The leading cause of morbidity and mortality in the world is ischemic heart disease. Physical activity is a major approach in prevention and therapy of cardiac diseases. Self-heart-rate-monitoring in daily life is an important point for health awareness of cardiac patients. Aim of this study was validation of measurement accuracy of seven different devices against ECG-monitoring during cardiac rehabilitation training on a bicycle ergometer. Tested devices were: Garmin Forerunner 35 (Garmin), Mio Fuse (Mio), Fitbit Charge HR (FitbitHR), Fitbit Surge (FitbitS), Withings Pulse™ Ox (Withings), Apple Watch Series 1 (Apple) and Pearl Fitness-Tracker (FBT-50.HR PRO.V4). All devices were tested on 35 participants with six timed measurements during 20 minutes constant load bicycle ergometer workout for each. Simultaneousely, ECG measurements were recorded. Pearson´s correlations were assessed. Apple, Mio, and Garmin showed excellent accuracy with close correlation to ECG for self-monitoring of heart rate (HR) during cycling. FitbitHR, Pearl and FitbitS presented reasonable results. In contrast, Withings showed poor correlation to ECG with significant differences. We found significant differences between the tested devices. Since accuracy is of major importance for cardiac patients, only Apple, Mio and Garmin could be recommended. However, further research within distinct clinical and non-clinical settings is necessary and should take different types of physical activities into account.