Related Experiment Video
Updated: Aug 5, 2026
Assessment of Physical Activity Intensity with Accelerometers and Oxygen Consumption
Published on: June 20, 2025
Absolute and relative physical activity intensity thresholds across diverse fitness levels: a cross-sectional study
Jonatan Fridolfsson1,2, Andreas Malmgren3,4, Petri Gudmundsson5,6
1Center for Lifestyle Intervention, Institute of Medicine, Sahlgrenska Academy, University of Gothenburg, Gothenburg, Sweden.
Introduction:
Physical activity (PA) intensity can be expressed in absolute terms (metabolic equivalents, METs) or relative to individual capacity (percentage of maximal oxygen consumption, VO₂max), and these two approaches yield markedly different interpretations of health-related PA. This study examined how time spent at absolute and relative PA intensities relates to cardiorespiratory fitness (CRF) in middle-aged women across a wide fitness range.
Methods:
In this cross-sectional study, 73 middle-aged women (38 recreational runners, 35 inactive controls) aged 50-65 years with diverse CRF levels (20-60 mL/min/kg) were assessed. CRF was assessed through maximal cycle ergometer testing. PA was measured via hip-worn accelerometry for seven days using the 4 Hz frequency extended method. Absolute intensity used traditional METs thresholds (3-6 METs moderate, ≥6 METs vigorous); relative intensity was expressed as percentage of individual VO₂max (46-64% moderate, ≥64% vigorous). Associations with CRF were analysed using partial least squares regression and cumulative intensity thresholds.
Results:
The active group had significantly higher CRF (41.4 vs. 27.7 mL/min/kg, p < 0.001) and accumulated substantially more vigorous activity (20.6 vs. 0.8 min/day, p < 0.001), whereas the inactive group accumulated more relative moderate intensity, illustrating the "intensity paradox." For absolute intensity, positive CRF associations emerged at approximately 4.5 METs, strengthening progressively and peaking in the vigorous range (>6 METs). For relative intensity, negative associations diminished progressively, reaching equilibrium near 64% VO₂max. The relative intensity model explained 70% of CRF variance compared to 53% for absolute intensity.
Discussion:
In this cross-sectional sample, traditional 3 METs moderate intensity thresholds appear inadequate for middle-aged women, with significant CRF associations occurring primarily at vigorous intensity. These findings suggest that vigorous intensity should be the primary target when CRF is the desired outcome, and support prescribing intensity relative to individual capacity.
