Evaluating Reliability and Validity of Heart Rate from the Garmin Instinct 2 During Deadlift Exercises at Different Intensities

Background and Objectives: Wearable heart rate monitors are widely used for performance monitoring and safety, yet their validity during resistance training is limited. This study evaluated the accuracy and reliability of wrist-worn Garmin Instinct 2 monitors during deadlift exercise across multiple intensities, using the Polar H10 chest strap as the criterion measure. Methods: Twenty-one recreationally active adults (four women, 17 men, 0 identified otherwise) completed deadlift sets at low (RPE 3–4) and moderate-to-high (RPE 6–8) intensities while heart rate (HR) was recorded using four Garmin Instinct 2 devices placed on the inner and outer wrists of each forearm and one Polar H10 chest strap. Inter-device reliability was examined using the Intraclass Correlation Coefficient (ICC [1,2]) and the Coefficient of Variation (CV). Validity was assessed via mean absolute percentage error (MAPE) and Lin’s Concordance Correlation Coefficient (CCC). Results: Outer wrist placement met the threshold for reliability and one measure of validity during low intensity exercise (average HR CV = 2.7%, ICC = 0.88, MAPE = 8.5% [left forearm], 9.5% [right forearm], CCC = 0.55 [left forearm], 0.52 [right forearm]; maximal HR CV = 2.9%, ICC = 0.88, MAPE = 3.2% [left forearm], 7.7% [right forearm], CCC = 0.61 [left forearm], 0.60 [right forearm]). No validity thresholds were met during moderate-to-high intensity exercise (average HR MAPE = 15.9% [left forearm], 15.9% [right forearm], CCC = 0.57 [left forearm], 0.60 [right forearm]; maximal HR MAPE = 30.1% [left forearm], 13.5% [right forearm], CCC = 0.53 [left forearm], 0.55 [right forearm]). Inner wrist placement did not meet any reliability or validity threshold for any condition. Conclusions: Outer wrist placement of the Garmin Instinct 2 achieved acceptable MAPE under low-intensity loading, but measures of accuracy appeared to decline with increased intensity. Inner wrist placement failed to return any conditions in which reliability or validity thresholds were met for either average or maximal heart rate. For training or occupational settings requiring precise HR monitoring, chest strap monitors or bicep-worn sensors are still recommended.