If you feel a tendancy to ride with locked joints, then your riding position is wrong, and no amount of technical add-ons will correct that.
1. Make sure your pedal-saddle orientation is good for you.
2. Bring your bars higher and closer.
3. If riding drops, consider a narrower bar. Too wide bars result in a triangulated position, where locked elbows are ineviatable. With narrower than normal bars, your arms appear bowed out at the elbows, and cannot lock up.
4. If on drops, tilt your brake levers onwards for a more neutral wrist position.

Often raising your bars results in a more comfortable position and a lower back.
If you find you need a really short stem to achieve comfort, then you need a shorter bike.
I ride a pretty short position, but find it impossible to lock my elbows.

If you can send a picture of you riding, taken from the side, maybe we could help.

see
http://www.peterwhitecycles.com/fitting.htm