Figure 2. Comparison of energy expenditure and macronutrient partitioning via Polar Grit X Pro and metabolic cart by group in each heart rate (HR) zone, stages 1-5. The Polar Grit X Pro watch had good to excellent reliability compared with the metabolic cart at estimating energy expenditure combined across all HR zones, but it underestimated percent fat and overestimated percent carbohydrate at higher intensities.