Although, I do not believe anyone is an "expert" on anything because exercise is ever-changing, progressing, and becoming increasingly scientific, this article from the NY Times is excellent.  

                

“There is a neuromuscular inhibitory response to static stretching,” says Malachy McHugh, the director of research at the Nicholas Institute of Sports Medicine and Athletic Trauma at Lenox Hill Hospital in New York City. The straining muscle becomes less responsive and stays weakened for up to 30 minutes after stretching, which is not how an athlete wants to begin a workout.

THE RIGHT WARM-UP should do two things: loosen muscles and tendons to increase the range of motion of various joints, and literally warm up the body. When you’re at rest, there’s less blood flow to muscles and tendons, and they stiffen. “You need to make tissues and tendons compliant before beginning exercise,” Knudson says.

Leave the old-fashioned static stretches for after your workout, they make an excellent cool-down without inhibiting the strength and power of your workout.