Figure 1. Comparison of energy expenditure and macronutrient partitioning via Polar Grit X Pro and metabolic cart by participant in each heart rate zone, stages 1-5. The Polar Grit X Pro watch had good to excellent reliability compared with the metabolic cart at estimating energy expenditure (intraclass correlation [ICC] = 0.8) and had fair to good reliability in estimating percent fat and percent carbohydrate breakdown (ICC = 0.49).