Final answer:
The simplest way to define workout intensity on an erg is through heart rate monitoring, as it directly relates to the body's physiological response to exercise and helps maintain the workout within the target heart rate range.
Step-by-step explanation:
The simplest way of defining the intensity of a workout on an erg (rowing machine) can be measured in several ways, but the measurement most closely related to the body's response to exercise intensity is heart rate monitoring. This method is effective because as exercise intensity increases, the heart rate also increases proportionally.
This correlates with the concept of cardiac output, which is the volume of blood the heart pumps in one minute, and is calculated by multiplying the stroke volume by the heart rate. Monitoring heart rate during a workout can help ensure that the individual is training within the target heart rate range, achieving maximum aerobic benefit without overexertion.