When I got on a swim team that tied us to the side with surgical tubing, my performances improved dramatically. We learned how many times each of us could wrap the tubing around the dive blocks to shorten them, making them harder. And then the goal was to try to swim to the other side. If you got it right, you were holding a maximum sprint for quite a while trying to just move one inch and grab the far wall.
For people that want to play with resistance but don't want to deal with all that pulley stuff, just order a drag chute off of Finis. They are around 20 to 30 bucks and work great. And they nearly double the effective length of your pool since you spend a lot more time swimming between turns.
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Zen and the Art of Triathlon.
Strava Workout Log Interviews with Chris McCormack, Helle Frederikson, Angela Naeth, and many more.
http://www.zentriathlon.com