Descending sets ask you to swim each repetition faster than the previous. For example, the following set:
6x 50: Descend 1->6
asks you to swim six 50’s with each one faster.
If the set read:
6x 50: Descend 1->3, 4->6
you are asked to descend the first 3 and the second 3 in the set. The fourth 50 should be slower than the third. The third and sixth are the fastest in the set. Unless specified, the interval stays the same, giving you a bit more rest as your swimming time descends.
Building is different from Descending in that the swimmer’s goal is to increase speed within the single swim distance(s). For example, the following set:
3x 100: Build
asks you to swim each 100 starting easy (with perfect technique) and increasing speed within each 100 to a fast finish (maintaining perfect technique throughout). As you may have guessed, the goal in a “Build” swim is to build speed while maintaining good stroke technique.
Swimming a Negative Split means that the second half of the distance is swum at a faster pace than the first half. For example, the following set:
1x 600: Negative Split
asks you to swim the second 300 yards at a faster pace and time than the first 300 yards. The idea is to control your pace at the beginning of the swim so that you have the energy necessary to swim faster at the end of the swim.
IM (Individual Medley)
An Individual Medley is comprised of swimming equal distances for all four strokes in the following order: Butterfly, Backstroke, Breaststroke, and Freestyle. For example:
100 IM = 25 Butterfly + 25 Backstroke + 25 Breaststroke + 25 Freestyle
400 IM = 100 Butterfly + 100 Backstroke + 100 Breaststroke + 100 Freestyle
FRIM (Freestyle Individual Medley)
A Freestyle Individual Medley substitutes Freestyle for Butterfly. Butterfly is a difficult stroke. By not swimming the Butterfly segment of an IM, FRIMs in workouts enable swimmers to spend more energy focusing on the middle part of the IM.