Figure 3. Correlation of energy expenditure via Polar Grit X Pro and metabolic cart in each heart rate (HR) zone, stages 1-5. When separated by HR zone, energy expenditure reported on the Polar Grit X Pro watch had negligible reliability compared with the metabolic cart in zones 1, 3, 4, and 5 (zone 1: intraclass correlation [ICC] = 0.15, zone 3: ICC = 0.26, zone 4: ICC = 0.09, zone 5: ICC = 0.07) and fair reliability in zone 2 (ICC = 0.58). Pearson’s R values provided for ease of interpretation.