The aim of this study was to evaluate the validity of heart rate (HR) measurements by a photoplethysmography (PPG) fitness device during a sustained isometric contraction that models resistance training. PPG technology relies on blood flow and volume changes to determine HR, and evidence suggests that sustained isometric contractions as low 15% can affect blood flow, thereby potentially affecting accuracy of HR measurement. It is unknown if PPG heart rate measurement validity is affected by isometric contractions similar to gripping an implement during resistance training. This study used a within-subjects repeated measures design. Subjects completed three hand-gripping protocols at 15%, 35%, and 55% of their maximal voluntary contraction (MVC) while completing a variable intensity cycle ergometer protocol designed to induce an increase in heart rate. Subjects wore sensors on both an experimental (handgrip) limb and control (non-gripping) limb at wrist and forearm locations to determine HR response. PPG device HR was compared to a criterion HR chest strap. The overall results of the study revealed differences in mean absolute percentage error (MAPE) and intraclass correlation coefficient (ICC) between wrist and forearm locations. Wrist locations produced MAPE values of 9.92% and 8.90% and ICC values of 0.58 and 0.63 at the experimental and control sides, respectively. Forearm locations produced lower MAPE values of 3.72% and 3.92% with higher ICC values of 0.91 and 0.86 at the experimental and control sides, respectively. MAPE values increased to 16.3% and ICC values decreased to .38 during specific phases at the wrist location. Conversely, at the forearm location, MAPE values increased at most to 6.92% with a lowest ICC value of .64 during specific phases. The specific MVC intensity did not affect validity (p=.621). These combined results suggest that sustained isometric contractions affect validity of PPG devices and result in lower validity at the wrist location compared to the forearm location which is not specific to the intensity of the isometric contraction. Individuals using PPG monitoring technology to track HR response during resistance training should consider using a forearm location versus a wrist-worn device if precision of the HR measurement device is of primary concern.