You can overexert your heart when you are working out. Working out for prolonged periods of time at or above your maximal heart rate is not recommended.
A basic formula for determining your maximal heart rate is 220 minus your age. For example, the maximal heart rate for a 35-year-old is 185. It is recommended to have a 5- to 10-minute warm-up before beginning a workout, and then a 5- to 10-minute cooldown after finishing exercising. This not only warms up your skeletal muscles to prevent injury, but also increases your heart rate gradually.
It is also recommended that you perform cardio respiratory workouts 3 to 5 times per week to improve your heart health. This allows a day between each workout for your body and heart to recover from intense workouts. Without this recovery time, you could cause fatigue and possible overuse injuries.
(This answer provided for NATA by the King College Athletic Training Education Program.)