Need to Revise Classification of Physical Activity Intensity in Older Adults? The Use of Estimated METs, Measured METs, and V̇O2 Reserve

Abstract

Background: Multiples of resting metabolic rate (RMR) are often used to classify physical activity intensity, a concept known as the metabolic equivalent of task (MET). However, the METs metrics may misclassify physical activity intensity in older adults because of age-related changes in RMR and maximal aerobic capacity (V˙O2max). This study aimed to (i) compare classifications of activity intensity by estimated (METsestimated) and measured (METsmeasured) METs and (ii) compare physical activity classified by absolute (METsmeasured) versus relative intensity (%V˙O2Reserve) in older adults.

Methods: Ninety-eight adults aged 75-90 years participated in the study. RMR and V˙O2 during sitting, standing, daily activities, and 6-minute walking test were measured. V˙O2Reserve was defined as the difference between V˙O2max and RMR. Moderate and vigorous intensity was classified as 3 and 6 METs and 40% and 60% of V˙O2Reserve, respectively. Paired t tests and a confusion matrix were used to investigate aims 1 and 2, respectively.

Results: METsmeasured was 24% lower than the standard 1 MET of 3.5 mL O2·min-1·kg-1. METsestimated underestimated the intensity during daily and walking activities when compared to METsmeasured. Nevertheless, when comparing METsmeasured to percentages of V˙O2Reserve, a mismatch was shown for moderate intensity in 47%-67% of the participants during daily activities and 21% of the participants during self-selected gait speed.

Conclusions: Applying METsestimated for older adults leads to potential underestimation of physical activity intensity, suggesting that current classification metrics should be revised for older adults. V˙O2Reserve is a candidate metric for establishing precise physical activity intensity cut points for older adults. Clinical Trials Registration Number: NCT04821713.

Keywords: Aerobic reserve capacity; Daily activities; Energy expenditure; Maximum oxygen uptake; Resting metabolic rate.

Figure 1.

Age-related loss of $\dot{V}{O}_2$Reserve and classification of physical activity intensity. (A) Best fitting line of age-related changes in RMR and $\dot{V}{O}_2$max of the ENGAGE participants. (B) Classification of moderate (3 METs, red line) and vigorous (6 METs, blue line) intensity with increasing age using estimated METs (3.5 mL O₂·min⁻¹·kg⁻¹) as 1 MET. (C) Classification of moderate (3 METs) and vigorous (6 METs) intensity with increasing age using measured METs (2.65 mL O₂·min⁻¹·kg⁻¹) as 1 MET. MET = metabolic equivalent of task; RMR = resting metabolic rate.

Figure 2.

$\dot{V}{O}_2$ during activities ranging from RMR to $\dot{V}{O}_2$max. Barplot with mean values of oxygen uptake and error bars as standard deviation. ADL = activities of daily living; 6-MWT—Max = 6-minute walking test at maximal gait speed; 6-MWT—Self = 6-minute walking test at self-selected gait speed; RMR = resting metabolic rate.

Figure 3.

Intensity of activities according to METs_measured and METs_estimated. Grouped barplot of measured METs (filled bars) and estimated METs (open bars) for ADL, walking activities, and $\dot{V}{O}_2$max. Error bars represent standard deviation. METs physical activity intensity classification: light (<3.0 METs); moderate (3.0–5.9 METs); and vigorous (≥6 METs) categories. Red line = 3 METs; blue line = 6 METs. ADL = activities of daily living; MET = metabolic equivalent of task; 6-MWT—Max = 6-minute walking test at maximal gait speed; 6-MWT—Self = 6-minute walking test at self-selected gait speed. *Significant difference between measured and estimated METs (p < .001).