Heart Rate Monitors

Effective aerobic conditioning requires that you maintain your heart rate
at the proper intensity level for at least 20 minutes per workout. If your heart rate is too high, your activity can become counter effective.

Use a heart rate monitor.

For most people, as your heart rate exceeds 85% (the upper limit), your workout causes the body to to become anaerobic and produce lactic acids.
This also burns less fat, which can cause a burning sensation and
strain within the muscles.

It's important to reach a level of intensity that is productive. A heart rate monitor will tune you into your body's internal activity level and help prevent injury.