cycling solid E under workout detail

I've been using Apple Fitness+ since it was released and it's fantastic.

I recently noticed an icon that I am not familiar with in the description under "indoor cycle" on the line with CC, SDH, and AH ... There's a solid block with a capital E inside it.

It is also after the title of the workout ie: cycling with Kym "E"

It's not on all the cycling workouts.

I've looked all over and can't find what this refers to.

Can anyone shed some light on this?