Here's a quote from Stretching Roundtable.
Catanzaro: Bigger muscles? Perhaps. But stronger? I say no way! In my stretching article for T-mag, I mentioned two methods of aggressive stretching advocated by John Parrillo and Torbjorn Akerfeldt where the object is to expand the fascial compartment thus allowing greater room for growth. The classic bird study you've probably heard about also proves that stretching may have some merit for muscle growth.
Now, as far as strength is concerned, that?s a whole 'nother story! Sure, dynamic stretching may increase strength temporarily, but static stretching will definitely weaken muscle. The proposed theories of force decrement with stretching (which breaks down to roughly 60% neural and 40% muscular/contractile) include decreased motor neuron excitability, increased tendon slack, decreased stiffness, and altered actin-myosin position.
As the length of the muscle increases, stiffness decreases. As stiffness decreases, force decreases, which means?drum roll please?strength decreases!