What causes beating in the left upper chest near armpit?

Beating in the left upper chest near the armpit can be caused by several factors, including muscle contractions, heart palpitations, or anxiety. It might also result from the movement of the heart or blood vessels, especially if you're physically active. In some cases, it could indicate an underlying condition, so if the sensation is persistent or accompanied by other symptoms, it’s important to consult a healthcare professional for evaluation.