One end of a lap

KNEES

Think of what bends when you sit down or starts a race—it’s not your elbows.

The word "lap" here isn’t about running—it’s the part of your body when you sit down, where your legs meet. The "end" of that lap would be the bony joints at the bottom, which are your knees. It’s a playful twist on the word, using "lap" as both a body part and a common phrase for a single circuit in racing.