Intensity Relative to Your Own Fitness Better Explained Fitness in Midlife Women

The work is a cross-sectional observational study of 73 women aged 50 to 65, published in Frontiers in Sports and Active Living in 2026. It looked at a single snapshot in time, so it can show associations, not cause and effect. Thirty-eight of the women were recreational endurance athletes and 35 were inactive controls. Fitness across the group ranged widely, from 20 to 60 mL/min/kg.

Researchers measured each woman's cardiorespiratory fitness, which is how well the heart, lungs and muscles deliver and use oxygen. Each woman did a maximal cycling test on an exercise bike, increasing in difficulty until exhaustion. The authors then estimated peak oxygen uptake, known as VO₂ max, from the peak power she reached, using a validated equation rather than measuring gases directly.

The women also wore a hip accelerometer for seven days. This is a small device that records body movement. The team then sorted the movement into intensity levels in two ways.

The first way is absolute intensity, using metabolic equivalents, or METs. A MET is a fixed unit of energy use. Under this system, 3 to 6 METs counts as moderate and 6 METs or more as vigorous, for everyone. The second way is relative intensity, which expresses effort as a percentage of a person's own VO₂ max. Here, 46% to 64% counts as moderate and 64% or more as vigorous.

The two groups differed sharply in fitness. The active women had higher fitness, 41.4 versus 27.7 mL/min/kg, which is what the study design was meant to produce.

The key comparison was which approach better explained differences in fitness. The relative intensity model explained 70% of the variation in fitness. The absolute intensity model explained 53%. With absolute measures, the link to fitness showed up mainly at vigorous levels, above 6 METs.

The authors point to a built-in mismatch. A 3 MET effort equals 46% of VO₂ max only for someone with a fitness level of about 22.8 mL/min/kg. For a typical middle-aged adult, with a VO₂ max of about 33, they note that 3 METs is only around 30% of capacity. That is light effort for that person. They write that the 3 MET threshold is too low for about 95% of middle-aged adults.

The study also found that light and lower-moderate relative intensities were negatively associated with fitness. These negative associations faded as intensity rose, levelling off near 64% of VO₂ max. In plain terms, in this sample, time spent at easier relative efforts went along with lower fitness, and the clearest positive signals came at vigorous effort.

The authors conclude that, in this sample, the traditional 3 MET moderate threshold appears inadequate for middle-aged women. They suggest vigorous intensity should be the main target when fitness is the goal, and support setting intensity relative to individual capacity. The paper notes that activity at all intensities has health benefits, and it focused on cardiorespiratory fitness only.

There are limits. The design cannot show that vigorous activity raises fitness, and fitter women may simply choose harder activity. The sample is small and Swedish. The comparison was between recreational endurance athletes and inactive women, two extremes. Peak oxygen uptake was estimated from a bike test, not measured directly. The women were also free of major disease and were not smokers, so the findings may not extend to others. This is where the argument is weakest.

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Studies that follow women over time, or test training at different relative intensities, to see whether the associations hold up.

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