What was that strap that Cotto was using on 24/7?

http://youtu.be/7pEnU0189L0 skip to the 4:00 minute mark. they explain what it is and what it does.

it's a heart monitor I was told Yes thats exactly what it is. If you've ever been in a marathon or any organized race, you would see 75% of the runners wearing one

is a monitor. I believe it monitors the heartrate and e few other things. I seen a vid with pacquiao a while back wearing the same thing while running during training. and a doctor explained it (ill see if i can find the video). but yea its basically a heart rate monitor

it's a heart monitor I was told

you mean a resistance band?