The sensation felt in the left arm is your pulse, a vibration in the arteries as blood moves through them that corresponds to the number of times the heart beats. A pulse can be felt at many points in the body, including inside the upper arm, near the elbow and the lower wrist.
An individual's pulse corresponds to his heart rate, which is the number of times the heart beats per minute. A person can check his own pulse by resting two fingers on one of the check points and counting the number of times that the heart beats within a minute. Do not use the thumb when checking the pulse, because the thumb has its own pulse, explains WebMD.
The pulse can be most prominently felt after exercising or when the body is under a condition of stress, which can explain why a pulse feels stronger at certain times. It is normal to feel a steady pulse, unless it beats abnormally fast, abnormally weak, or in an unusual pattern for an extended period of time. Under those circumstances, it is advised to seek medical attention, notes WebMD. Consult a doctor if the heart rate is below 60 beats per minute or above 100 beats per minute, suggests Mayo Clinic.