Lamberts, RP and Lambert, MI. Day-to-day variation in heart rate at different levels of submaximal exertion: implications for monitoring training. J Strength Cond Res 23(3): 1005-1010, 2009-The HIMS test, which consists of controlled exercise at increasing workloads, has been developed to monitor changes in training status and accumulative fatigue in athletes. As the workload can influence the day-to-day variation in heart rate, the exercise intensity, which is associated with the highest sensitivity, needs to be established with the goal of refining the interpretability of these heart rate measurements. The aim of the study was to determine the within-subject day-to-day variation of submaximal and recovery heart rate in subjects who reached different exercise intensities. Thirty-eight subjects participated in this study and after familiarization were allocated to 1 of 4 groups based on the percentage of predicted heart rate maximum that was elicited during the first test (i.e., groups: <85, 85-90, 90-95, and >95% maximum heart rate). Variation in heart rate was determined for the following 4 days at a range of intensities (61-98% of maximum heart rate) and recovery periods. Variation in heart rate decreased with increasing exercise intensity in all groups. The lowest variation in heart rate was found at the end of the last stage of the test in the 85-90% group (3 ± 1 b·min−1) and >95% group (3 ± 2 b·min−1). The lowest variation during the recovery periods occurred at the first minute after the last stage. Although there were no significant differences between the groups, the 85-90% group showed a tendency to have the lowest variation in heart rate. If changes in heart rate and heart rate recovery are to be monitored in athletes, a submaximal protocol should elicit heart rate between 85 and 90% of maximum heart rate, because this intensity is associated with the least day-to-day variation.