Improper technique? Tricep curls from a low bench

Hi all,

I've been working out with a trainer since Dec. but I had to switch because my first trainer quit.  He had taught me to do something that I don't really know the proper term for so I'm calling it tricep curls. 

Anyway, I've tried this twice since he left and both times I've hurt my mid back.  Here's what I'm trying to do.....(any advice on correct posture/positioning is appreciated)....Lying on my back, feet on the ground and using a 20 lb weight bar (curved grips, not a straight bar).  With the bar overhead I slowly lower the bar, bending at the elbows with my elbows tucked inward.  Then I press the bar back up. 
I'm guessing that I am either lowering the bar to far or arching my back which is causing the injury.  Should I have my feet up on the bench so that my back stays flat against the bench?  Maybe the bench is just too tall???

Thanks!

http://www.myfit.ca/exercisedatabase/viewanex ercise.asp?table=exercises&ID=57

Skull crushers?

I'd suggest less targeted work  but that's me.

Feet on the floor.

Feet on the bench can help you push your back into the bench, just keep a close 'eye' on how your form feels so you aren't digging too much.  I do these often on a step bench, as opposed to a weight bench, so your bench can be relatively low to the ground.

Definitely keep your abdominals nice and tight so your back is pressed against the bench.  And do not go lower than 90 degrees with your arms.  Certainly no further than your forehead - if you are extending past your head, you are probably going to far, and you are probably also losing the good angle with your elbow to where it becomes greater than 90.

If your back is at all sensitive on the first two reps, switch to a lighter weight.  It could just mean you are compensating with the rest of your body because your arms cannot handle the weight.

maybe go lighter til you get the form right. i love skull crushers. i put my feet on the bench, knees bent (i have lower back issues).

other tripcep options

I like doing those - have always called them "nose breakers."  (but that is probably something I made up in my own head?...)  If you are having trouble with form, however, there are lots of ways to work your triceps - just choose something that works for you.

Thanks for all the advice.  I looked at the pic for "skull crushers" and that is what I was trying to do.  I believe I may have had my arms a little too far over/behind my head at the start (so that I wouldn't crush my skull :-).  Anyway, that may have been what caused me to arch my back as well as the fact that I was probably going past the 90 degree mark.

Thanks to all.  I think I'll just avoid this one and do some of the alternative exercises for triceps.