Gymwatch

wearable strap that not only captures data on pretty much any exercise you’d like to try out — it also helps you train. The device captures a full range of motion and can tell you if you’re doing a certain exercise too fast or too slow, and it can also help make sure you’re doing it with the correct form.

See also: http://www.trainwithpush.com/

finally a watch for the gym! Enough running detectors already! Looking to trying this one!