Verified Answer :

If your heart isn't strong enough it's is hard to stretch, lift weights, breathe, perspire.

Answer:

Breath.

Explanation:

Lungs and heart depend on each other, they are closely linked. So, if the heart is not strong enough, breathing process is compromised, because lungs functions rely on the heart, and vice versa. All this happens because oxygen is carried by bloodstream, therefore, if the heart is not strong enough, cells will not get enough oxygen.