Cardio loads, in principle, are created in order to strengthen the heart, cardiovascular system . They are not harmful if you follow a few simple rules:
If your load matches your level of health and physical fitness, then sport is only beneficial. The question is how to find out the limit of your physical capabilities and regulate the load. The question is aggravated by the fact that your physical capabilities are not constant, but constantly changing. For example, if you change your diet or you have a "blockage at work" with increased stress, then the total load and stress have increased and you need to change the level of physical activity.
Professional athletes use RMSSD to monitor exercise as part of the heart rate variability methodology. It was created precisely in order to be able to adjust the level of load on a daily basis, avoid "overtraining" and do not to harm health, but to its benefit
If you do not focus on a particular sport, heavy cardio exerts the main harm to the heart. And this is training with a pulse of more than 170 beats / min for more than five minutes.
The fact is that the heart muscle feeds through the precardial valve, which opens when the heart rests between impulses to push blood. So, with a pulse of 170 beats / min. this valve simply does not have time to open and the heart begins to starve. And as you know, cells without oxygen die in five minutes.
If you don't do hard physical training, there is nothing to fear. Cardio will only be beneficial.
Excessive exercise can lead to athletic heart phenomena. Stretching of the walls of the heart leads to its hypertrophy. (link below). How to reduce harm? Know your indications for running / cycling. This is not everyone's shown activity for certain health parameters. Monitor your heart rate with a heart rate monitor. Do not jog / cycle for more than one and a half to two hours a day, unless you are a professional athlete.
These types of workouts - light running, cycling - are called "cardio" because they are good for the heart. Its walls gradually stretch without thickening, which makes it possible to pump more blood for the same number of contractions, i.e. As a result, your pulse becomes calmer with the same loads that made your heart pounding before.
"Cardio" is done in order to reduce the harm to the heart from strength exercises.