Step-rate thresholds for moderate and vigorous-intensity activity in persons with Down syndrome

Objectives: Monitoring physical activity intensity in persons with Down syndrome (DS) may be affected by an altered relationship between metabolic equivalent units (METs) and step-rate. This study examined whether the relationship between METs and step-rate is altered in persons with DS and developed step-rate thresholds for activity intensity for these persons. Design: Cross-sectional. Methods: Eighteen persons with DS (25±7years; 8 women) and 22 persons without DS (26±5years; 9 women) completed six over-ground walking trials each lasting 6min at their preferred speed and at 0.5, 0.75, 1.0, 1.25, and 1.5ms^-1. METs were measured with portable spirometry and step-rate with hand-tally. Random effects models were used to predict METs from step-rate, squared step-rate, height, presence of DS, sex, and body mass index (BMI). Results: Step-rate, squared step-rate, height, and presence of DS contributed significantly to the model (SE=0.20 METs; R²=0.63); sex and BMI did not contribute. As height increased, step-rate thresholds for moderate and vigorous-intensity activity decreased. For a given height, participants with DS had lower step-rate at the moderate-intensity threshold than participants without DS. Across participant heights, the moderate-intensity cut-off ranged between 101 and 76stepsmin^-1 in persons with DS and between 103 and 80stepsmin^-1 in persons without DS. For persons with DS, step-rate at the vigorous-intensity threshold ranged between 136 and 126stepsmin^-1. Conclusions: Persons with DS showed altered relationship between METs and step-rate and had lower step-rate thresholds for moderate-intensity activity than persons without DS.