Originally Posted by Flexor
Technically, there is no stretch of the biceps during preacher curls, this occurs during hyperextension of the shoulder with the upper arm behind the back. However, there is a definite stretch feeling in the biceps with preacher curls and I believe this is because with the shoulder flexed, the bicep is already shortened partially at the beginning of the preacher. So in esence you are performing an abbreviated or partial curl in which stresses on the muscle are far more constant than a standing curl. Biomechanics and muscle tension is different because you have moved your position relative to the pull of gravity.