Definitely felt engagement of my core muscles when doing armstroke today. One of my 200's was a 2:57, which is pretty good for me considering I'm babying my ankle while kicking and I did not feel as out of breath as some equivalent efforts.

Warm up
2 sets as:
300 free
200 kick
100 choice
all on 20 sec recovery

Main Set
5x200 free on 15 sec recovery
2x400 pull buoy on 30 sec recovery

Cool Down
12x50 choice easy