Do You Need to Go All-Out to Measure Your Cardio Fitness?

A cardio fitness test can sound like a test of how much discomfort you can tolerate: run harder, pedal faster, keep going until you cannot continue. That picture raises an obvious question. If you stop short of your absolute limit, can the result still tell you something useful?

Exercise tests can be maximal or submaximal. A maximal test typically increases the workload progressively until a person reaches or closely approaches their maximal exercise capacity; it does not mean sprinting all-out from the first moment. A submaximal test ends below maximal effort and uses responses at lower workloads to estimate cardiorespiratory fitness or maximal oxygen uptake.¹

Submaximal does not necessarily mean effortless. Depending on the protocol, an estimate may use heart rate, workload, perceived exertion, oxygen uptake measured at lower intensities, or performance on a standardized task. Those responses are then used to predict a higher-level fitness measure rather than observing maximal capacity directly.¹

The accuracy of those predictions depends on the method. One systematic review examined 43 prediction equations that used oxygen uptake measured during submaximal exercise to predict VO₂max in healthy adults. Overall, the equations showed moderate-to-high accuracy, but no single approach was consistently best. Performance varied with factors including exercise mode, the amount of exertion required, the population in which an equation was developed, and the variables included in the prediction.²

Practical walking tests provide another example. A systematic review of field-based fitness tests in adults aged 19 to 64 without known pathology found strong evidence of criterion-related validity for both the 2-kilometer walk test and the 6-minute walk test. The review indicated that these submaximal walking tests were particularly useful for adults with low or moderate fitness, while their ability to estimate cardiorespiratory fitness was more limited at very high fitness levels.³

That means a test does not become inaccurate simply because it stops below maximal effort, and a harder test is not automatically better for every purpose. A well-validated submaximal test can provide useful information, but its estimate remains dependent on the protocol, prediction method, and population in which it is used.^2,3

Those assumptions matter in practice. If an estimate relies heavily on heart rate, for example, factors that alter the heart-rate response, including some medications, can affect the prediction. Exercise mode, the amount of effort required, the equation used, and characteristics of the person being tested can also influence how closely a submaximal estimate reflects maximal capacity. Reviews therefore emphasize matching the method to the person and the purpose rather than treating prediction equations as interchangeable.^1,2

The purpose of testing also changes how much direct measurement is needed. If the goal is a practical estimate of cardiorespiratory fitness, a validated submaximal test may provide useful information without requiring maximal effort. If the goal is to characterize maximal physiological capacity directly or to answer a clinical exercise-testing question, an estimate from a walk, step, or other submaximal test is not equivalent to a directly measured maximal assessment.^1,3

Being easier to perform also does not mean that every field test has equally strong evidence for safety. A 2025 systematic review found that research on the feasibility and safety of many commonly used field fitness tests was much thinner than the literature on validity and reliability. Among adult tests, strong safety evidence was identified specifically for the 2-kilometer walk test. For many other tests, limited safety evidence reflected a lack of research rather than clear evidence that the tests were unsafe.⁴

Maximal and submaximal tests therefore answer related but different questions. A validated submaximal test can estimate cardiorespiratory fitness without requiring someone to reach maximal effort, while a maximal assessment can provide directly measured physiological information that a submaximal estimate does not reproduce in the same way.^1,3

A test does not earn its value by making you work harder than necessary. What matters is whether the test is suited to the question you wanted it to answer.