Predicting METs from the heart rate index in persons with Down syndrome

Persons with Down syndrome (DS) have altered heart rate modulation and very low aerobic fitness. These attributes may impact the relationship between metabolic equivalent units (METs) and the heart rate index (HR_index-the ratio between heart rate during activity and resting heart rate), thereby altering the HR_index thresholds for moderate- and vigorous-intensity physical activity. This study examined whether the relationship between METs and HR_index differs between persons with and without DS and attempted to develop thresholds for activity intensity based on the HR_index for persons with DS. METs were measured with portable spirometry and heart rate with a monitor in 18 persons with DS (25±7 years; 10 women) and 18 persons without DS (26±5 years; 10 women) during 6 over-ground walking trials, each lasting 6min, at the preferred walking speed and at 0.5, 0.75, 1.0, 1.25, and 1.5m/s. The relationship between METs and HR_index in the two groups was analyzed with multi-level modeling with random intercepts and slopes. Group, HR_index, and the square of HR_index were significant predictors of METs (p<0.001; R²=0.65). Absolute percent error did not differ significantly between groups across speeds (DS: 19.6±14.4%; non-DS: 21.0±14.5%). Bland-Altman plots demonstrated somewhat greater variability in the difference between actual and predicted METs in participants with than without DS. The HR_index threshold for moderate-intensity activity was 1.32 and 1.20 for persons with and without DS, respectively. The HR_index threshold for vigorous-intensity activity was 1.80 and 1.65 for persons with and without DS, respectively. Persons with DS have an altered relationship between METs and HR_index and higher HR_index thresholds for moderate- and vigorous-intensity physical activity.