Determine the metric to use for executing your session

In order to execute your sessions as they are intended, it is important to use the primary metric to guide your effort. The primary metric can be either power(lightning bolt), pace(clock), or heart rate(heart). Typically, bike workouts have a primary metric of power, and swims and runs have a primary metric of pace. Easy sessions typically have heart rate as the primary metric. 

To identify what the primary metric is for a session, look at your Sessions page for your upcoming workout and notice the icon above the potential NTS score.