Since messing up my thighs playing a soccer game, I have been using a pull float to immobilze my legs (otherwise, my injured thigh hurts like hell!). I am amazed at how this helps me improve my arm stroke and strength. I also wear my heart rate monitor, and without using my legs, I have to pull really hard with my arms in order to get my heart rate up to the desired zone.
What a great workout! Even after my thigh heals, I will keep doing some swim workouts like this. š