What thoracic cavity is the heart located in

The thoracic cavity medial to the lungs and posterior to the sternum

The heart is located in the mediastinum.

Well if you pick a machine to work on and that's gonna be your exercise if you don't stick to that machine your muscle strength your muscle mass rather will change because you haven't been doing it you change something in your exercise routine which changes your body's form of dealing with things for example you might become weaker or stronger depending on what kind of exercises you do in whether or not the machine you picked actually your muscles grow.

Keep your breathing calm and try to ignore it