What is positive elevation gain (D+) in trail running?

The more it climbs, the more it hurts
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What is positive elevation gain (D+) in trail running?

Positive elevation gain (D+) in trail running is the total cumulative ascent: the sum of all the meters you climb over a course, climb after climb. It is not the difference between the lowest and highest point, but the addition of each ascent. The higher the D+, the tougher the trail is.