If your heart is not strong enough or efficient enough, it is difficult to
stretch
lift weights
breathe
perspire

If your heart is weak and inefficient it can struggle to pump blood to the muscles being used. Without this blood oxygen cannot be delivered to the muscles hence the exercises become very difficult. This is why it is important to look after your heart, and ensure it is healthy before undertaking strenuous exercise.

The best way is to swollow and hold in your breath