Final answer:
An increase in muscle mass does not affect the heart rate during steady-state cardio as it is a static characteristic that does not fluctuate with exercise intensity.
Step-by-step explanation:
An increase in muscle mass would not change the intensity of heart rate for steady-state cardio. During steady-state cardio, the body's demand for oxygen, blood pressure, and body temperature may change to meet the energy requirements of the activity. However, muscle mass is a more static characteristic that doesn't fluctuate with the intensity of exercise like the heart rate does. Therefore, while an individual's muscle mass might contribute to their overall fitness level, it won't directly affect the heart rate during a steady-state cardiovascular exercise.