My swim training today says the following:
16 x 50, descend 1-4, 5-8, 9-12, 13-16
Can someone describe what that means please? Thanks!
Descend = Get faster as you go (descend the pace).
So your set is: 4 is faster than 3 which is faster than 2 which is faster than 1. Same for the other groups of 4.
Take a look-see right here to get a nearly exhaustive list of swim training definitions you might encounter in the future.
Chris