number of specialist nutrition clinics. Some practitioners offer a combined RMR and exercise calorimetry session that gives you the full picture in a single visit. It is particularly worth doing before a major training cycle and after any significant change in body composition or training volume.

1RM - The Untapped Strength Potential

The 1RM (one-repetition maximum) test is the one test on this list that most endurance runners dismiss entirely because they do not think of themselves as strength athletes. And this tiny mistake leads to most runners leaving untapped performance upsides on the table.

The 1RM test is considered the gold standard for assessing strength capacity in non-laboratory environments, and has been proven as a valid and reliable measure across a wide range of populations and exercises. What it gives you specifically, as an endurance runner, is an honest answer to a question you probably cannot answer accurately from feel alone: how strong are you, relative to your bodyweight, in the movements that matter most for running performance?

The movements worth testing are the back squat and the Romanian deadlift, both of which have direct transfer to running economy, climbing efficiency, and descent control. A meaningful strength target for a trail runner is roughly bodyweight on the squat for 1RM and 1 to 1.2 times bodyweight on the Romanian deadlift. Most runners who have never tested their 1RM are significantly below these targets without knowing it, because training in the 8 to 12 rep range gives you no reliable information about where your maximum actually sits. Training at 60 to 80% of 1RM produces hypertrophy, while training above 85% of 1RM develops maximal strength - and without knowing your 1RM, you cannot accurately target either zone.

The second reason to test is progress tracking. Strength adaptation from a programme of 8 to 10 weeks is meaningless to evaluate if you have no baseline to measure it against. A 1RM test before and after a training block gives you a concrete before-and-after comparison that tells you whether the programme is actually working. Subjective feel - "my squats feel better" - is a poor substitute for this.

Research confirms that the 1RM test has good-to-excellent test-retest reliability regardless of exercise selection, training experience, or whether the assessment is of upper or lower body. It is not a dangerous test when conducted correctly. The safety precautions are standard: one familiarisation session before the test proper, a thorough warm-up protocol, progressive loading to maximum rather than jumping to heavy weight, and a spotter for squats and pressing movements. For runners with no previous maximal testing experience, estimating 1RM from a submaximal set - the Epley or Brzycki formula applied to a heavy set of 3 to 5 reps - is a reasonable alternative that gives you most of the same information with meaningfully less fatigue and risk.

Why These Three Together

Each of these tests closes a different blind spot that blood tests and general health panels leave open. Blood tests tell you about your biochemical status - what is or is not available in your system at a given point in time. They are useful, but they say nothing about what your body does with those inputs under the specific demands of endurance performance.

VO2max sets your ceiling and your thresholds - the top and bottom of useful intensity. The metabolic test shows how you burn fuel across that range, and whether you're eating enough to support it. And the 1RM? That's the one telling you how much free speed you're leaving in the gym.

Done together, ideally at the start of a major training cycle and repeated at intervals as the cycle progresses, these three tests give you a data-driven picture of yourself as an athlete that no combination of wearable metrics, RPE diaries, or blood panels can replicate. It only costs a few hours and a lab fee, but what you get is a clear read on where you actually stand - and what's worth fixing first.

For more on how to apply VO2max data and metabolic testing to your actual training, read our posts on Running Economy and Strength Training and The Aerobic Spectrum.