Heart rate monitors are recommended during use to prevent over-training and maximally improve lactate threshold via lactate threshold training programs.  If you are not educated on lactate threshold training and VO2 Max training, you should read up before using this device for such activity. Polar H10 is the recommended HR Monitor for use with this device.  Go to polar.com and search for Lactate or VO2 Max training.Fitness watches with HR monitors are helpful; however, an H10 Polar HR monitor is recommended for the most accurate HR data.HR monitoring is highly recommended to help prevent overtraining.