Aquick and easy way to monitor intensity level?

Your wrist or the side of your neck.

Explanation:

According to https://myturningpoint.org, the two ways to monitor your heart rate are either on your wrist at the base of the thumb, or at the side of your neck. To do this, you need to use two fingers to press lightly on the artery. Once you feel a pulse beat, you count the number of beats in a time period of 6 seconds.

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