In the workout, each set name is followed by bk such as:
ZNMXZ it appears those are the initials for the person writing the workout Brenda Kottke (bk) for the Basic Training workouts in the USMS Workout Library. www.usms.org/.../ladder-again-bk-18253
Ah! That makes sense. I had the same qeustions from looking up workouts.
